path: root/src/acmefs.zig

//! ACME'S CONTROL FILESYSTEM, as a pure transaction over the core.
//!
//! plan9's acme serves `/mnt/acme`: a directory per window holding `addr`,
//! `body`, `ctl`, `data`, `event`, `tag`..., and a program that opens those
//! files IS an editor extension — no plugin API, no embedded interpreter, no
//! rebuild. `pardes --fs` serves the same tree over Linux FUSE (src/fuse.zig),
//! and this file is the whole of what the files MEAN. Read it beside acme's
//! `fsys.c` (the tree) and `xfid.c` (the handlers).
//!
//! THE SHAPE, and why it is this shape. A filesystem is a request/response
//! protocol driven by other processes, i.e. exactly the kind of concurrency
//! the core does not have and must not grow. acme answers it with a thread per
//! in-flight request (`xfidallocthread`, a `Channel` per `Xfid`, a `QLock` per
//! window); pardes cannot and should not, so:
//!
//!   * This module is a PURE MAIN-THREAD TRANSACTION: `handle(p, req) Reply`.
//!     No thread, no waiting, no callback, no allocation on the hot path. It
//!     is freestanding-safe (no libc, no OS) and unit-testable with no FUSE
//!     anywhere near it — the tests below post requests and read replies.
//!   * Requests arrive as an ordinary `Event.fs_req` and answers leave as an
//!     ordinary `Effect.fs_reply`, so the transport is the queue every other
//!     host<->core message already uses. A backend with no threads at all
//!     (the browser, a test) is not a special case: it either never sends a
//!     request, or sends one from its own frame loop.
//!   * BLOCKING — acme's `event` file, whose read waits for the user to do
//!     something (acme parks the `Xfid` in `w->eventx` and a later `winevent`
//!     sends it a message) — is `Status.again` here: "nothing consumed, ask me
//!     again". The waiting lives in the host, which is where the kernel's
//!     request already is. The core keeps no waiter list and no wakeups.
//!
//! DIVERGENCE FROM ACME, deliberate: acme counts RUNES, pardes counts BYTES
//! (clamped to grapheme boundaries). Every offset in this filesystem — `addr`,
//! `data`, the event records' q0/q1, `index`'s lengths — is a byte offset,
//! because pardes is byte-addressed end to end (selections, look spots, LSP
//! offsets) and a second coordinate system would mean an O(n) conversion at
//! every boundary and a lossy `addr=dot`. acme pays that cost the other way
//! round: it keeps the document as `Rune*` and converts on every utf read
//! (`xfidutfread`, which carries a "BUG: stupid code: scan from beginning"
//! comment for its cache miss). Identical for ASCII, which is what scripts
//! compute with.
const std = @import("std");
const mvzr = @import("mvzr");
const pardes = @import("pardes.zig");
const config = @import("config.zig");
const modal = @import("modal.zig");
const file_pane = @import("file_pane.zig");
const output_pane = @import("output_pane.zig");
/// only for the scrollback read below: a terminal's `body` is a grid, and
/// term_pane owns how a grid becomes bytes (and whether there is one at all).
const term_pane = @import("term_pane.zig");
/// only for `look.readFile`, which is what a `get` verb IS — the same
/// synchronous path-backed read `file_pane.open` does, and the one place this
/// module touches a disk.
const look = @import("look.zig");
const Pardes = pardes.Pardes;
const Pane = pardes.Pane;
const MAX_PANES = pardes.MAX_PANES;

// ============================================================================
// THE ABI — what a transport hands in and gets back.
// ============================================================================

/// The filesystem operations the core answers. Protocol-neutral on purpose:
/// FUSE opcodes, 9P messages and a unit test all reduce to these.
pub const Op = enum(u8) {
    /// resolve `data` (a name) inside the directory `node`
    lookup,
    getattr,
    /// only `truncate` is honoured; a filesystem of live editor state has no
    /// mode, owner or timestamps to set
    setattr,
    open,
    read,
    write,
    release,
    readdir,
    statfs,
};

pub const Status = enum(u8) {
    ok,
    /// NO DATA YET, nothing consumed: the transport must hold this request and
    /// re-submit it unchanged on a later frame. The one blocking primitive,
    /// and the reason the core needs no waiters (see the header).
    again,
    err,
};

/// One operation. `data` is BORROWED for the length of the single
/// `update(.{ .fs_req = ... })` call that carries it — the same rule as
/// `.pty_read`'s bytes — so this never goes through `postEvent`.
pub const Req = struct {
    /// opaque echo token; the transport's request id (FUSE `unique`)
    tag: u64,
    op: Op,
    node: u64,
    /// from `.open`, on read/write/release
    handle: u32 = 0,
    /// read/write: byte offset. readdir: how many entries to skip.
    off: u64 = 0,
    /// read/readdir: bytes wanted. Writes carry their length in `data`.
    size: u32 = 0,
    /// lookup: the name. write: the bytes.
    data: []const u8 = &.{},
    /// setattr: a size was set (only 0 means anything here)
    truncate: bool = false,
};

pub const Reply = struct {
    tag: u64,
    status: Status = .ok,
    /// positive errno when `status == .err`
    errno: u16 = 0,
    /// lookup/getattr/setattr answer this; `open` leaves it zeroed
    attr: Attr = .{},
    /// `open` answers this; every later read/write/release repeats it
    handle: u32 = 0,
    payload: Payload = .none,
    /// write: how many of the offered bytes were taken. A short count is a
    /// real answer (`data` refusing a partial grapheme), not an error.
    written: u32 = 0,

    pub const Attr = struct {
        node: u64 = 0,
        dir: bool = false,
        size: u64 = 0,
        /// permission bits only; the transport adds the format bits
        mode: u16 = 0o600,
    };

    /// WHERE THE ANSWER'S BYTES ARE. Resolved by `pardes.fsPayload` inside the
    /// effect drain — a borrow window identical to `.save_text`'s — so reading
    /// a megabyte of body copies nothing.
    pub const Payload = union(enum) {
        none,
        /// `State.out[0..len]`: formatted answers (ctl, addr, index, dirents,
        /// event records). Valid until the next `handle` call.
        staged: u32,
        /// a slice of a live pane's text. `serial` rejects a reused slot
        /// exactly like `.save_text` does.
        region: struct { pane: u8, serial: u32, off: u32, len: u32 },
    };

    pub fn fail(tag: u64, e: u16) Reply {
        return .{ .tag = tag, .status = .err, .errno = e };
    }
};

/// The errno values this filesystem returns, standing in for acme's error
/// strings (`fsys.c`/`xfid.c`: Eperm, Ebadctl, Ebadaddr, Ebadevent, Edel...).
/// A filesystem has one channel for "no": the number.
pub const E = struct {
    pub const PERM: u16 = 1;
    pub const NOENT: u16 = 2;
    pub const IO: u16 = 5;
    pub const NOMEM: u16 = 12;
    pub const NOTDIR: u16 = 20;
    pub const INVAL: u16 = 22;
    pub const NFILE: u16 = 23;
    /// the one FILE here with a fixed capacity: a pane's editable tag tail is
    /// a bounded one-line buffer, so a write with no room left is full rather
    /// than refused (`writeTag`)
    pub const NOSPC: u16 = 28;
    pub const NOSYS: u16 = 38;
};

// ============================================================================
// THE TREE — nodes, names, and the packing that makes both cheap.
// ============================================================================

/// One file inside a pane's directory: acme's `dirtabw` minus the plan9
/// compatibility stubs (`editout` needs acme's Edit language; `draw`,
/// `consctl` and `label` are rio artefacts acme keeps for other programs'
/// sake), plus nothing.
pub const PaneFile = enum(u4) {
    dir = 0,
    addr,
    body,
    ctl,
    data,
    errors,
    event,
    tag,
    xdata,
    rdsel,
    wrsel,

    /// Every name IS the variant's name; only the directory itself is spelled
    /// differently, because `.` is not an identifier.
    pub fn name(f: PaneFile) []const u8 {
        return if (f == .dir) "." else @tagName(f);
    }

    /// acme's dirtabw modes: 0400 read, 0200 write, 0600 both.
    pub fn mode(f: PaneFile) u16 {
        return switch (f) {
            .dir => 0o500,
            .errors, .wrsel => 0o200,
            .rdsel => 0o400,
            else => 0o600,
        };
    }
};

/// The files at the root, and the root itself. `new` is a directory whose
/// every lookup CREATES a pane (acme(4): "Accessing any file in new creates a
/// new window"), which is how a script opens one without a keystroke.
pub const TopFile = enum(u4) {
    root = 1,
    index = 2,
    cons = 3,
    new = 4,

    pub fn name(f: TopFile) []const u8 {
        return if (f == .root) "." else @tagName(f);
    }

    pub fn mode(f: TopFile) u16 {
        return switch (f) {
            .root, .new => 0o500,
            .index => 0o400,
            .cons => 0o200,
        };
    }

    pub fn dir(f: TopFile) bool {
        return f == .root or f == .new;
    }
};

/// A NODE ID, which is one integer to the kernel and two fields to us: the
/// file within a pane's directory, and the pane's SERIAL — never reused, so a
/// node id can never come to mean a different pane. acme does the same packing
/// with `QID(w->id, f)` / `WIN(q)` / `FILE(q)` macros over an int; a packed
/// struct is the same bits with the shifts and masks checked by the compiler,
/// and `serial == 0` (no pane) is what keeps the top-level ids 1..5 out of the
/// way with no separate range check.
pub const Node = packed struct(u64) {
    file: u4 = 0,
    serial: u60 = 0,

    pub fn of(serial: u32, file: PaneFile) u64 {
        std.debug.assert(serial != 0);
        return @bitCast(Node{ .file = @intFromEnum(file), .serial = serial });
    }

    /// What this id points at, or null when it names neither a top-level file
    /// nor a possible pane file. Validity is decided HERE so no handler has to.
    pub fn target(node: u64) ?Target {
        const n: Node = @bitCast(node);
        if (n.serial == 0) {
            return .{ .top = std.enums.fromInt(TopFile, n.file) orelse return null };
        }
        return .{ .pane = .{
            .serial = std.math.cast(u32, n.serial) orelse return null,
            .file = std.enums.fromInt(PaneFile, n.file) orelse return null,
        } };
    }
};

/// What a node id points at.
pub const Target = union(enum) {
    top: TopFile,
    pane: struct { serial: u32, file: PaneFile },
};

// ============================================================================
// STATE — everything the filesystem remembers between requests.
// ============================================================================

/// What a formatted answer starts out able to hold before it grows: `index`
/// over every pane, a directory listing, one event record. The buffer is kept
/// between requests and cleared, not freed, so the steady state allocates
/// nothing and nothing is capped by a number picked here.
pub const out_reserve = 4 * 1024;

/// Records a reader has not taken yet, per pane. Beyond this the oldest are
/// dropped: an editor must not stall or grow without bound because a script
/// stopped reading, and a reader that fell this far behind has already lost
/// the thread — it can re-read `body` and resynchronise. (acme grows
/// `w->events` with `realloc` and has no bound at all.)
pub const queue_cap = 64 * 1024;

/// A byte queue of formatted event records, each framed by its length so a
/// record whose TEXT contains newlines still comes out whole.
pub const Queue = struct {
    buf: std.ArrayList(u8) = .empty,
    /// How much of `buf` has been consumed. Popping moves this instead of
    /// sliding the remainder down: a full queue holds thousands of ~20-byte
    /// records, and a memmove per pop made draining one quadratic. The space
    /// is reclaimed when the head passes half the buffer, so the amortised
    /// cost of a pop is a pointer bump.
    head: usize = 0,

    pub fn deinit(q: *Queue, gpa: std.mem.Allocator) void {
        q.buf.deinit(gpa);
        q.head = 0;
    }

    pub fn push(q: *Queue, gpa: std.mem.Allocator, record: []const u8) void {
        if (record.len > std.math.maxInt(u32)) return;
        while (q.buf.items.len - q.head + record.len + 4 > queue_cap) {
            if (q.peek() == null) return;
            q.pop();
        }
        q.compact();
        var head: [4]u8 = undefined;
        std.mem.writeInt(u32, &head, @intCast(record.len), .little);
        q.buf.appendSlice(gpa, &head) catch return;
        q.buf.appendSlice(gpa, record) catch {
            q.buf.shrinkRetainingCapacity(q.buf.items.len - 4);
            return;
        };
    }

    /// The oldest record, or null when empty. Does not consume.
    pub fn peek(q: *const Queue) ?[]const u8 {
        const rest = q.buf.items[@min(q.head, q.buf.items.len)..];
        if (rest.len < 4) return null;
        const len = std.mem.readInt(u32, rest[0..4], .little);
        if (rest.len < 4 + len) return null;
        return rest[4 .. 4 + len];
    }

    pub fn pop(q: *Queue) void {
        const record = q.peek() orelse return;
        q.head += 4 + record.len;
        if (q.head == q.buf.items.len) {
            q.buf.clearRetainingCapacity();
            q.head = 0;
        }
    }

    fn compact(q: *Queue) void {
        if (q.head == 0 or q.head * 2 < q.buf.items.len) return;
        const rest = q.buf.items.len - q.head;
        std.mem.copyForwards(u8, q.buf.items[0..rest], q.buf.items[q.head..]);
        q.buf.shrinkRetainingCapacity(rest);
        q.head = 0;
    }

    pub fn empty(q: *const Queue) bool {
        return q.peek() == null;
    }
};

/// Per-pane filesystem state, indexed by pane SLOT (not serial): it dies with
/// the pane, and a reused slot must start clean.
pub const PaneFs = struct {
    /// acme's `w->addr`: where `data`/`xdata` read and write. Byte offsets.
    addr: Range = .{},
    /// `limit=addr`: the range regex searches are confined to, or none.
    limit: ?Range = null,
    /// how many opens of this pane's `event` file are live. Non-zero means the
    /// pane is SCRIPT-DRIVEN: its Look and Exec are reported, not performed.
    readers: u16 = 0,
    events: Queue = .{},
    /// `nomark`: writes stop pushing an undo point each, so a script's batch
    /// of edits is one Undo (acme: `w->nomark`).
    nomark: bool = false,
    /// `noscroll`: a body write does not drag the view to the new text.
    noscroll: bool = false,
    /// The tag as it was at the end of the last update, so tag edits can be
    /// reported without a hook in every tag mutation. Only kept while somebody
    /// is listening.
    tag_snap: std.ArrayList(u8) = .empty,

    pub const Range = struct { q0: u32 = 0, q1: u32 = 0 };

    fn deinit(pf: *PaneFs, gpa: std.mem.Allocator) void {
        pf.events.deinit(gpa);
        pf.tag_snap.deinit(gpa);
        pf.* = .{};
    }
};

/// The core's filesystem state. Lives on `Pardes`; zero-initialised, so a core
/// that never serves a filesystem pays one branch per frame and no memory
/// beyond this struct.
pub const State = struct {
    /// Formatted answers, valid until the next `handle` call (Payload.staged).
    /// Kept and cleared rather than freed: after the first few requests the
    /// capacity is there and staging an answer allocates nothing.
    out: std.ArrayList(u8) = .empty,
    panes: [MAX_PANES]PaneFs = @splat(.{}),
    /// How many `event` files are open anywhere. The one gate every recording
    /// hook in the core is behind: nobody listening, nothing recorded, no diff
    /// computed, no bytes copied.
    listeners: u16 = 0,
    /// Which input the core is handling, as acme's origin character: `K`
    /// keyboard, `M` mouse, `E` a write to body/tag through this filesystem,
    /// `F` an action through one of its other files. Set once per update.
    origin: u8 = 'K',

    pub fn deinit(st: *State, gpa: std.mem.Allocator) void {
        for (&st.panes) |*pf| pf.deinit(gpa);
        st.out.deinit(gpa);
    }

    /// Start a fresh answer. The previous one's bytes are dead the moment the
    /// next request arrives, which is exactly the borrow window `fsPayload`
    /// documents.
    pub fn stage(st: *State, gpa: std.mem.Allocator) *std.ArrayList(u8) {
        st.out.clearRetainingCapacity();
        st.out.ensureTotalCapacity(gpa, out_reserve) catch {};
        return &st.out;
    }

    /// A pane died: drop its filesystem state, and with it any listener count
    /// it held, so a script killed with its pane cannot leave the editor
    /// suppressing button actions forever.
    pub fn forget(st: *State, gpa: std.mem.Allocator, id: usize) void {
        if (id >= MAX_PANES) return;
        st.listeners -= @min(st.listeners, st.panes[id].readers);
        st.panes[id].deinit(gpa);
    }

    /// Is anybody reading this pane's events? The suppression rule and every
    /// recording hook ask this.
    pub fn scripted(st: *const State, id: usize) bool {
        return id < MAX_PANES and st.panes[id].readers != 0;
    }
};

// ============================================================================
// EVENT RECORDS — what the core reports, in acme's wire format.
// ============================================================================

/// acme's record, byte for byte: origin char, type char, then four
/// blank-separated decimals (q0, q1, flag, text length), a blank, the text,
/// and a newline — `winevent`'s `"%c%d %d %d %d %.*S\n"` with the owner char
/// pushed in front (`wind.c`). Text of 256 bytes or more is elided (the
/// reader fetches it from `data`), which is also what bounds this buffer.
pub const max_record_text = 256;

/// The action characters. Lower case is the tag, upper case the body, which is
/// how a reader tells them apart with no extra field.
pub const Action = enum(u8) {
    body_delete = 'D',
    tag_delete = 'd',
    body_insert = 'I',
    tag_insert = 'i',
    body_look = 'L',
    tag_look = 'l',
    body_exec = 'X',
    tag_exec = 'x',

    /// The enum IS the character, the way acme's `winevent` takes a `char` and
    /// prints `%c` (`wind.c`) — so these three are expressions rather than the
    /// three parallel switches that spelled the same alphabet out again.
    pub fn char(a: Action) u8 {
        return @intFromEnum(a);
    }

    pub fn fromChar(c: u8) ?Action {
        return std.enums.fromInt(Action, c);
    }

    /// Lower case is the tag, upper case the body: acme's whole encoding of
    /// WHICH TEXT a record is about, with no extra field.
    pub fn onTag(a: Action) bool {
        return @intFromEnum(a) >= 'a';
    }
};

/// Flag bits, acme(4). Look and exec are different vocabularies at the same
/// bit positions, so they get separate names rather than one enum.
pub const flag_builtin: u32 = 1;
pub const flag_expansion: u32 = 2;
pub const flag_filename: u32 = 4;
pub const flag_chorded: u32 = 8;

/// Format one record into `buf` and return the bytes.
pub fn formatRecord(
    buf: []u8,
    origin: u8,
    action: Action,
    q0: u32,
    q1: u32,
    flag: u32,
    text: []const u8,
) []const u8 {
    const sent = if (text.len >= max_record_text) text[0..0] else text;
    return std.fmt.bufPrint(buf, "{c}{c}{d} {d} {d} {d} {s}\n", .{
        origin,
        action.char(),
        q0,
        q1,
        flag,
        sent.len,
        sent,
    }) catch buf[0..0];
}

/// THE SPAN TWO VERSIONS OF A TEXT DIFFER IN: everything outside their common
/// prefix and common suffix.
///
/// Chunked through `std.mem.eql`, which lowers to vectorised compares. That is
/// not premature: this runs on EVERY edit of a scripted pane, over the whole
/// buffer, and the byte-at-a-time loop it replaces cost 2.4x per keystroke on
/// a 40 KB body (`zig build fs-bench`, the two `keystroke` rows).
pub const Span = struct { at: u32, removed: u32, inserted: u32 };

pub fn diffSpan(old: []const u8, new: []const u8) Span {
    const both = @min(old.len, new.len);
    const stride = 64;
    var head: usize = 0;
    while (head + stride <= both and
        std.mem.eql(u8, old[head..][0..stride], new[head..][0..stride])) head += stride;
    while (head < both and old[head] == new[head]) head += 1;
    var tail: usize = 0;
    const rest = both - head;
    while (tail + stride <= rest and std.mem.eql(
        u8,
        old[old.len - tail - stride ..][0..stride],
        new[new.len - tail - stride ..][0..stride],
    )) tail += stride;
    while (tail < rest and old[old.len - 1 - tail] == new[new.len - 1 - tail]) tail += 1;
    return .{
        .at = @intCast(head),
        .removed = @intCast(old.len - tail - head),
        .inserted = @intCast(new.len - tail - head),
    };
}

/// Report a whole-text replacement the way acme reports an edit: the deletion
/// first and then the insertion, because that is the order `textdelete` and
/// `textinsert` would have run in. pardes replaces whole buffers, so the pair
/// is recovered here — one implementation, one place that knows the order, and
/// the only cost paid by an unscripted editor is the `scripted` check.
pub fn noteReplace(p: *Pardes, id: usize, on_tag: bool, old: []const u8, new: []const u8) void {
    if (!p.fs.scripted(id)) return;
    const span = diffSpan(old, new);
    if (span.removed == 0 and span.inserted == 0) return;
    if (span.removed > 0) _ = noteAction(
        p,
        id,
        if (on_tag) .tag_delete else .body_delete,
        span.at,
        span.at + span.removed,
        0,
        "",
    );
    if (span.inserted > 0) _ = noteAction(
        p,
        id,
        if (on_tag) .tag_insert else .body_insert,
        span.at,
        span.at + span.inserted,
        0,
        new[span.at..][0..span.inserted],
    );
}

/// Record a Look or an Exec, and say whether THE CORE MUST NOT PERFORM IT.
///
/// That inversion is acme's whole extension model: while a script holds a
/// pane's `event` file open, buttons 2 and 3 in that pane belong to the script
/// — the words in its tag are its commands, not pardes's. A script that dies
/// closes the file and the pane goes back to being an editor.
pub fn noteAction(
    p: *Pardes,
    id: usize,
    action: Action,
    q0: u32,
    q1: u32,
    flag: u32,
    text: []const u8,
) bool {
    if (!p.fs.scripted(id)) return false;
    var buf: [max_record_text + 64]u8 = undefined;
    const record = formatRecord(&buf, p.fs.origin, action, q0, q1, flag, text);
    p.fs.panes[id].events.push(p.gpa, record);
    return true;
}

// ============================================================================
// THE TRANSACTION.
// ============================================================================

/// Answer one filesystem request against the live editor. The only entry
/// point: `Event.fs_req` lands here and the `Reply` leaves as
/// `Effect.fs_reply`.
pub fn handle(p: *Pardes, req: Req) Reply {
    const target = Node.target(req.node) orelse return Reply.fail(req.tag, E.NOENT);
    // THE ORIGIN CHARACTER for everything this request goes on to cause —
    // including the TAG DIFF the core takes at the end of the update, after
    // this function has returned. acme sets `w->owner` in `winlock` and calls
    // `winsettag` before `winunlock`, so the tag change a body write provokes
    // (the dirty marker appearing) is attributed to that write and not to
    // whatever touched the editor last. Set once, here, for the same reason.
    //
    // Nothing restores it: `Pardes.update` sets the origin afresh on every
    // keystroke and every mouse event, which is what owns it the rest of the
    // time. A read cannot cause a record, so only the mutating ops set it.
    if (req.op == .write or req.op == .setattr) p.fs.origin = switch (target) {
        // acme's `xfidwrite` opens with exactly this: `c = 'F'; if(qid==QWtag
        // || qid==QWbody) c = 'E';` — `E` is "writes to the body or tag file",
        // `F` is "actions through the window's other files" (acme(4)).
        .pane => |t| @as(u8, if (t.file == .body or t.file == .tag) 'E' else 'F'),
        .top => 'F',
    };
    return switch (req.op) {
        .lookup => lookup(p, req, target),
        .getattr => switch (attrOf(p, target)) {
            .ok => |a| .{ .tag = req.tag, .attr = a },
            .missing => Reply.fail(req.tag, E.NOENT),
        },
        .setattr => setattr(p, req, target),
        .open => open(p, req, target),
        .release => release(p, req),
        .readdir => readdir(p, req, target),
        .read => read(p, req, target),
        .write => write(p, req, target),
        // A synthetic filesystem has no blocks. Answering successfully with
        // zeros keeps `df` and anything that stats the mount working.
        .statfs => .{ .tag = req.tag },
    };
}

const AttrResult = union(enum) { ok: Reply.Attr, missing };

fn attrOf(p: *Pardes, target: Target) AttrResult {
    switch (target) {
        .top => |f| return .{ .ok = .{
            .node = @intFromEnum(f),
            .dir = f.dir(),
            .mode = f.mode(),
            .size = topSize(p, f),
        } },
        .pane => |t| {
            const id = p.paneBySerial(t.serial) orelse return .missing;
            return .{ .ok = .{
                .node = Node.of(t.serial, t.file),
                .dir = t.file == .dir,
                .mode = t.file.mode(),
                .size = paneFileSize(p, id, t.file),
            } };
        },
    }
}

/// A size for `stat`. Exact where it is cheap and honest (`body`, `tag`), zero
/// where the file is a stream whose length is not a property (`event`, `log`);
/// FUSE serves these with direct IO, so a zero-length file still reads.
fn topSize(p: *Pardes, f: TopFile) u64 {
    return switch (f) {
        .root, .new, .cons => 0,
        .index => indexLen(p),
    };
}

fn paneFileSize(p: *Pardes, id: usize, f: PaneFile) u64 {
    const pane = p.panes[id] orelse return 0;
    return switch (f) {
        .body, .data, .xdata => bodyLen(p, pane),
        .tag => tagLen(p, pane),
        .dir, .addr, .ctl, .errors, .event, .rdsel, .wrsel => 0,
    };
}

// ---------------------------------------------------------------------------
// PER-FILE SEMANTICS. Everything above is the frame: the ABI, the tree, the
// state, the records. Everything below is what acme's xfid.c does.
// ---------------------------------------------------------------------------

// ===========================================================================
// THE TWO TEXTS A PANE HAS. Every handler below asks these, so "what is this
// pane's body" has one answer here and not eleven answers scattered about.
// ===========================================================================

/// A pane's BODY, BORROWED. A file pane — which includes every output buffer
/// — lends its content, and that is the whole reason a `body` read costs
/// nothing (`Payload.region`). A terminal has no such buffer: its body is the
/// emulator's scrollback, which has to be RENDERED before it is bytes, so it
/// is not lendable and `readBody` produces one instead. Empty here therefore
/// means "nothing to lend", which for a terminal is not "empty document".
fn bodyOf(pane: *const Pane) []const u8 {
    if (pane.file) |*f| return f.content;
    return "";
}

/// ...and the writable side of the same question. Null is "this pane has no
/// document", which is every terminal and the answer to every write that
/// would need one.
fn fileOf(pane: *Pane) ?*file_pane.State {
    return if (pane.file) |*f| f else null;
}

/// The pane's TAG exactly as it is drawn: the live read-only prefix (the path,
/// the dirty marker, the pane's builtin words, the alignment gap) then the
/// editable tail.
///
/// Scratch-owned — and `Pardes.update` resets that arena before the transport
/// ever reads a payload, so every tag answer is COPIED into `State.out`.
/// `body` is the only text lent out, because it is the only one that is a
/// buffer rather than a rendering.
fn tagOf(p: *Pardes, pane: *Pane) []const u8 {
    return p.tagText(p.scratch.allocator(), pane) catch "";
}

/// The directory a pane belongs to: acme's "the directory currently named in
/// the tag", which is where this pane's `+Errors` goes.
fn dirOf(pane: *Pane) []const u8 {
    if (pane.file) |*f| return std.fs.path.dirname(f.path) orelse "/";
    const cwd = pane.cwdSlice();
    return if (cwd.len > 0) cwd else "/";
}

/// acme's `w->dirty`: the body differs from what is on disk. A terminal and an
/// output buffer have nothing on disk, so they are never dirty — the same
/// `saves` trait the tag's `*` marker already asks.
fn dirtyOf(pane: *const Pane) bool {
    const f = if (pane.file) |*x| x else return false;
    if (!output_pane.fileTraits(f.output).saves) return false;
    return f.revision != f.saved_revision;
}

/// A byte offset as a `Range` field. A pane holding four gigabytes of text is
/// not something this editor does; saturating is honest where a silent wrap
/// would hand a script an address pointing at the wrong end of the file.
fn clip(n: usize) u32 {
    return std.math.cast(u32, n) orelse std.math.maxInt(u32);
}

fn cellOf(row: i32, col: i32) modal.Cursor {
    return .{ .row = @intCast(@max(0, row)), .col = @intCast(@max(0, col)) };
}

fn firstLine(s: []const u8) []const u8 {
    return s[0 .. std.mem.indexOfScalar(u8, s, '\n') orelse s.len];
}

/// acme's DOT — the user's selection — as a byte range over the body.
///
/// pardes keeps the selection as two (row, col) cells with a HELIX block
/// cursor, i.e. the head cell is INSIDE the range; acme's dot is gap to gap.
/// This is the one place that conversion lives and `setDot` is its inverse, so
/// `addr=dot` followed by `dot=addr` is the identity rather than a range that
/// creeps by one grapheme each round trip.
fn dotOf(pane: *Pane) PaneFs.Range {
    const text = bodyOf(pane);
    const head = modal.hxOff(text, cellOf(pane.cur_row, pane.cur_col));
    if (!pane.vsel.active) return .{ .q0 = clip(head), .q1 = clip(head) };
    const anchor = modal.hxOff(text, cellOf(pane.vsel.row, pane.vsel.col));
    var hi = @max(head, anchor);
    if (hi < text.len) hi = modal.nextGrapheme(text, hi);
    return .{ .q0 = clip(@min(head, anchor)), .q1 = clip(hi) };
}

/// acme's `textsetselect`. The head lands ON the last grapheme of the range,
/// never one past it, because that is where every pardes motion leaves it and
/// a cursor sitting one cell right of its own selection is a selection the
/// acme chords will not act on.
fn setDot(pane: *Pane, r: PaneFs.Range) void {
    const text = bodyOf(pane);
    const q0 = @min(@as(usize, r.q0), text.len);
    const q1 = @max(q0, @min(@as(usize, r.q1), text.len));
    const a = modal.hxPos(text, q0);
    pane.vsel = .{ .active = q1 > q0, .row = @intCast(a.row), .col = @intCast(a.col), .explicit = true };
    const h = modal.hxPos(text, if (q1 > q0) modal.prevGrapheme(text, q1) else q0);
    pane.cur_row = @intCast(h.row);
    pane.cur_col = @intCast(h.col);
    pane.cur_pinned = true;
    pane.sticky_col = -1;
    pane.msel.active = false;
    pane.ensureCursorVisible();
}

/// acme's `textshow`: put a spot on screen. Suppressed by `noscroll`.
fn showOffset(pane: *Pane, off: usize) void {
    const text = bodyOf(pane);
    const c = modal.hxPos(text, @min(off, text.len));
    pane.cur_row = @intCast(c.row);
    pane.cur_col = @intCast(c.col);
    pane.cur_pinned = true;
    pane.sticky_col = -1;
    pane.ensureCursorVisible();
}

/// acme's `clampaddr`. Its `Range` is signed and it clamps both ends; ours is
/// unsigned, so only the top can be wrong — and it can, the moment a pane's
/// body shrinks under a stored address.
fn clampAddr(pf: *PaneFs, len: usize) void {
    const n = clip(len);
    pf.addr.q0 = @min(pf.addr.q0, n);
    pf.addr.q1 = @min(pf.addr.q1, n);
    if (pf.limit) |*l| {
        l.q0 = @min(l.q0, n);
        l.q1 = @min(l.q1, n);
    }
}

/// Move a range across an edit at `at` that replaced `removed` bytes with
/// `inserted` — acme's `if(tq0 >= q0) tq0 += nr;`, applied to both ends, which
/// is what keeps a script rewriting text under your cursor from dragging the
/// cursor onto a different word.
fn shiftBy(r: PaneFs.Range, at: u32, removed: u32, inserted: u32) PaneFs.Range {
    return .{ .q0 = shiftOne(r.q0, at, removed, inserted), .q1 = shiftOne(r.q1, at, removed, inserted) };
}

fn shiftOne(v: u32, at: u32, removed: u32, inserted: u32) u32 {
    if (v <= at) return v;
    if (v <= at +| removed) return at +| inserted;
    return v - removed +| inserted;
}

/// How many of these bytes end on a character boundary.
///
/// acme buffers a partial rune on the Fid (`fullrunewrite` plus `f->rpart`)
/// and stitches it onto the next write. A SHORT COUNT is the POSIX spelling of
/// the same promise — the writer's libc retries with the tail — and it needs
/// no per-handle state at all. Never zero for a
/// non-empty write: a writer handed 0 retries the same bytes forever.
fn wholeUtf8(data: []const u8) usize {
    var i = data.len;
    var back: usize = 0;
    while (i > 0 and back < 4) : (back += 1) {
        i -= 1;
        const c = data[i];
        if (c < 0x80) return data.len; // an ASCII tail is always complete
        if (c & 0xC0 == 0xC0) { // a lead byte: is its sequence all here?
            const need = std.unicode.utf8ByteSequenceLength(c) catch return data.len;
            if (i + need <= data.len or i == 0) return data.len;
            return i;
        }
    }
    // four trailing continuation bytes and no lead: not UTF-8 at all. acme's
    // `cvttorunes` substitutes for bad bytes rather than refusing them, and so
    // does storing them verbatim.
    return data.len;
}

// ===========================================================================
// LOOKUP — acme's `fsyswalk`, minus 9P's fid bookkeeping.
// ===========================================================================

/// The files inside a pane's directory, by name. `.dir` is the directory
/// itself and is never a name to resolve.
/// A name inside a pane's directory. `.` and `..` are the kernel's business,
/// never ours, and the directory variant is not nameable — so a hit on the
/// variant names is the whole lookup.
fn paneFileNamed(name: []const u8) ?PaneFile {
    const f = std.meta.stringToEnum(PaneFile, name) orelse return null;
    return if (f == .dir) null else f;
}

fn topFileNamed(name: []const u8) ?TopFile {
    const f = std.meta.stringToEnum(TopFile, name) orelse return null;
    return if (f == .root) null else f;
}

/// acme: "is it a numeric name? yes: it's a directory". A pane's directory is
/// named by its SERIAL, which is never reused, so a stale path can go stale
/// but can never come to mean a different pane.
fn serialNamed(name: []const u8) ?u32 {
    if (name.len == 0 or name.len > 10) return null;
    for (name) |c| if (c < '0' or c > '9') return null;
    return std.fmt.parseInt(u32, name, 10) catch null;
}

/// The smallest live serial greater than `after`, so a caller can walk every
/// pane in ascending serial without sorting anything. O(panes) per step over
/// at most sixteen slots, and no allocation — the alternative was a scratch
/// array in a function that must not allocate.
fn nextSerialAfter(p: *Pardes, after: u32) ?u32 {
    var best: ?u32 = null;
    for (p.panes) |slot| {
        const pane = slot orelse continue;
        if (pane.serial <= after) continue;
        if (best == null or pane.serial < best.?) best = pane.serial;
    }
    return best;
}

/// Create a pane the way the `New` builtin does — an empty scratch below the
/// active one, in its column — and answer its serial.
///
/// acme has `newwindowthread` sitting on a channel for exactly this, and its
/// windows go wherever `rowadd` puts them. Going through `newScratchBelow`
/// means a pane a script opened is in every respect a pane you opened: same
/// tag, same builtins, same undo, same Del.
fn newPane(p: *Pardes) ?u32 {
    const slot = p.freeSlot() orelse return null;
    p.newScratchBelow(p.active);
    const pane = p.panes[slot] orelse return null;
    return pane.serial;
}

fn lookup(p: *Pardes, req: Req, target: Target) Reply {
    const name = req.data;
    if (name.len == 0 or std.mem.indexOfScalar(u8, name, '/') != null) return Reply.fail(req.tag, E.NOENT);
    const node: u64 = switch (target) {
        .top => |f| switch (f) {
            .root => root: {
                if (topFileNamed(name)) |t| break :root @intFromEnum(t);
                const serial = serialNamed(name) orelse return Reply.fail(req.tag, E.NOENT);
                _ = p.paneBySerial(serial) orelse return Reply.fail(req.tag, E.NOENT);
                break :root Node.of(serial, .dir);
            },
            .new => new: {
                // acme(4): "Accessing any file in new creates a new window."
                //
                // acme creates it one component EARLIER — `fsyswalk` sends on
                // `cnewwindow` the moment it walks the name `new` itself. That
                // cannot work over FUSE: the kernel CACHES the dentry for
                // `new`, so a lookup there would fire once per mount and never
                // again. Creating at the CHILD keeps the promise the man page
                // makes (`echo hi > $PARDES_FS/new/body` opens a pane holding
                // `hi`) under a protocol that caches.
                //
                // The name is checked BEFORE the pane is made, so a stat of
                // `new/nosuchfile` leaves no litter. acme's walk creates the
                // window first and then fails the second component, which
                // leaves an empty window behind for every typo.
                const want = paneFileNamed(name) orelse return Reply.fail(req.tag, E.NOENT);
                const serial = newPane(p) orelse return Reply.fail(req.tag, E.NFILE);
                break :new Node.of(serial, want);
            },
            else => return Reply.fail(req.tag, E.NOTDIR),
        },
        .pane => |t| pane: {
            if (t.file != .dir) return Reply.fail(req.tag, E.NOTDIR);
            _ = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
            const f = paneFileNamed(name) orelse return Reply.fail(req.tag, E.NOENT);
            break :pane Node.of(t.serial, f);
        },
    };
    // A lookup answers with the TARGET's attributes, which is exactly what a
    // getattr of that node would say — one spelling, so the two can never
    // disagree about a size or a mode.
    return switch (attrOf(p, Node.target(node) orelse return Reply.fail(req.tag, E.NOENT))) {
        .ok => |a| .{ .tag = req.tag, .attr = a },
        .missing => Reply.fail(req.tag, E.NOENT),
    };
}

// ===========================================================================
// READDIR
// ===========================================================================

/// One directory entry in the transport-neutral staging format `src/fuse.zig`
/// decodes: node id, kind, name length, name — packed, little-endian, no
/// padding. A readdir answer is that record repeated.
///
/// `node` travels because it becomes the `d_ino` a `getdents64` reports, and a
/// `d_ino` that disagrees with the later `st_ino` is a filesystem that lies to
/// `find -inum`.
fn stageDirent(out: *std.ArrayList(u8), gpa: std.mem.Allocator, node: u64, dir: bool, name: []const u8) void {
    if (name.len == 0 or name.len > 255) return;
    var head: [10]u8 = undefined;
    std.mem.writeInt(u64, head[0..8], node, .little);
    head[8] = @intFromBool(dir);
    head[9] = @intCast(name.len);
    out.appendSlice(gpa, &head) catch return;
    out.appendSlice(gpa, name) catch return;
}

/// The pane files, for a pane directory and for `new/`. `serial == 0` is
/// `new/`: there is no pane yet — the LOOKUP is what creates one — so there is
/// no id to report, and the transport substitutes one.
fn stagePaneFiles(p: *Pardes, out: *std.ArrayList(u8), serial: u32, skip: *u64) void {
    inline for (comptime std.enums.values(PaneFile)) |f| {
        if (f != .dir) {
            if (skip.* > 0) skip.* -= 1 else stageDirent(out, p.gpa, Node.of(serial, f), false, f.name());
        }
    }
}

fn readdir(p: *Pardes, req: Req, target: Target) Reply {
    const out = p.fs.stage(p.gpa);
    var skip = req.off;
    switch (target) {
        .top => |f| switch (f) {
            .root => {
                inline for (.{ TopFile.index, TopFile.cons, TopFile.new }) |t| {
                    if (skip > 0) skip -= 1 else stageDirent(out, p.gpa, @intFromEnum(t), t.dir(), t.name());
                }
                // Ascending serial: serials are never reused, so this order is
                // stable across a create and a delete — which is what a script
                // that walks the tree twice and diffs the two walks needs.
                // acme lists windows in SCREEN order (column by column), which
                // changes when you drag a window and says nothing a script can
                // rely on.
                var last: u32 = 0;
                while (nextSerialAfter(p, last)) |s| {
                    last = s;
                    if (skip > 0) {
                        skip -= 1;
                        continue;
                    }
                    var buf: [16]u8 = undefined;
                    const name = std.fmt.bufPrint(&buf, "{d}", .{s}) catch continue;
                    stageDirent(out, p.gpa, Node.of(s, .dir), true, name);
                }
            },
            // `new/` ENUMERATES NOTHING, and that is a guarantee rather than a
            // shrug: the names it could list are exactly the names whose LOOKUP
            // creates a pane, and every tool that lists a directory then stats
            // what it found — `ls -l`, `ls --color`, `find`, a shell completing
            // `$PARDES_FS/new/` — would make one pane per name. acme never
            // lists it either. Naming a file here is what creates one; see
            // `lookup`.
            .new => {},
            else => return Reply.fail(req.tag, E.NOTDIR),
        },
        .pane => |t| {
            if (t.file != .dir) return Reply.fail(req.tag, E.NOTDIR);
            _ = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
            stagePaneFiles(p, out, t.serial, &skip);
        },
    }
    // Zero bytes is END OF DIRECTORY, never an error: the transport stops
    // asking, and re-staging from scratch on every call is what makes a
    // partially consumed answer safe to ask for again at a higher cookie.
    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(out.items.len) } };
}

// ===========================================================================
// OPEN / RELEASE / SETATTR
// ===========================================================================

/// Open carries no per-open state, because there is none to carry: `addr` and
/// `limit` belong to the pane (as they do in acme, where they are Window
/// fields), and every read brings its own offset. What an open DOES do is
/// arm the two things acme arms on open, and count event readers.
///
/// So there is no fid table. acme needs one because 9P walks to a fid and
/// every later message names only that fid; FUSE puts the nodeid on every
/// request, RELEASE included, so the handle is decoration. It is answered
/// non-zero only because the transport spells "no handle" as zero.
fn open(p: *Pardes, req: Req, target: Target) Reply {
    switch (target) {
        .top => {},
        .pane => |t| {
            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
            const pf = &p.fs.panes[id];
            switch (t.file) {
                // acme(4): "When the ctl file is first opened, regular
                // expression context searches in addr addresses examine the
                // whole file"; `limit=addr` narrows them again.
                .ctl => pf.limit = null,
                // acme resets both on the FIRST open (`w->nopen[QWaddr]++ ==
                // 0`) and keeps a per-file open count to know. There is none
                // here: `addr` is one piece of per-pane state that a second
                // opener would be sharing anyway, so the honest reading of
                // "first" is "whenever somebody opens it" — and a script's
                // first act on `addr` is always to write one.
                .addr => {
                    pf.addr = .{};
                    pf.limit = null;
                },
                // THE SUPPRESSION GATE. While this is non-zero the pane is
                // script-driven: its Look and Exec are reported, not
                // performed (`noteAction`). Counted per OPEN, not per pane, so
                // two readers means the second one closing leaves the first
                // still in charge.
                .event => {
                    pf.readers +|= 1;
                    p.fs.listeners +|= 1;
                },
                else => {},
            }
        },
    }
    return .{ .tag = req.tag, .handle = 1 };
}

fn release(p: *Pardes, req: Req) Reply {
    const target = Node.target(req.node) orelse return .{ .tag = req.tag };
    switch (target) {
        .top => {},
        .pane => |t| {
            if (t.file != .event) return .{ .tag = req.tag };
            // The pane may have DIED while this was open. `State.forget` has
            // then already taken its whole reader count out of `listeners`
            // (the core calls it from `deinitPane`), so a serial that no
            // longer resolves must not be decremented a second time — that
            // underflow is exactly what would leave the editor suppressing
            // button actions forever with no script left to interpret them.
            const id = p.paneBySerial(t.serial) orelse return .{ .tag = req.tag };
            const pf = &p.fs.panes[id];
            if (pf.readers == 0) return .{ .tag = req.tag };
            pf.readers -= 1;
            p.fs.listeners -|= 1;
            // The LAST reader leaving takes the tag snapshot with it. It is
            // only ever compared against while somebody is listening, so
            // keeping it would let the tag drift unobserved and then hand the
            // NEXT reader a `d`/`i` pair for a change it never saw.
            if (pf.readers == 0) pf.tag_snap.clearAndFree(p.gpa);
        },
    }
    return .{ .tag = req.tag };
}

fn setattr(p: *Pardes, req: Req, target: Target) Reply {
    // acme has NO equivalent: 9P has no truncate-on-open, so nothing in
    // `xfid.c` answers a Twstat carrying a length. Linux does — `> body` is
    // O_TRUNC — and refusing it would make the shell's most natural way to
    // REPLACE a pane's text (rather than append to it) fail with EPERM on the
    // redirect, before a single byte was written. So exactly one field is
    // honoured, only the value zero means anything, and everything else a
    // `stat` structure can carry (mode, owner, times) is silently accepted and
    // ignored the way a filesystem of live editor state has to.
    if (req.truncate) switch (target) {
        .pane => |t| switch (t.file) {
            .body, .data, .xdata => {
                const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
                const pane = p.panes[id].?;
                if (fileOf(pane) != null) {
                    _ = spliceBody(p, id, pane, 0, bodyOf(pane).len, "") orelse
                        return Reply.fail(req.tag, E.NOMEM);
                    p.fs.panes[id].addr = .{};
                    setDot(pane, .{});
                }
            },
            else => {},
        },
        else => {},
    };
    return switch (attrOf(p, target)) {
        .ok => |a| .{ .tag = req.tag, .attr = a },
        .missing => Reply.fail(req.tag, E.NOENT),
    };
}

// ===========================================================================
// READ
// ===========================================================================

/// Answer with a WINDOW onto what was just staged. `Payload.staged` is a
/// LENGTH from the start of the buffer, so a read at an offset slides the
/// bytes down rather than growing the payload union with a second field
/// nothing else would ever use.
fn staged(p: *Pardes, req: Req) Reply {
    const out = &p.fs.out;
    const off = @min(req.off, out.items.len);
    const n = @min(out.items.len - off, req.size);
    if (off > 0) std.mem.copyForwards(u8, out.items[0..n], out.items[off..][0..n]);
    out.shrinkRetainingCapacity(n);
    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(n) } };
}

fn read(p: *Pardes, req: Req, target: Target) Reply {
    switch (target) {
        .top => |f| return switch (f) {
            .index => readIndex(p, req),
            // acme's dirtab: `cons` is 0200 and a directory is not read(2)able.
            .cons, .root, .new => Reply.fail(req.tag, E.PERM),
        },
        .pane => |t| {
            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
            const pane = p.panes[id].?;
            const pf = &p.fs.panes[id];
            return switch (t.file) {
                .addr => readAddr(p, req, pf, pane),
                .body => readBody(p, req, id, pane),
                .ctl => readCtl(p, req, pane),
                .data => readData(req, id, pane, pf, false),
                .xdata => readData(req, id, pane, pf, true),
                .tag => readTag(p, req, pane),
                .event => readQueue(p, req, &pf.events),
                .rdsel => readRdsel(req, id, pane),
                .dir, .errors, .wrsel => Reply.fail(req.tag, E.PERM),
            };
        },
    }
}

/// acme's `Ctlsize`: five `%11d ` fields = 60 bytes, before the tag.
const ctl_fields = 5 * 12;

/// acme's `winctlprint(w, buf, 0)` — the five numbers `index` and `ctl` share.
///
/// COST: acme reads the tag's length off `w->tag.file->nc` for free, because
/// acme's tag IS a buffer. pardes's is COMPUTED every time it is asked for
/// (path, dirty marker, builtins, and the alignment gap, which is measured
/// against every other pane in the same layout column), so these five numbers
/// cost one tag render — a couple of microseconds and a few bumps of the
/// per-update scratch arena, which `Pardes.update` resets. That is the price
/// of the second field being the number a `tag` read will actually hand back;
/// a cheaper approximation that disagreed with `read tag` would be worse than
/// slow, it would be wrong.
fn stageCtlNumbers(p: *Pardes, out: *std.ArrayList(u8), pane: *Pane) void {
    out.print(p.gpa, "{d:>11} {d:>11} {d:>11} {d:>11} {d:>11} ", .{
        pane.serial,
        tagOf(p, pane).len,
        bodyOf(pane).len,
        // acme's `isdir` marks a window holding a DIRECTORY LISTING. pardes
        // never opens one — a Look at a directory spawns a shell there
        // (look.zig) — so this is structurally zero, not unimplemented.
        @as(u32, 0),
        @intFromBool(dirtyOf(pane)),
    }) catch {};
}

/// acme's `xfidindexread`: one line per pane, the five numbers then the tag up
/// to its first newline. Seekable, so a script can pread the middle of it —
/// "at character position 5×12 starts the name of the window" (acme(4)).
fn readIndex(p: *Pardes, req: Req) Reply {
    const out = p.fs.stage(p.gpa);
    var last: u32 = 0;
    while (nextSerialAfter(p, last)) |s| {
        last = s;
        const pane = p.panes[p.paneBySerial(s).?].?;
        stageCtlNumbers(p, out, pane);
        out.appendSlice(p.gpa, firstLine(tagOf(p, pane))) catch {};
        out.append(p.gpa, '\n') catch {};
    }
    return staged(p, req);
}

/// acme: `sprint(buf, "%11d %11d ", w->addr.q0, w->addr.q1)`. acme's numbers
/// are RUNE offsets; these are bytes (see the header). "Thus a regular
/// expression may be evaluated by writing it to addr and reading it back."
fn readAddr(p: *Pardes, req: Req, pf: *PaneFs, pane: *Pane) Reply {
    clampAddr(pf, bodyOf(pane).len);
    const out = p.fs.stage(p.gpa);
    out.print(p.gpa, "{d:>11} {d:>11} ", .{ pf.addr.q0, pf.addr.q1 }) catch {};
    return staged(p, req);
}

fn readBody(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
    if (pane.file != null) {
        // ZERO COPY: `.region` is resolved by `fsPayload` during the effect
        // drain, so reading a megabyte of body moves no bytes in here at all.
        // This is the whole reason `Payload` is a union and not a slice.
        const text = bodyOf(pane);
        const off = @min(req.off, text.len);
        const n = @min(text.len - off, req.size);
        return .{ .tag = req.tag, .payload = .{ .region = .{
            .pane = @intCast(id),
            .serial = pane.serial,
            .off = clip(off),
            .len = clip(n),
        } } };
    }
    // A TERMINAL has no such buffer. acme's body is always a `Text`; pardes's
    // is a terminal emulator, and its "body" is the scrollback — which only
    // becomes bytes when somebody renders the pages into lines. So it is
    // produced, staged, and paid for per read. `win`'s transcript, read side.
    const text = term_pane.screenTextAlloc(pane, p.gpa) catch
        return Reply.fail(req.tag, E.NOMEM);
    defer p.gpa.free(text);
    const out = p.fs.stage(p.gpa);
    out.appendSlice(p.gpa, text) catch return Reply.fail(req.tag, E.NOMEM);
    return staged(p, req);
}

/// The face the shell was last asked to wear. acme owns its fonts and prints
/// the real one; the core only knows what it REQUESTED — on a tty the font
/// belongs to the terminal emulator and in the browser to the page — so it
/// prints that, or `default`, which is the same word the Debug overlay shows
/// for the same reason.
fn fontName(p: *Pardes) []const u8 {
    const name = p.settings.font.effective_name.get();
    return if (name.len == 0) "default" else name;
}

/// plan9's `%q` (`quotestrfmt`): a string with nothing special in it prints
/// bare, anything else is wrapped in single quotes with internal quotes
/// doubled. Load-bearing rather than decoration — a script splits the ctl line
/// into shell words, and a font name with a space in it is one word.
fn stageQuoted(out: *std.ArrayList(u8), gpa: std.mem.Allocator, s: []const u8) void {
    const plain = s.len > 0 and for (s) |c| {
        if (c <= ' ' or c == '\'') break false;
    } else true;
    if (plain) {
        out.appendSlice(gpa, s) catch {};
        return;
    }
    out.append(gpa, '\'') catch {};
    for (s) |c| {
        if (c == '\'') out.append(gpa, '\'') catch {};
        out.append(gpa, c) catch {};
    }
    out.append(gpa, '\'') catch {};
}

/// acme's `winctlprint(w, buf, 1)`: index's five numbers plus three more.
fn readCtl(p: *Pardes, req: Req, pane: *Pane) Reply {
    const out = p.fs.stage(p.gpa);
    stageCtlNumbers(p, out, pane);
    // acme prints `Dx(w->body.r)` — the body's width in PIXELS — and
    // `w->body.maxtab`, a tab's width in pixels too. pardes is a CELL GRID:
    // on a tty there is no pixel width to report at all, and on the two pixel
    // shells the number a script actually wants is still how many characters
    // fit. So both are CELLS. A script that would have divided by the font
    // width to get columns gets columns without dividing.
    out.print(p.gpa, "{d:>11} ", .{pane.cols}) catch {};
    stageQuoted(out, p.gpa, fontName(p));
    out.print(p.gpa, " {d:>11} ", .{config.tab_width}) catch {};
    return staged(p, req);
}

fn readTag(p: *Pardes, req: Req, pane: *Pane) Reply {
    const out = p.fs.stage(p.gpa);
    out.appendSlice(p.gpa, tagOf(p, pane)) catch {};
    return staged(p, req);
}

/// acme's `xfidruneread`: hand back whole characters from the START of `addr`
/// and move `addr` to the null string just after them; `xdata` additionally
/// stops at the END of `addr` (acme passes `w->addr.q1` where `data` passes
/// `nc`). The file offset is ignored — `addr` is the position.
///
/// "Whole characters" is acme's partial-rune rule; here it is a GRAPHEME
/// boundary, which is strictly stronger and is what every other offset in
/// pardes already respects. A read too small for the next grapheme returns
/// zero bytes rather than half of one — acme's `if(m == 0) break`.
fn readData(req: Req, id: usize, pane: *Pane, pf: *PaneFs, stop_at_end: bool) Reply {
    const text = bodyOf(pane);
    clampAddr(pf, text.len);
    const q0: usize = pf.addr.q0;
    // acme carries a "BUG: what should happen if q1 > q0?" here and answers by
    // reading nothing. An inverted address is a legal thing to have written
    // (`address()` never normalises), so the empty read is the answer.
    const hi: usize = if (stop_at_end) @max(q0, @as(usize, pf.addr.q1)) else text.len;
    var end = @min(hi, q0 +| req.size);
    end = @max(q0, modal.graphemeStart(text, end));
    // `data` collapses the address onto the point it read up to; `xdata` moves
    // only q0 and KEEPS q1, because q1 is the stop address the man page
    // promises ("reads stop at the end address") and the next chunked read has
    // to be able to continue from where this one stopped. acme spells the same
    // difference at xfid.c:331-341: QWdata assigns both, QWxdata only q0.
    pf.addr.q0 = clip(end);
    if (!stop_at_end) pf.addr.q1 = clip(end);
    if (pane.file == null) return .{ .tag = req.tag };
    return .{ .tag = req.tag, .payload = .{ .region = .{
        .pane = @intCast(id),
        .serial = pane.serial,
        .off = clip(q0),
        .len = clip(end - q0),
    } } };
}

/// acme copies the selection into a TEMP FILE at open, with a comment
/// apologising for it, so a `|sort` cannot see the text change underneath.
/// There is no such window here: the whole request is one main-thread
/// transaction, nothing can run between the open and the read, and the bytes
/// go out of the pane unmoved.
fn readRdsel(req: Req, id: usize, pane: *Pane) Reply {
    if (pane.file == null) return .{ .tag = req.tag };
    const text = bodyOf(pane);
    const d = dotOf(pane);
    const lo = @min(@as(usize, d.q0), text.len);
    const hi = @max(lo, @min(@as(usize, d.q1), text.len));
    const off = @min(req.off, hi - lo);
    const n = @min(hi - lo - off, req.size);
    return .{ .tag = req.tag, .payload = .{ .region = .{
        .pane = @intCast(id),
        .serial = pane.serial,
        .off = clip(lo + off),
        .len = clip(n),
    } } };
}

/// ONE RECORD PER READ, and `Status.again` when there is none.
///
/// This is the whole of what acme's blocking `event` read becomes. acme parks
/// the `Xfid` in `w->eventx` and `winevent` sends it a message to wake it up;
/// the waiting lives in a thread per in-flight request, and `xfidflush` exists
/// to cancel one. Here nothing is consumed and nothing is remembered: the
/// transport still holds the kernel's request and asks again. No waiter list,
/// no wakeup, no flush bookkeeping, and no loop anywhere in the core.
fn readQueue(p: *Pardes, req: Req, q: *Queue) Reply {
    const record = q.peek() orelse return .{ .tag = req.tag, .status = .again };
    // acme hands back as much of its event buffer as the count allows and
    // keeps the rest, which can split a record down the middle; a reader is
    // simply expected never to ask for less than one. Refusing is the honest
    // version of that contract — half a record is unparseable and silently
    // desynchronises the reader for the rest of the session.
    if (req.size < record.len) return Reply.fail(req.tag, E.INVAL);
    const out = p.fs.stage(p.gpa);
    out.appendSlice(p.gpa, record) catch return Reply.fail(req.tag, E.NOMEM);
    q.pop();
    return .{ .tag = req.tag, .payload = .{ .staged = @intCast(out.items.len) } };
}

// ===========================================================================
// WRITE
// ===========================================================================

fn write(p: *Pardes, req: Req, target: Target) Reply {
    switch (target) {
        .top => |f| return switch (f) {
            // acme(4): text written to `cons` appears in `dir/+Errors`, where
            // `dir` is the directory the command ran in — acme knows which
            // from the mount the writer inherited (`x->f->mntdir`, one per
            // `win`). A FUSE mount is ONE directory for the whole editor, so
            // the writing process is anonymous and the only defensible owner
            // is the pane the user is in. A script that wants a specific
            // pane's errors writes `<id>/errors`, which is unambiguous.
            .cons => if (appendErrors(p, p.active, req.data)) |took|
                .{ .tag = req.tag, .written = @intCast(took) }
            else
                Reply.fail(req.tag, E.IO),
            else => Reply.fail(req.tag, E.PERM),
        },
        .pane => |t| {
            const id = p.paneBySerial(t.serial) orelse return Reply.fail(req.tag, E.NOENT);
            const pane = p.panes[id].?;
            return switch (t.file) {
                .addr => writeAddr(p, req, id, pane),
                .body => writeBody(p, req, id, pane),
                .ctl => writeCtl(p, req, t.serial),
                // acme's `data` and `xdata` differ only in what a READ stops
                // at; the writes are the same code path there and here.
                .data, .xdata => writeData(p, req, id, pane),
                .tag => writeTag(p, req, pane),
                .event => writeEvent(p, req, id),
                .wrsel => writeWrsel(p, req, id, pane),
                .errors => if (appendErrors(p, id, req.data)) |took|
                    .{ .tag = req.tag, .written = @intCast(took) }
                else
                    Reply.fail(req.tag, E.IO),
                .dir, .rdsel => Reply.fail(req.tag, E.PERM),
            };
        },
    }
}

/// THE ONE BODY SPLICE every writing file goes through: replace `[q0, q1)`
/// with `bytes`, via `file_pane.setContent` — which is where the core diffs
/// out the insert/delete event records, so a script's edit is reported exactly
/// once and in exactly the same shape as a keystroke's. One swap per write for
/// the same reason: two swaps would be two `D`/`I` pairs for one write.
///
/// The origin character the records carry is `handle`'s, set once per request
/// (acme's winlock owner), so nothing here has to know which file it is
/// serving.
fn spliceBody(p: *Pardes, id: usize, pane: *Pane, q0: usize, q1: usize, bytes: []const u8) ?usize {
    const f = fileOf(pane) orelse return null;
    const take = if (bytes.len == 0) 0 else wholeUtf8(bytes);
    const lo = @min(q0, f.content.len);
    const hi = @max(lo, @min(q1, f.content.len));
    const new = p.gpa.alloc(u8, f.content.len - (hi - lo) + take) catch return null;
    @memcpy(new[0..lo], f.content[0..lo]);
    @memcpy(new[lo..][0..take], bytes[0..take]);
    @memcpy(new[lo + take ..], f.content[hi..]);
    // acme: `if(w->nomark == FALSE){ seq++; filemark(t->file); }` — `nomark`
    // is how a script makes a batch of edits one Undo.
    //
    // COST, and the reason `nomark` matters more here than it does in acme:
    // acme's `filemark` is a sequence number on a log-structured, disk-backed
    // Buffer, so it is O(1). pardes's undo is a SNAPSHOT of the whole body
    // (`file_pane.pushUndo` compares and then duplicates it), so a script that
    // appends a line at a time to a megabyte body pays a megabyte per line and
    // keeps 256 of them. That is exactly the same cost one KEYSTROKE pays on
    // the same body — this is not a filesystem tax, it is the core's edit
    // model — but a script can do it ten thousand times a second where a
    // typist cannot. `nomark` is the documented remedy and the reason acme
    // gave scripts the verb.
    if (!p.fs.panes[id].nomark) file_pane.pushUndo(p, pane);
    file_pane.setContent(p, f, new);
    return take;
}

/// acme(4): "Text written to body is always appended; the file offset is
/// ignored." So `req.off` is deliberately never read here.
fn writeBody(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
    if (req.data.len == 0) return .{ .tag = req.tag, .written = 0 };
    // A TERMINAL's body is not a document, it is a program's transcript — and
    // the only way to put text into a transcript is to TYPE it. So a body
    // write to a terminal pane is a pty write: `echo ls > $PARDES_FS/3/body`
    // runs ls in pane 3's shell. That is `win`'s semantics in acme (the shell
    // reads what you write to its window's body), reached through the effect
    // the core already has instead of through a pipe.
    //
    // Nothing is RECORDED for it: the insert/delete diff lives in
    // `file_pane.setContent`, and a terminal has no `file` to swap. The
    // program's output comes back as ordinary `.output` bytes.
    if (pane.file == null) {
        const take = wholeUtf8(req.data);
        p.emitWrite(id, req.data[0..take]);
        return .{ .tag = req.tag, .written = @intCast(take) };
    }
    const at = bodyOf(pane).len;
    const take = spliceBody(p, id, pane, at, at, req.data) orelse
        return Reply.fail(req.tag, E.NOMEM);
    if (!p.fs.panes[id].noscroll) showOffset(pane, at + take);
    return .{ .tag = req.tag, .written = @intCast(take) };
}

/// acme's tag is one `Text` and a write appends to all of it. pardes's tag is
/// PREFIX ++ TAIL: the prefix is chrome the core recomputes every frame (the
/// path, the dirty marker, the builtin words, the alignment gap), so bytes
/// appended to it would be gone by the next render. A tag write therefore
/// appends to the TAIL — which is the part that is a buffer, and the part a
/// script means when it writes ` Undo` into a tag.
///
/// The tail is a fixed one-line buffer (`Pane.tag_tail`), so a write that does
/// not fit is short, and one with no room at all is ENOSPC rather than a zero
/// count the writer would retry forever.
fn writeTag(p: *Pardes, req: Req, pane: *Pane) Reply {
    if (req.data.len == 0) return .{ .tag = req.tag, .written = 0 };
    // the laid-out default tail becomes real bytes on first touch, exactly as
    // it does when you click into the tag
    p.seedTail(pane);
    const room = pane.tag_tail.len - pane.tag_tail_len;
    if (room == 0) return Reply.fail(req.tag, E.NOSPC);
    const take = wholeUtf8(req.data[0..@min(req.data.len, room)]);
    @memcpy(pane.tag_tail[pane.tag_tail_len..][0..take], req.data[0..take]);
    pane.tag_tail_len += take;
    pane.tag_init = true;
    return .{ .tag = req.tag, .written = @intCast(take) };
}

/// acme(4): text written to `data` "replaces the characters addressed by the
/// addr file and sets the address to the null string at the end of the written
/// text". The file offset is ignored.
fn writeData(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
    if (fileOf(pane) == null) return Reply.fail(req.tag, E.INVAL);
    const pf = &p.fs.panes[id];
    clampAddr(pf, bodyOf(pane).len);
    const q0: usize = pf.addr.q0;
    const q1: usize = @max(q0, @as(usize, pf.addr.q1));
    const before = dotOf(pane);
    // acme's winlock(w, 'F'): everything but body and tag is "an action
    // through the window's other files".
    const take = spliceBody(p, id, pane, q0, q1, req.data) orelse
        return Reply.fail(req.tag, E.NOMEM);
    setDot(pane, shiftBy(before, clip(q0), clip(q1 - q0), clip(take)));
    pf.addr = .{ .q0 = clip(q0 + take), .q1 = clip(q0 + take) };
    if (!pf.noscroll) showOffset(pane, q0 + take);
    return .{ .tag = req.tag, .written = @intCast(take) };
}

/// acme's `wrsel` cuts the selection when the file is OPENED and inserts each
/// write at a running point after it (`w->wrselrange`). Same result, no
/// open-time mutation: each write REPLACES the selection, and because the
/// selection is left collapsed just after the inserted text, a second write
/// appends to the first exactly as `wrselrange` does. The only difference is
/// what an open and close with NO write does — acme has already emptied the
/// selection by then, this leaves the pane untouched. A filesystem that edits
/// your document when you `stat` it is a filesystem you cannot explore.
///
/// acme also forces `nomark` for the file's lifetime so the whole stream is
/// one Undo. That needs open-time state we do not keep; a script that wants it
/// writes `nomark` to `ctl`, which is the same button with a name on it.
fn writeWrsel(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
    if (fileOf(pane) == null) return Reply.fail(req.tag, E.INVAL);
    const d = dotOf(pane);
    const q0: usize = d.q0;
    const q1: usize = @max(q0, @as(usize, d.q1));
    const take = spliceBody(p, id, pane, q0, q1, req.data) orelse
        return Reply.fail(req.tag, E.NOMEM);
    setDot(pane, .{ .q0 = clip(q0 + take), .q1 = clip(q0 + take) });
    return .{ .tag = req.tag, .written = @intCast(take) };
}

/// acme's `xfidwrite` QWaddr. Two failures, and acme has two error strings for
/// them: `Ebadaddr` (the parser stopped before the end of the expression) and
/// `Eaddr` (it parsed but did not evaluate — out of range, or no match). A
/// filesystem has one channel for "no", so both are EINVAL.
fn writeAddr(p: *Pardes, req: Req, id: usize, pane: *Pane) Reply {
    const pf = &p.fs.panes[id];
    const text = bodyOf(pane);
    clampAddr(pf, text.len);
    // acme's parser stops at a newline of its own accord (`\n` reaches the
    // `default:` arm), which is what lets `echo '/foo/' > addr` work from a
    // shell. Trimming says the same thing without threading it through every
    // arm of the state machine.
    const expr = std.mem.trimEnd(u8, req.data, "\n");
    var a: Addr = .{ .text = text, .lim = pf.limit, .expr = expr };
    const r = a.address(pf.addr) orelse return Reply.fail(req.tag, E.INVAL);
    if (a.i < expr.len) return Reply.fail(req.tag, E.INVAL);
    pf.addr = r;
    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
}

// ===========================================================================
// THE ADDRESS LANGUAGE — acme's addr.c, byte-addressed.
// ===========================================================================

/// mvzr PANICS on a pattern that ends inside an escape: `parseCharSet` slices
/// `in[i+1..]` and `valueFor` indexes `[0]` of it, so a trailing backslash is
/// an out-of-bounds read rather than a compile failure (pardes.zig's
/// `applySelRegex` carries the same warning about the prefix `[^\`). A live
/// typist can only reach that by accident; a SCRIPT's regex is untrusted
/// input, so it is screened here before the engine ever sees it.
fn safePattern(pat: []const u8) bool {
    var i: usize = 0;
    while (i < pat.len) : (i += 1) {
        if (pat[i] != '\\') continue;
        if (i + 1 >= pat.len) return false;
        i += 1;
    }
    return true;
}

/// THE ADDRESS PARSER, in acme's shape: one left-to-right pass with three
/// pieces of state — a running range, a DIRECTION (`+`/`-`/none) and a SIZE
/// (line or character) — recursing once per `,` or `;`.
///
/// What is gone is the C. acme reads the expression through a `getc` callback
/// over a `Rune*` so one parser can serve both the filesystem and the Edit
/// language; it reports failure through two out-parameters (`evalp` for "did
/// not evaluate", `qp` for "stopped here") because it cannot return three
/// things; and it grows the regex pattern with `runerealloc` one rune at a
/// time. Here the expression is a slice, the cursor is a field, a pattern is a
/// subslice of the expression, and "did not evaluate" is `null`.
const Addr = struct {
    text: []const u8,
    /// `limit=addr`: regex context searches are confined to this. acme applies
    /// it FORWARDS only, and so does this.
    lim: ?PaneFs.Range,
    expr: []const u8,
    i: usize = 0,
    /// One frame per `,` or `;`. acme recurses without a bound, which is fine
    /// when the expression came from a person typing into a tag and is a
    /// STACK OVERFLOW when it came from a script: `,,,,,...` a hundred
    /// thousand deep is one write(2). A compound address deeper than this is
    /// not an address anybody meant.
    depth: u8 = 0,

    const max_depth = 32;
    const Size = enum { char, line };

    /// acme's `address()`. `ar` is what `.` means — and `xfidwrite` passes
    /// `w->addr`, NOT the user's selection, so `.` is the CURRENT ADDRESS and
    /// `addr=dot` is the only door the selection comes in by. (acme(4)
    /// describes the language as "the format understood by button 3", where
    /// `.` is dot; the code is the authority and this follows the code.)
    fn address(a: *Addr, ar_in: PaneFs.Range) ?PaneFs.Range {
        const start = a.i;
        var ar = ar_in;
        var r = ar_in;
        var dir: u8 = 0;
        var size: Size = .line;
        var c: u8 = 0;
        while (a.i < a.expr.len) {
            const prevc = c;
            c = a.expr[a.i];
            a.i += 1;
            switch (c) {
                ',', ';' => {
                    // `;` differs from `,` in one way: it makes the RIGHT side
                    // relative to the left one.
                    if (c == ';') ar = r;
                    if (prevc == 0) r.q0 = 0; // lhs defaults to 0
                    if (a.i >= a.expr.len) {
                        r.q1 = clip(a.text.len); // rhs defaults to $
                    } else {
                        if (a.depth >= max_depth) return null;
                        a.depth += 1;
                        const nr = a.address(ar) orelse return null;
                        a.depth -= 1;
                        r.q1 = nr.q1;
                    }
                    return r;
                },
                '+', '-' => {
                    // a pending `+`/`-` with no count of its own means one
                    // line, unless what follows is itself an operand
                    if (prevc == '+' or prevc == '-') {
                        const nc = if (a.i < a.expr.len) a.expr[a.i] else 0;
                        if (nc != '#' and nc != '/' and nc != '?')
                            r = a.number(r, 1, prevc, .line) orelse return null;
                    }
                    dir = c;
                },
                '.', '$' => {
                    // both are only meaningful as the FIRST character of a
                    // (sub)expression; anywhere else they end the parse
                    if (a.i != start + 1) {
                        a.i -= 1;
                        return r;
                    }
                    r = if (c == '.') ar else .{ .q0 = clip(a.text.len), .q1 = clip(a.text.len) };
                    dir = if (a.i < a.expr.len) '+' else 0;
                },
                '#', '0'...'9' => {
                    var digit = c;
                    if (c == '#') {
                        if (a.i >= a.expr.len or a.expr[a.i] < '0' or a.expr[a.i] > '9') {
                            a.i -= 1;
                            return r;
                        }
                        digit = a.expr[a.i];
                        a.i += 1;
                        size = .char;
                    }
                    var n: u64 = digit - '0';
                    while (a.i < a.expr.len) : (a.i += 1) {
                        const d = a.expr[a.i];
                        if (d < '0' or d > '9') break;
                        n = @min(n * 10 + (d - '0'), std.math.maxInt(u32));
                    }
                    r = a.number(r, @intCast(n), dir, size) orelse return null;
                    dir = 0;
                    size = .line;
                },
                '/', '?' => {
                    const back = c == '?';
                    r = a.regexp(r, a.pattern(c), back) orelse return null;
                    dir = 0;
                    size = .line;
                },
                else => {
                    a.i -= 1;
                    return r;
                },
            }
        }
        // a trailing `+` or `-` with nothing after it: one line that way
        if (dir != 0) r = a.number(r, 1, dir, .line) orelse return null;
        return r;
    }

    /// The pattern between the delimiters, with the backslash of an escape
    /// KEPT (it belongs to the regex engine, not to this parser).
    ///
    /// DIVERGENCE: acme closes both `/re/` and `?re?` on a `/` — its scanner
    /// has no `case '?'` at all, so `?foo?` yields the pattern `foo?`, which
    /// as a regex means `fo` plus an optional `o`. That is a bug you can only
    /// find by reading addr.c. Here the OPENING delimiter closes.
    fn pattern(a: *Addr, delim: u8) []const u8 {
        const s = a.i;
        while (a.i < a.expr.len) {
            const c = a.expr[a.i];
            if (c == '\n') break;
            a.i += 1;
            if (c == '\\') {
                if (a.i < a.expr.len) a.i += 1;
                continue;
            }
            if (c == delim) return a.expr[s .. a.i - 1];
        }
        return a.expr[s..a.i];
    }

    /// acme's `number()`, byte for byte — including its two oddities: a `-`
    /// count from offset 0 wraps to the END of the file, and `:1-1` is legal
    /// (it means `#0`) while `:1-2` is an error.
    fn number(a: *Addr, r_in: PaneFs.Range, n: u32, dir: u8, size: Size) ?PaneFs.Range {
        var r = r_in;
        if (size == .char) {
            var off: i64 = n;
            if (dir == '+') {
                off = @as(i64, r.q1) + n;
            } else if (dir == '-') {
                if (r.q0 == 0 and n > 0) r.q0 = clip(a.text.len);
                off = @as(i64, r.q0) - n;
            }
            if (off < 0 or off > @as(i64, @intCast(a.text.len))) return null;
            // BYTES, and a byte offset can land inside a grapheme where acme's
            // rune offset never could. Clamped to the boundary at or before
            // it, which is the rule every other offset in pardes follows.
            const g = clip(modal.graphemeStart(a.text, @intCast(off)));
            return .{ .q0 = g, .q1 = g };
        }
        var line: i64 = n;
        var q0: usize = r.q0;
        var q1: usize = r.q1;
        switch (dir) {
            '-' => {
                if (q0 < a.text.len) while (q0 > 0 and a.text[q0 - 1] != '\n') {
                    q0 -= 1;
                };
                q1 = q0;
                while (line > 0 and q0 > 0) {
                    if (a.text[q0 - 1] == '\n') {
                        line -= 1;
                        q1 = q0;
                    }
                    q0 -= 1;
                }
                if (line > 1) return null;
                while (q0 > 0 and a.text[q0 - 1] != '\n') q0 -= 1;
                return .{ .q0 = clip(q0), .q1 = clip(q1) };
            },
            '+' => {
                if (q1 > 0) while (q1 < a.text.len and a.text[q1 - 1] != '\n') {
                    q1 += 1;
                };
                q0 = q1;
            },
            else => {
                q0 = 0;
                q1 = 0;
            },
        }
        while (line > 0 and q1 < a.text.len) {
            const ch = a.text[q1];
            q1 += 1;
            if (ch == '\n' or q1 == a.text.len) {
                line -= 1;
                if (line > 0) q0 = q1;
            }
        }
        if (line > 0) return null;
        return .{ .q0 = clip(q0), .q1 = clip(q1) };
    }

    /// acme's `regexp()`. Forward runs from the END of the running range to
    /// the limit (`limit=addr`, else the end of the file); backward runs from
    /// its START back to the beginning.
    ///
    /// The engine is mvzr, the one `%s` and the selection previews already
    /// use. Two of its properties come along and cannot be fixed here: `^` and
    /// `$` assert against the SLICE being searched rather than against a line,
    /// and `.` matches a newline like any other byte. Both are already waived
    /// in pardes.zig; an address that needs a line anchor matches `\n`.
    fn regexp(a: *Addr, r: PaneFs.Range, pat: []const u8, back: bool) ?PaneFs.Range {
        // acme reuses the LAST compiled expression for an empty pattern
        // (`rxnull`). There is no such global here — one more piece of hidden
        // state for a script to guess wrong about — so `//` is not an address.
        if (pat.len == 0 or !safePattern(pat)) return null;
        const re = mvzr.compile(pat) orelse return null;
        if (back) {
            const hi = @min(@as(usize, r.q0), a.text.len);
            var best: ?mvzr.Match = null;
            var at: usize = 0;
            while (at < hi) {
                const m = re.matchPos(at, a.text[0..hi]) orelse break;
                best = m;
                at = if (m.end > m.start) m.end else m.end + 1;
            }
            const m = best orelse return null;
            return .{ .q0 = clip(m.start), .q1 = clip(m.end) };
        }
        const hi = if (a.lim) |l| @min(@as(usize, l.q1), a.text.len) else a.text.len;
        const from = @min(@as(usize, r.q1), hi);
        const m = re.match(a.text[from..hi]) orelse return null;
        return .{ .q0 = clip(from + m.start), .q1 = clip(from + m.end) };
    }
};

// ===========================================================================
// CTL VERBS — acme's xfidctlwrite.
// ===========================================================================

/// The verbs that mean something here. acme matches PREFIXES with `strncmp`
/// and advances by the matched length, which is why its arms have to be
/// ordered `delete` before `del`, `nomark` before `mark`, `noscroll` before
/// `scroll` — get that ordering wrong and a verb is silently truncated into a
/// different one. Splitting on the newline the man page already requires and
/// matching WHOLE tokens makes that class of bug unrepresentable.
const Verb = enum {
    @"addr=dot",
    clean,
    cleartag,
    del,
    delete,
    dirty,
    @"dot=addr",
    get,
    @"limit=addr",
    mark,
    nomark,
    noscroll,
    put,
    scroll,
    show,
};

/// ...and the ones acme has that pardes REFUSES. Loudly, because a silently
/// accepted no-op is the worse failure: the script believes it holds the lock.
///
///   menu / nomenu — acme maintains `Undo Redo Put` in the LEFT HALF of the
///     tag and these switch that off. pardes's tag prefix is computed chrome
///     (the path, the dirty marker, the pane's own builtins) with no halves
///     and no writable menu region, so there is nothing to switch.
///   dump / dumpdir — acme's dump file stores a COMMAND that recreates a
///     window. pardes's dump (src/dump.zig) stores the window's TEXT, so a
///     recreation command has nowhere to be kept and nothing to run it.
///   font — the face belongs to the SHELL, not the core: on a tty it is the
///     terminal emulator's and in the browser it is the page's. The `Font`
///     builtin only ASKS; a ctl verb that looked like it set one would be a
///     lie on three of the four platforms.
///   lock / unlock — acme's exclusive-use lock is a `QLock` held against a 9P
///     fid. There is no fid here and the core is single-threaded, so a lock
///     would promise a mutual exclusion nothing can violate and nothing
///     provides.
const refused_verbs = [_][]const u8{ "dump", "dumpdir", "font", "lock", "menu", "nomenu", "unlock" };

fn verbIs(line: []const u8, word: []const u8) bool {
    if (!std.mem.startsWith(u8, line, word)) return false;
    return line.len == word.len or line[word.len] == ' ';
}

/// acme's ctl write is NOT atomic: it applies verbs until one fails, then
/// answers `Ebadctl` with a count of the bytes it got through, so
/// `dirty\nbogus\n` leaves the window dirty and the write "fails". A short
/// count on a Linux write is not read as "the rest failed" by anybody, so the
/// only honest translation is all-or-nothing: validate every verb first, then
/// apply. `ctlVerb` answers the same yes/no in both passes.
fn writeCtl(p: *Pardes, req: Req, serial: u32) Reply {
    for ([2]bool{ false, true }) |apply| {
        // `del`'s guard is the one predicate that reads state EARLIER VERBS IN
        // THE SAME WRITE change, so the validation pass has to model it or the
        // two passes disagree: `clean\ndel` (acme's own idiom, and what
        // examples/acmefs/life.py sends on the way out) would fail validation
        // while `dirty\ndel` would pass it and then fail half-applied.
        var dirty = if (p.paneBySerial(serial)) |id| dirtyOf(p.panes[id].?) else false;
        var it = std.mem.splitScalar(u8, req.data, '\n');
        while (it.next()) |raw| {
            const line = std.mem.trim(u8, raw, " \t\r");
            if (line.len == 0) continue;
            // `del` and `delete` remove the pane, and the verbs after them in
            // the same write have nothing left to act on.
            const live = p.paneBySerial(serial) orelse if (apply) break else return Reply.fail(req.tag, E.NOENT);
            if (!ctlVerb(p, live, line, apply, &dirty)) return Reply.fail(req.tag, E.INVAL);
        }
    }
    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
}

/// One verb. `apply` false is the validation pass and must change nothing but
/// `dirty`, which both passes advance identically so that `del`'s guard sees
/// the same answer in each.
fn ctlVerb(p: *Pardes, id: usize, line: []const u8, apply: bool, dirty: *bool) bool {
    const pane = p.panes[id] orelse return false;
    const pf = &p.fs.panes[id];

    // The one verb with an argument. acme rejects a name containing any
    // character `<= ' '` and an empty one; so does this.
    if (verbIs(line, "name")) {
        if (line.len <= 5) return false;
        const name = std.mem.trim(u8, line[5..], " \t");
        if (name.len == 0) return false;
        for (name) |c| if (c <= ' ') return false;
        if (!apply) return true;
        const f = fileOf(pane) orelse return true; // a terminal has no name to set
        const copy = p.gpa.dupe(u8, name) catch return true;
        p.gpa.free(f.path);
        f.path = copy;
        return true;
    }
    for (refused_verbs) |w| if (verbIs(line, w)) return false;

    const v = std.meta.stringToEnum(Verb, line) orelse return false;
    // acme: `del` is "delete, but check dirty", `delete` is "delete for sure".
    // pardes's `Del` builtin is unconditional (the guard there is the `*` you
    // can see in the tag), so `del` gets acme's guard here and `delete` does
    // not — which is the whole difference between the two words.
    if (v == .del and dirty.*) return false;
    switch (v) {
        .dirty => dirty.* = true,
        .clean, .get, .put => dirty.* = false,
        else => {},
    }
    if (!apply) return true;

    switch (v) {
        .@"addr=dot" => pf.addr = dotOf(pane),
        .@"dot=addr" => {
            clampAddr(pf, bodyOf(pane).len);
            setDot(pane, pf.addr);
        },
        .@"limit=addr" => {
            clampAddr(pf, bodyOf(pane).len);
            pf.limit = pf.addr;
        },
        // acme marks the window clean by resetting the file's sequence number;
        // pardes's equivalent is "the revision on screen IS the saved one".
        .clean => if (fileOf(pane)) |f| {
            f.saved_revision = f.revision;
        },
        .dirty => if (fileOf(pane)) |f| {
            f.saved_revision = f.revision -% 1;
        },
        // acme: "wipe tag right of bar". pardes's bar is the boundary between
        // the computed prefix and the editable tail, so this empties the tail
        // — and leaves it SEEDED, or the next render would put the default
        // builtins straight back.
        .cleartag => {
            pane.tag_tail_len = 0;
            pane.tag_init = true;
        },
        .del, .delete => _ = p.executeBuiltinLine(id, "Del"),
        .put => _ = p.executeBuiltinLine(id, "Save"),
        // acme's `get`: "Equivalent to the Get interactive command with no
        // arguments". pardes has no such builtin, so this is what Get would
        // be — the same synchronous read `file_pane.open` does, through the
        // same content swap, with an undo point in front of it so a script
        // cannot discard your edits irrecoverably.
        .get => if (fileOf(pane)) |f| {
            if (output_pane.fileTraits(f.output).saves) {
                if (look.readFile(p.gpa, f.path)) |bytes| {
                    file_pane.pushUndo(p, pane);
                    file_pane.setContent(p, f, bytes);
                    f.saved_revision = f.revision;
                } else |_| {}
            }
        },
        // acme's `mark` both cancels `nomark` AND pushes a mark, so the edits
        // made while nomark was on stay one Undo and the next one starts fresh.
        .mark => {
            pf.nomark = false;
            file_pane.pushUndo(p, pane);
        },
        .nomark => pf.nomark = true,
        .noscroll => pf.noscroll = true,
        .scroll => pf.noscroll = false,
        .show => showOffset(pane, dotOf(pane).q0),
    }
    return true;
}

// ===========================================================================
// EVENT WRITE-BACK — acme's xfideventwrite.
// ===========================================================================

const EventRecord = struct { action: Action, q0: u32, q1: u32 };

/// `{origin}{type}{q0} {q1}\n`, acme's `xfideventwrite` parse: two characters,
/// two blank-separated decimals, a newline. Everything a full record carries
/// after that — the flag, the count, the text — is omitted on the way back in,
/// which is what acme(4) means by "with the flag, count, and text omitted".
///
/// acme walks this with `strtoul`, pointer arithmetic and `goto Rescue`; here
/// the failure is `null` and the position stays in the struct, so the caller
/// can tell "ran out cleanly" from "stopped on garbage" by looking at `i`.
const EventReader = struct {
    data: []const u8,
    i: usize = 0,

    fn next(er: *EventReader) ?EventRecord {
        if (er.i >= er.data.len) return null;
        var i = er.i;
        if (i + 2 > er.data.len) return null;
        // acme stores the first character as `w->owner` (with a
        // `/* disgusting */` beside it) so later records inherit whatever the
        // writer claimed. Read and dropped here — see `writeEvent`.
        i += 1;
        const action = Action.fromChar(er.data[i]) orelse return null;
        i += 1;
        const q0 = scanNumber(er.data, &i) orelse return null;
        const q1 = scanNumber(er.data, &i) orelse return null;
        while (i < er.data.len and er.data[i] == ' ') i += 1;
        if (i >= er.data.len or er.data[i] != '\n') return null;
        er.i = i + 1;
        return .{ .action = action, .q0 = q0, .q1 = q1 };
    }
};

fn scanNumber(data: []const u8, i: *usize) ?u32 {
    while (i.* < data.len and data[i.*] == ' ') i.* += 1;
    const s = i.*;
    var n: u64 = 0;
    while (i.* < data.len and data[i.*] >= '0' and data[i.*] <= '9') : (i.* += 1)
        n = @min(n * 10 + (data[i.*] - '0'), std.math.maxInt(u32));
    if (i.* == s) return null;
    return @intCast(n);
}

/// Writing a record back PERFORMS the action it names, "exactly as it would
/// have been if the event file had not been open" (acme(4)). This is the
/// documented remote-control door and the point of the whole suppression rule:
/// a script reads an `X` record, decides the text is not one of its own tag
/// commands, and hands it back for pardes to run.
///
/// It is also, deliberately, arbitrary code execution — an `X` record is an
/// Exec — which is why the mount is 0700 under the user's runtime directory.
///
/// NOTHING in the write applies unless all of it parses: acme validates each
/// record just before executing it and leaves the earlier ones done, which
/// makes a malformed batch half-applied and unrepeatable.
fn writeEvent(p: *Pardes, req: Req, id: usize) Reply {
    const pane0 = p.panes[id] orelse return Reply.fail(req.tag, E.NOENT);
    const serial = pane0.serial;
    {
        const body = bodyOf(pane0);
        const tag = tagOf(p, pane0);
        var check: EventReader = .{ .data = req.data };
        while (check.next()) |r| {
            switch (r.action) {
                // acme accepts only `xXlL` on the way back in. A `D` or an `I`
                // is a REPORT, not a request; writing one back would mean
                // "pretend the user typed this", which nothing implements and
                // acme's switch rejects with `Ebadevent`.
                .body_look, .tag_look, .body_exec, .tag_exec => {},
                else => return Reply.fail(req.tag, E.INVAL),
            }
            // lower case is the tag, upper case the body — how a reader tells
            // the two texts apart with no extra field
            const n = if (r.action.onTag()) tag.len else body.len;
            if (r.q0 > r.q1 or r.q1 > n) return Reply.fail(req.tag, E.INVAL);
        }
        if (check.i != req.data.len) return Reply.fail(req.tag, E.INVAL);
    }
    // acme(4): `F` is "actions through the window's other files", which is
    // exactly what this is, and `handle` has already set it. acme takes the
    // origin from the RECORD instead (`w->owner = *p++`, with a
    // `/* disgusting */` beside it), so a writer can attribute its own action
    // to the keyboard; the character is parsed here and dropped, because a
    // record saying where it came from is worth nothing if the sender picks.
    var run: EventReader = .{ .data = req.data };
    while (run.next()) |r| {
        // an earlier action in this same write may have deleted the pane
        const now = p.paneBySerial(serial) orelse break;
        const pane = p.panes[now].?;
        const whole = if (r.action.onTag()) tagOf(p, pane) else bodyOf(pane);
        const lo = @min(@as(usize, r.q0), whole.len);
        const hi = @max(lo, @min(@as(usize, r.q1), whole.len));
        // the action can replace the very text it is reading from
        const text = p.scratch.allocator().dupe(u8, whole[lo..hi]) catch continue;
        switch (r.action) {
            .body_exec, .tag_exec => _ = p.execute(now, text),
            .body_look, .tag_look => p.lookAt(now, text),
            else => unreachable,
        }
    }
    return .{ .tag = req.tag, .written = @intCast(req.data.len) };
}

// ===========================================================================
// +Errors — acme's `errorwin`.
// ===========================================================================

/// acme(4): writing to `errors` "appends to the body of the dir/+Errors
/// window, where dir is the directory currently named in the tag. The window
/// is created if necessary, but not until text is actually written."
///
/// One buffer per DIRECTORY, not per pane — which is why a search for an
/// existing one matches on the dirname and not on the writer. Answers HOW
/// MANY BYTES WERE TAKEN, or null for failure.
///
/// The count matters because the append goes through `spliceBody`, which stops
/// at a whole-character boundary: the kernel splits a large `write(2)` at
/// `max_write` wherever it lands, so a multi-byte character straddling that
/// boundary must be reported short and retried by the writer's libc, exactly
/// as `body` and `data` do. Acknowledging the whole buffer would drop it.
fn appendErrors(p: *Pardes, id: usize, text: []const u8) ?usize {
    if (text.len == 0) return 0;
    const pane = p.panes[id] orelse return null;
    const dir = dirOf(pane);
    for (p.panes, 0..) |slot, i| {
        const q = slot orelse continue;
        const qf = fileOf(q) orelse continue;
        const o = qf.output orelse continue;
        if (std.meta.activeTag(o.from) != .errors) continue;
        if (!std.mem.eql(u8, std.fs.path.dirname(qf.path) orelse "", dir)) continue;
        return spliceBody(p, i, q, qf.content.len, qf.content.len, text);
    }
    const free = p.freeSlot() orelse return null;
    const content = p.gpa.dupe(u8, text) catch return null;
    const np = output_pane.open(p, free, dir, .errors, "", content) catch {
        p.gpa.free(content);
        return null;
    };
    p.placeDoc(id, free, np);
    return text.len;
}

// ===========================================================================
// SIZES — what `stat` reports.
// ===========================================================================

/// The whole `index`, measured. acme's `xfidindexread` walks every window to
/// size its buffer too; the tag of each is FORMATTED to be measured, into the
/// per-update scratch arena that is reset anyway, so this is bump allocation
/// rather than sixteen allocations a frame.
fn indexLen(p: *Pardes) u64 {
    var n: u64 = 0;
    for (p.panes) |slot| {
        const pane = slot orelse continue;
        n += ctl_fields + firstLine(tagOf(p, pane)).len + 1;
    }
    return n;
}

/// A terminal's body has no length that is cheap AND honest — measuring it
/// means rendering the whole scrollback — so it reports zero and is served
/// with direct IO, exactly like `event` and `log`.
fn bodyLen(p: *Pardes, pane: *Pane) u64 {
    _ = p;
    return bodyOf(pane).len;
}

fn tagLen(p: *Pardes, pane: *Pane) u64 {
    return tagOf(p, pane).len;
}

// ===========================================================================
// TESTS.
//
// The whole point of the split: every one of these drives `handle()` through
// the ordinary event queue with NO FUSE, NO mount, NO thread and no /dev/fuse
// anywhere. A filesystem whose semantics are a pure function of the core is a
// filesystem you can unit-test at the speed of a function call, and one whose
// blocking is a return value is one you can test without a scheduler.
// ===========================================================================

const testing = std.testing;

/// What a transport sees: the reply, and the bytes `fsPayload` resolved for it
/// inside the drain's borrow window. The two effects a filesystem operation
/// can additionally cause are captured too, because for `put` and for a write
/// to a terminal's body THE EFFECT IS THE ANSWER.
const Answer = struct {
    reply: Reply = .{ .tag = 0, .status = .err, .errno = E.IO },
    bytes: []const u8 = "",
    saved: bool = false,
    pty_buf: [256]u8 = undefined,
    pty_len: usize = 0,

    fn pty(a: *const Answer) []const u8 {
        return a.pty_buf[0..a.pty_len];
    }

    fn errno(a: Answer) u16 {
        return if (a.reply.status == .err) a.reply.errno else 0;
    }
};

/// One request in, one answer out. Effects are DRAINED but not performed: a
/// `put` must be observable as a `.save_file` without a test writing to the
/// real filesystem.
fn call(p: *Pardes, req: Req) Answer {
    p.update(.{ .fs_req = req });
    var ans: Answer = .{};
    while (p.nextEffect()) |e| switch (e) {
        .fs_reply => |r| {
            ans.reply = r;
            ans.bytes = p.fsPayload(r);
        },
        .save_file, .save_text => ans.saved = true,
        .write => |w| {
            const b = w.bytes.slice();
            const n = @min(b.len, ans.pty_buf.len - ans.pty_len);
            @memcpy(ans.pty_buf[ans.pty_len..][0..n], b[0..n]);
            ans.pty_len += n;
        },
        else => {},
    };
    return ans;
}

fn rd(p: *Pardes, node: u64, off: u64, size: u32) Answer {
    return call(p, .{ .tag = 1, .op = .read, .node = node, .off = off, .size = size });
}

fn wr(p: *Pardes, node: u64, data: []const u8) Answer {
    return call(p, .{ .tag = 2, .op = .write, .node = node, .data = data });
}

fn rdir(p: *Pardes, node: u64, skip: u64) Answer {
    return call(p, .{ .tag = 4, .op = .readdir, .node = node, .off = skip, .size = 4096 });
}

fn look_up(p: *Pardes, dir: u64, name: []const u8) Answer {
    return call(p, .{ .tag = 3, .op = .lookup, .node = dir, .data = name });
}

/// A core with one FILE pane holding `text`, which is what most of acme's
/// window files are about. Slot 0, and its serial is the directory name.
fn withFile(gpa: std.mem.Allocator, text: []const u8) !*Pardes {
    const p = try Pardes.init(gpa, .{ .tty_only = true, .cols = 80, .rows = 24 });
    errdefer p.deinit();
    while (p.nextEffect()) |_| {}
    _ = try p.hxOpenFileContent(text);
    while (p.nextEffect()) |_| {}
    return p;
}

fn serialOf(p: *Pardes) u32 {
    return p.panes[0].?.serial;
}

const Dirent = struct { node: u64, dir: bool, name: []const u8 };

/// Decode the readdir staging format `src/fuse.zig` agreed to.
fn dirents(bytes: []const u8, out: []Dirent) []Dirent {
    var n: usize = 0;
    var i: usize = 0;
    while (i + 10 <= bytes.len and n < out.len) {
        const node = std.mem.readInt(u64, bytes[i..][0..8], .little);
        const kind = bytes[i + 8];
        const len = bytes[i + 9];
        i += 10;
        if (i + len > bytes.len) break;
        out[n] = .{ .node = node, .dir = kind == 1, .name = bytes[i .. i + len] };
        i += len;
        n += 1;
    }
    return out[0..n];
}

fn nameAt(list: []const Dirent, want: []const u8) ?Dirent {
    for (list) |d| if (std.mem.eql(u8, d.name, want)) return d;
    return null;
}

test "readdir lists the root, a pane directory, and new/ without creating anything" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "hello\n");
    defer p.deinit();
    const serial = serialOf(p);
    var buf: [32]Dirent = undefined;

    const root = rdir(p, @intFromEnum(TopFile.root), 0);
    try testing.expectEqual(Status.ok, root.reply.status);
    const top = dirents(root.bytes, &buf);
    try testing.expectEqual(@as(usize, 4), top.len);
    try testing.expectEqualStrings("index", top[0].name);
    try testing.expectEqualStrings("cons", top[1].name);
    try testing.expectEqualStrings("new", top[2].name);
    try testing.expect(top[2].dir and !top[0].dir);
    var idbuf: [16]u8 = undefined;
    try testing.expectEqualStrings(try std.fmt.bufPrint(&idbuf, "{d}", .{serial}), top[3].name);
    try testing.expect(top[3].dir);
    // the id a readdir reports is the id a getattr will report
    try testing.expectEqual(Node.of(serial, .dir), top[3].node);

    // `off` skips entries, and past the end is EOF, not an error
    const rest = rdir(p, @intFromEnum(TopFile.root), 3);
    try testing.expectEqual(@as(usize, 1), dirents(rest.bytes, &buf).len);
    const eof = rdir(p, @intFromEnum(TopFile.root), 99);
    try testing.expectEqual(Status.ok, eof.reply.status);
    try testing.expectEqual(@as(usize, 0), eof.bytes.len);

    const dir = rdir(p, Node.of(serial, .dir), 0);
    const files = dirents(dir.bytes, &buf);
    try testing.expectEqual(@as(usize, 10), files.len); // dirtabw minus "."
    try testing.expect(nameAt(files, "addr") != null);
    try testing.expect(nameAt(files, "xdata") != null);
    try testing.expect(nameAt(files, ".") == null);
    try testing.expectEqual(Node.of(serial, .body), nameAt(files, "body").?.node);

    // acme(4) says accessing a file in `new` creates a window, so LISTING it
    // must enumerate nothing at all: every name it could report is a name
    // whose lookup creates a pane, and `ls -l` stats what a listing reported.
    const before = p.next_serial;
    const new = rdir(p, @intFromEnum(TopFile.new), 0);
    try testing.expectEqual(Status.ok, new.reply.status);
    try testing.expectEqual(@as(usize, 0), new.bytes.len);
    try testing.expectEqual(before, p.next_serial);

    // a file is not a directory
    try testing.expectEqual(E.NOTDIR, rdir(p, Node.of(serial, .body), 0).errno());
}

test "lookup resolves top files, pane serials and pane files" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "hello\n");
    defer p.deinit();
    const serial = serialOf(p);
    const root = @intFromEnum(TopFile.root);

    try testing.expectEqual(@as(u64, @intFromEnum(TopFile.index)), look_up(p, root, "index").reply.attr.node);
    try testing.expect(look_up(p, root, "new").reply.attr.dir);
    try testing.expectEqual(E.NOENT, look_up(p, root, "nosuchthing").errno());

    var idbuf: [16]u8 = undefined;
    const dir = look_up(p, root, try std.fmt.bufPrint(&idbuf, "{d}", .{serial}));
    try testing.expectEqual(Node.of(serial, .dir), dir.reply.attr.node);
    try testing.expect(dir.reply.attr.dir);
    // a serial that is not a live pane, and a serial that never existed
    try testing.expectEqual(E.NOENT, look_up(p, root, "99999").errno());

    const body = look_up(p, Node.of(serial, .dir), "body");
    try testing.expectEqual(Node.of(serial, .body), body.reply.attr.node);
    // a lookup answers exactly what a getattr of the same node would
    const stat = call(p, .{ .tag = 4, .op = .getattr, .node = Node.of(serial, .body) });
    try testing.expectEqual(body.reply.attr.size, stat.reply.attr.size);
    try testing.expectEqual(@as(u64, "hello\n".len), stat.reply.attr.size);
    try testing.expectEqual(E.NOENT, look_up(p, Node.of(serial, .dir), "editout").errno());
    try testing.expectEqual(E.NOTDIR, look_up(p, Node.of(serial, .body), "x").errno());
}

test "a lookup inside new/ creates a pane and resolves that pane's file" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "first\n");
    defer p.deinit();
    const before = serialOf(p);

    // a name that is not a pane file creates nothing
    try testing.expectEqual(E.NOENT, look_up(p, @intFromEnum(TopFile.new), "bogus").errno());
    try testing.expectEqual(before, p.next_serial);

    const a = look_up(p, @intFromEnum(TopFile.new), "body");
    try testing.expectEqual(Status.ok, a.reply.status);
    const made: Node = @bitCast(a.reply.attr.node);
    try testing.expect(made.serial != before);
    try testing.expectEqual(@intFromEnum(PaneFile.body), made.file);

    // ...and it is a real pane: `echo hi > new/body` leaves a pane holding hi
    _ = wr(p, a.reply.attr.node, "hi");
    const id = p.paneBySerial(@intCast(made.serial)).?;
    try testing.expectEqualStrings("hi", p.panes[id].?.file.?.content);
}

test "index prints winctlprint's five fields then the tag" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "hello\nthere\n");
    defer p.deinit();
    const pane = p.panes[0].?;

    const a = rd(p, @intFromEnum(TopFile.index), 0, 4096);
    try testing.expectEqual(Status.ok, a.reply.status);
    var got: [512]u8 = undefined;
    @memcpy(got[0..a.bytes.len], a.bytes);
    const line = got[0..a.bytes.len];

    const tag = tagOf(p, pane);
    var want: std.ArrayList(u8) = .empty;
    defer want.deinit(gpa);
    try want.print(gpa, "{d:>11} {d:>11} {d:>11} {d:>11} {d:>11} {s}\n", .{
        pane.serial, tag.len, @as(usize, "hello\nthere\n".len), 0, 0, firstLine(tag),
    });
    try testing.expectEqualStrings(want.items, line);
    // acme(4): "at character position 5x12 starts the name of the window"
    try testing.expectEqual(@as(usize, 60), std.mem.indexOf(u8, line, firstLine(tag)).?);

    // seekable: a script may pread the middle of it
    const mid = rd(p, @intFromEnum(TopFile.index), 60, 5);
    try testing.expectEqualStrings(firstLine(tag)[0..5], mid.bytes);

    // ...and a dirty pane says so in the fifth field
    pane.file.?.saved_revision = pane.file.?.revision -% 1;
    const dirty = rd(p, @intFromEnum(TopFile.index), 48, 12);
    try testing.expectEqualStrings("          1 ", dirty.bytes);
}

test "ctl read is index's five fields plus width in cells, font and tab width" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "x\n");
    defer p.deinit();
    const pane = p.panes[0].?;

    const a = rd(p, Node.of(pane.serial, .ctl), 0, 4096);
    try testing.expectEqual(Status.ok, a.reply.status);
    var want: std.ArrayList(u8) = .empty;
    defer want.deinit(gpa);
    try want.print(gpa, "{d:>11} {d:>11} {d:>11} {d:>11} {d:>11} {d:>11} {s} {d:>11} ", .{
        pane.serial, tagOf(p, pane).len, @as(usize, 2), 0, 0, pane.cols, "default", config.tab_width,
    });
    try testing.expectEqualStrings(want.items, a.bytes);

    // plan9 %q: a name with a space in it becomes one shell word
    var quoted: std.ArrayList(u8) = .empty;
    defer quoted.deinit(gpa);
    stageQuoted(&quoted, gpa, "DejaVu Sans Mono");
    try testing.expectEqualStrings("'DejaVu Sans Mono'", quoted.items);
    quoted.clearRetainingCapacity();
    stageQuoted(&quoted, gpa, "it's");
    try testing.expectEqualStrings("'it''s'", quoted.items);
}

test "body reads at any offset and writes append" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\n");
    defer p.deinit();
    const serial = serialOf(p);
    const body = Node.of(serial, .body);

    try testing.expectEqualStrings("one\ntwo\n", rd(p, body, 0, 100).bytes);
    try testing.expectEqualStrings("two\n", rd(p, body, 4, 100).bytes);
    try testing.expectEqualStrings("wo", rd(p, body, 5, 2).bytes);
    try testing.expectEqualStrings("", rd(p, body, 999, 2).bytes);
    // zero copy: the answer points INTO the pane, it is not a staged copy
    try testing.expect(rd(p, body, 0, 100).bytes.ptr == p.panes[0].?.file.?.content.ptr);

    // acme(4): "Text written to body is always appended; the file offset is
    // ignored" — so a write at offset 0 still lands at the end.
    const w = call(p, .{ .tag = 5, .op = .write, .node = body, .off = 0, .data = "three\n" });
    try testing.expectEqual(@as(u32, 6), w.reply.written);
    try testing.expectEqualStrings("one\ntwo\nthree\n", p.panes[0].?.file.?.content);

    // a write cut mid-character is SHORT, never split
    const short = wr(p, body, "a\xC3");
    try testing.expectEqual(@as(u32, 1), short.reply.written);
    try testing.expectEqualStrings("one\ntwo\nthree\na", p.panes[0].?.file.?.content);
}

test "a body write to a terminal pane types at its shell" {
    const gpa = testing.allocator;
    const p = try Pardes.init(gpa, .{ .tty_only = true, .cols = 40, .rows = 10 });
    defer p.deinit();
    while (p.nextEffect()) |_| {}
    const pane = p.panes[0].?;
    try testing.expect(pane.isTerminal());

    // `win`'s transcript semantics: the only way into a program's transcript
    // is to type at it, so a body write becomes a pty write.
    const a = wr(p, Node.of(pane.serial, .body), "ls -l\r");
    try testing.expectEqual(@as(u32, 6), a.reply.written);
    try testing.expectEqualStrings("ls -l\r", a.pty());

    // and a body READ renders the scrollback rather than lending a buffer
    const r = rd(p, Node.of(pane.serial, .body), 0, 64);
    try testing.expectEqual(Status.ok, r.reply.status);
}

test "tag reads the whole tag and writes append to the editable tail" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "x\n");
    defer p.deinit();
    const pane = p.panes[0].?;
    const node = Node.of(pane.serial, .tag);

    const whole = rd(p, node, 0, 4096);
    try testing.expect(std.mem.startsWith(u8, whole.bytes, "/hxcase.txt"));
    try testing.expect(std.mem.indexOf(u8, whole.bytes, "Del") != null);

    const before = rd(p, node, 0, 4096).bytes.len;
    const w = wr(p, node, " Mine");
    try testing.expectEqual(@as(u32, 5), w.reply.written);
    try testing.expect(std.mem.endsWith(u8, pane.tag_tail[0..pane.tag_tail_len], " Mine"));
    const after = rd(p, node, 0, 4096);
    try testing.expectEqual(before + 5, after.bytes.len);
    try testing.expect(std.mem.endsWith(u8, after.bytes, " Mine"));

    // the tail is one bounded line; with no room left the file is FULL
    pane.tag_tail_len = pane.tag_tail.len;
    try testing.expectEqual(E.NOSPC, wr(p, node, "x").errno());
}

test "the address language, form by form" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\nthree\n"); // 14 bytes, three lines
    defer p.deinit();
    const serial = serialOf(p);
    const addr = Node.of(serial, .addr);

    const Case = struct { expr: []const u8, q0: u32, q1: u32 };
    for ([_]Case{
        .{ .expr = "#0", .q0 = 0, .q1 = 0 },
        .{ .expr = "#5", .q0 = 5, .q1 = 5 },
        .{ .expr = "0", .q0 = 0, .q1 = 0 },
        .{ .expr = "1", .q0 = 0, .q1 = 4 },
        .{ .expr = "2", .q0 = 4, .q1 = 8 },
        .{ .expr = "$", .q0 = 14, .q1 = 14 },
        .{ .expr = ",", .q0 = 0, .q1 = 14 },
        .{ .expr = "1,2", .q0 = 0, .q1 = 8 },
        .{ .expr = "#1,#4", .q0 = 1, .q1 = 4 },
        .{ .expr = "2+1", .q0 = 8, .q1 = 14 },
        .{ .expr = "$-1", .q0 = 8, .q1 = 14 },
        .{ .expr = "/two/", .q0 = 4, .q1 = 7 },
        .{ .expr = "/t.o/", .q0 = 4, .q1 = 7 },
        // a trailing newline is what a shell redirect leaves behind
        .{ .expr = "1\n", .q0 = 0, .q1 = 4 },
    }) |c| {
        // every case starts from a known address, so `.` and `+`/`-` are
        // measured against the same place each time
        _ = wr(p, addr, "#0");
        const w = wr(p, addr, c.expr);
        try testing.expectEqual(Status.ok, w.reply.status);
        const got = rd(p, addr, 0, 64);
        var want: [32]u8 = undefined;
        try testing.expectEqualStrings(
            try std.fmt.bufPrint(&want, "{d:>11} {d:>11} ", .{ c.q0, c.q1 }),
            got.bytes,
        );
    }

    // `.` is the CURRENT ADDRESS (acme passes w->addr as `ar`), not the
    // selection: set it, then ask for it back.
    _ = wr(p, addr, "1");
    _ = wr(p, addr, ".");
    try testing.expectEqual(@as(u32, 0), p.fs.panes[0].addr.q0);
    try testing.expectEqual(@as(u32, 4), p.fs.panes[0].addr.q1);

    // `?re?` searches BACKWARD from the start of the running range and takes
    // the LAST match before it — acme's `rxbexecute`.
    _ = wr(p, addr, "$");
    _ = wr(p, addr, "?o?");
    try testing.expectEqual(@as(u32, 6), p.fs.panes[0].addr.q0); // the `o` in "two"
    try testing.expectEqual(@as(u32, 7), p.fs.panes[0].addr.q1);

    // limit=addr confines a forward search
    _ = wr(p, addr, "1");
    _ = wr(p, Node.of(serial, .ctl), "limit=addr\n");
    _ = wr(p, addr, "#0");
    try testing.expectEqual(E.INVAL, wr(p, addr, "/three/").errno());
    _ = wr(p, Node.of(serial, .ctl), "clean\n"); // any ctl write; limit stays
    // ...and opening ctl clears it again (acme(4))
    _ = call(p, .{ .tag = 6, .op = .open, .node = Node.of(serial, .ctl) });
    try testing.expect(p.fs.panes[0].limit == null);
    _ = wr(p, addr, "#0");
    try testing.expectEqual(Status.ok, wr(p, addr, "/three/").reply.status);

    // refusals
    for ([_][]const u8{ "zzz", "#", "//", "/nomatch/", "1 2", "99", "/a\\" }) |bad| {
        _ = wr(p, addr, "#0");
        try testing.expectEqual(E.INVAL, wr(p, addr, bad).errno());
    }

    // acme recurses once per `,` with no bound at all, which a script turns
    // into a stack overflow with one write(2). Refused, not crashed.
    const nested = "," ** 4096;
    _ = wr(p, addr, "#0");
    try testing.expectEqual(E.INVAL, wr(p, addr, nested).errno());
}

test "data and xdata read from addr, move it, and write through it" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\n");
    defer p.deinit();
    const serial = serialOf(p);
    const addr = Node.of(serial, .addr);
    const data = Node.of(serial, .data);
    const xdata = Node.of(serial, .xdata);

    _ = wr(p, addr, "#0");
    try testing.expectEqualStrings("one", rd(p, data, 0, 3).bytes);
    // ...and the address is now the null string after what was returned
    try testing.expectEqual(@as(u32, 3), p.fs.panes[0].addr.q0);
    try testing.expectEqual(@as(u32, 3), p.fs.panes[0].addr.q1);

    // xdata stops at the END of the address where data would run on
    _ = wr(p, addr, "1");
    try testing.expectEqualStrings("one\n", rd(p, xdata, 0, 100).bytes);
    _ = wr(p, addr, "1");
    try testing.expectEqualStrings("one\ntwo\n", rd(p, data, 0, 100).bytes);

    // a write REPLACES the addressed text and leaves the address after it
    _ = wr(p, addr, "1");
    const w = wr(p, data, "ONE\n");
    try testing.expectEqual(@as(u32, 4), w.reply.written);
    try testing.expectEqualStrings("ONE\ntwo\n", p.panes[0].?.file.?.content);
    try testing.expectEqual(@as(u32, 4), p.fs.panes[0].addr.q0);
}

test "data never splits a grapheme, in either direction" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "\u{00e9}x\n"); // é is two bytes
    defer p.deinit();
    const serial = serialOf(p);
    _ = wr(p, Node.of(serial, .addr), "#0");
    // one byte is not enough for the first character: acme's `if(m == 0) break`
    try testing.expectEqualStrings("", rd(p, Node.of(serial, .data), 0, 1).bytes);
    _ = wr(p, Node.of(serial, .addr), "#0");
    try testing.expectEqualStrings("\u{00e9}", rd(p, Node.of(serial, .data), 0, 2).bytes);

    // and a write ending mid-character is short rather than corrupting
    _ = wr(p, Node.of(serial, .addr), "#0");
    try testing.expectEqual(@as(u32, 1), wr(p, Node.of(serial, .data), "a\xC3").reply.written);
}

test "rdsel reads the selection and wrsel replaces it" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\n");
    defer p.deinit();
    const serial = serialOf(p);
    const ctl = Node.of(serial, .ctl);

    _ = wr(p, Node.of(serial, .addr), "#0,#3");
    try testing.expectEqual(Status.ok, wr(p, ctl, "dot=addr\n").reply.status);
    try testing.expectEqualStrings("one", rd(p, Node.of(serial, .rdsel), 0, 100).bytes);

    // ...and the round trip back out is the identity, not a range that creeps
    _ = wr(p, ctl, "addr=dot\n");
    try testing.expectEqual(@as(u32, 0), p.fs.panes[0].addr.q0);
    try testing.expectEqual(@as(u32, 3), p.fs.panes[0].addr.q1);

    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .wrsel), "ONE").reply.status);
    try testing.expectEqualStrings("ONE\ntwo\n", p.panes[0].?.file.?.content);
    // a second write appends after the first, acme's `wrselrange`
    _ = wr(p, Node.of(serial, .wrsel), "!");
    try testing.expectEqualStrings("ONE!\ntwo\n", p.panes[0].?.file.?.content);
}

test "every ctl verb, and every refusal" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\n");
    defer p.deinit();
    const serial = serialOf(p);
    const ctl = Node.of(serial, .ctl);
    const pane = p.panes[0].?;
    const pf = &p.fs.panes[0];

    // several verbs in one write, which is what the man page promises
    try testing.expectEqual(Status.ok, wr(p, ctl, "nomark\nnoscroll\ndirty\n").reply.status);
    try testing.expect(pf.nomark and pf.noscroll and dirtyOf(pane));
    try testing.expectEqual(Status.ok, wr(p, ctl, "mark\nscroll\nclean\n").reply.status);
    try testing.expect(!pf.nomark and !pf.noscroll and !dirtyOf(pane));

    _ = wr(p, ctl, "cleartag\n");
    try testing.expectEqual(@as(usize, 0), pane.tag_tail_len);

    _ = wr(p, Node.of(serial, .addr), "2");
    _ = wr(p, ctl, "limit=addr\n");
    try testing.expectEqual(@as(u32, 4), pf.limit.?.q0);
    _ = wr(p, ctl, "dot=addr\nshow\n");
    try testing.expectEqual(@as(i32, 1), pane.cur_row);

    try testing.expectEqual(Status.ok, wr(p, ctl, "name /tmp/renamed.txt\n").reply.status);
    try testing.expectEqualStrings("/tmp/renamed.txt", pane.file.?.path);
    // acme rejects a name with any character <= ' ' in it
    try testing.expectEqual(E.INVAL, wr(p, ctl, "name two words\n").errno());
    try testing.expectEqual(E.INVAL, wr(p, ctl, "name\n").errno());
    try testing.expectEqualStrings("/tmp/renamed.txt", pane.file.?.path);

    // `put` is acme's Put, which is pardes's Save
    try testing.expect(wr(p, ctl, "put\n").saved);

    // REFUSED, each for a reason that is not "unimplemented" — see
    // `refused_verbs`. Silently accepting these is the worse failure.
    for ([_][]const u8{
        "menu", "nomenu", "dump echo hi", "dumpdir /tmp", "font Go Mono", "lock", "unlock", "bogus", "DEL",
    }) |bad| try testing.expectEqual(E.INVAL, wr(p, ctl, bad).errno());

    // ATOMIC, which acme is not: an unknown verb aborts the WHOLE write.
    try testing.expect(!dirtyOf(pane));
    try testing.expectEqual(E.INVAL, wr(p, ctl, "dirty\nbogus\n").errno());
    try testing.expect(!dirtyOf(pane));
}

test "ctl get reloads the pane from disk and del honours a dirty body" {
    const gpa = testing.allocator;
    var tmp = testing.tmpDir(.{});
    defer tmp.cleanup();
    try tmp.dir.writeFile(testing.io, .{ .sub_path = "note.txt", .data = "from disk\n" });
    var path_buf: [256]u8 = undefined;
    const path = try std.fmt.bufPrint(&path_buf, ".zig-cache/tmp/{s}/note.txt", .{tmp.sub_path});

    const p = try withFile(gpa, "in memory\n");
    defer p.deinit();
    const serial = serialOf(p);
    const ctl = Node.of(serial, .ctl);
    const pane = p.panes[0].?;

    var name: [std.fs.max_path_bytes + 8]u8 = undefined;
    _ = wr(p, ctl, try std.fmt.bufPrint(&name, "name {s}\n", .{path}));
    try testing.expectEqual(Status.ok, wr(p, ctl, "get\n").reply.status);
    try testing.expectEqualStrings("from disk\n", pane.file.?.content);
    // Get leaves the pane clean and the previous text one Undo away
    try testing.expect(!dirtyOf(pane));
    try testing.expect(pane.file.?.undo_len > 0);

    // acme: `del` is "delete, but check dirty"; `delete` is "delete for sure"
    _ = wr(p, ctl, "dirty\n");
    try testing.expectEqual(E.INVAL, wr(p, ctl, "del\n").errno());
    try testing.expect(p.paneBySerial(serial) != null);
    // ...and a second pane so the last one closing does not quit the editor
    _ = look_up(p, @intFromEnum(TopFile.new), "body");
    try testing.expectEqual(Status.ok, wr(p, ctl, "delete\n").reply.status);
    try testing.expect(p.paneBySerial(serial) == null);
}

test "errors and cons append to one +Errors buffer per directory" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "x\n");
    defer p.deinit();
    const serial = serialOf(p);

    const live = for (p.panes) |slot| {
        if (slot) |q| if (q.file) |f| if (f.output) |o| if (std.meta.activeTag(o.from) == .errors) break q;
    } else null;
    try testing.expect(live == null); // "not until text is actually written"

    try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .errors), "boom\n").reply.status);
    _ = wr(p, @intFromEnum(TopFile.cons), "again\n");

    var found: usize = 0;
    for (p.panes) |slot| {
        const q = slot orelse continue;
        const f = q.file orelse continue;
        const o = f.output orelse continue;
        if (std.meta.activeTag(o.from) != .errors) continue;
        found += 1;
        try testing.expectEqualStrings("boom\nagain\n", f.content);
        try testing.expectEqualStrings("/+Errors", f.path);
    }
    try testing.expectEqual(@as(usize, 1), found);
}

test "setattr truncation empties the body and answers fresh attributes" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\n");
    defer p.deinit();
    const serial = serialOf(p);

    const a = call(p, .{ .tag = 7, .op = .setattr, .node = Node.of(serial, .body), .truncate = true });
    try testing.expectEqual(Status.ok, a.reply.status);
    try testing.expectEqual(@as(u64, 0), a.reply.attr.size);
    try testing.expectEqualStrings("", p.panes[0].?.file.?.content);

    // `> body` then a write is the shell's way of REPLACING a pane's text
    _ = wr(p, Node.of(serial, .body), "new text\n");
    try testing.expectEqualStrings("new text\n", p.panes[0].?.file.?.content);

    // a setattr that sets no size changes nothing
    const noop = call(p, .{ .tag = 8, .op = .setattr, .node = Node.of(serial, .body) });
    try testing.expectEqual(@as(u64, 9), noop.reply.attr.size);
}

test "event records are acme's bytes, one per read, and .again when empty" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "Msg fs-ran\n");
    defer p.deinit();
    const serial = serialOf(p);
    const event = Node.of(serial, .event);

    // nothing is recorded while nobody is listening
    _ = noteAction(p, 0, .body_exec, 1, 4, flag_builtin, "sg ");
    try testing.expect(p.fs.panes[0].events.empty());

    const h = call(p, .{ .tag = 10, .op = .open, .node = event });
    try testing.expect(h.reply.handle != 0);
    try testing.expectEqual(@as(u16, 1), p.fs.listeners);

    // an empty queue is `.again`: nothing consumed, ask me later. NEVER an
    // error, and never a loop.
    try testing.expectEqual(Status.again, rd(p, event, 0, 4096).reply.status);

    p.fs.origin = 'M';
    _ = noteAction(p, 0, .body_exec, 1, 4, flag_builtin, "ell");
    _ = noteAction(p, 0, .body_delete, 0, 3, 0, "");
    // `%c%c%d %d %d %d %s\n`, wind.c's winevent with the owner char in front
    try testing.expectEqualStrings("MX1 4 1 3 ell\n", rd(p, event, 0, 4096).bytes);
    try testing.expectEqualStrings("MD0 3 0 0 \n", rd(p, event, 0, 4096).bytes);
    try testing.expectEqual(Status.again, rd(p, event, 0, 4096).reply.status);

    // one record per read: a read too small to hold one is refused rather
    // than answered with half a record the reader cannot resynchronise from
    _ = noteAction(p, 0, .body_look, 0, 3, flag_filename, "one");
    try testing.expectEqual(E.INVAL, rd(p, event, 0, 4).errno());
    try testing.expectEqualStrings("ML0 3 4 3 one\n", rd(p, event, 0, 4096).bytes);

    // text of 256 bytes or more is elided; the reader fetches it from `data`
    const big = "z" ** max_record_text;
    _ = noteAction(p, 0, .body_exec, 0, max_record_text, 0, big);
    try testing.expectEqualStrings("MX0 256 0 0 \n", rd(p, event, 0, 4096).bytes);

    _ = call(p, .{ .tag = 11, .op = .release, .node = event, .handle = h.reply.handle });
    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
}

/// Drain a queue into `store` and return the records. Reading is destructive
/// and a `.staged` answer is only valid until the next request, so each record
/// is copied out as it arrives.
fn drainEvents(p: *Pardes, node: u64, store: []u8, out: [][]const u8) [][]const u8 {
    var used: usize = 0;
    var n: usize = 0;
    while (n < out.len) {
        const a = rd(p, node, 0, 4096);
        if (a.reply.status != .ok) break;
        @memcpy(store[used..][0..a.bytes.len], a.bytes);
        out[n] = store[used..][0..a.bytes.len];
        used += a.bytes.len;
        n += 1;
    }
    return out[0..n];
}

test "a write through the filesystem is reported once, attributed to the file it came through" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "one\ntwo\n");
    defer p.deinit();
    const serial = serialOf(p);
    const event = Node.of(serial, .event);
    _ = call(p, .{ .tag = 40, .op = .open, .node = event });
    var store: [4096]u8 = undefined;
    var slots: [16][]const u8 = undefined;
    _ = drainEvents(p, event, &store, &slots);

    // A body write is acme's `E`: "writes to the body or tag file". ONE pair
    // per write, because the diff lives in `file_pane.setContent` and a write
    // is one content swap — that is the contract with the core's hook, and
    // emitting records from the handler as well is what it forbids.
    _ = wr(p, Node.of(serial, .body), "three\n");
    const body_recs = drainEvents(p, event, &store, &slots);
    try testing.expect(body_recs.len >= 1);
    // ...and the record's TEXT here contains a newline of its own, which is
    // exactly why `Queue` frames records by length instead of by line
    try testing.expectEqualStrings("EI8 14 0 6 three\n\n", body_recs[0]);
    // the write also made the pane dirty, so its TAG changed — and acme
    // attributes that to the write too (`winsettag` runs inside the same
    // `winlock(w, 'E')`), which is why the origin is set for the whole
    // request and not just for the mutation.
    for (body_recs[1..]) |r| {
        try testing.expectEqual(@as(u8, 'E'), r[0]);
        try testing.expect(Action.fromChar(r[1]).?.onTag());
    }

    // A `data` write is acme's `F`: "actions through the window's other
    // files" — and a replacement is a delete then an insert, acme's order,
    // with no text on the delete.
    _ = wr(p, Node.of(serial, .addr), "1");
    _ = wr(p, Node.of(serial, .data), "ONE\n");
    const data_recs = drainEvents(p, event, &store, &slots);
    try testing.expectEqual(@as(usize, 2), data_recs.len);
    try testing.expectEqualStrings("FD0 3 0 0 \n", data_recs[0]);
    try testing.expectEqualStrings("FI0 3 0 3 ONE\n", data_recs[1]);
    try testing.expectEqual(Status.again, rd(p, event, 0, 4096).reply.status);
}

test "two event readers each count once, and the second closing leaves the first" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "x\n");
    defer p.deinit();
    const event = Node.of(serialOf(p), .event);

    _ = call(p, .{ .tag = 12, .op = .open, .node = event });
    _ = call(p, .{ .tag = 13, .op = .open, .node = event });
    try testing.expectEqual(@as(u16, 2), p.fs.panes[0].readers);
    try testing.expectEqual(@as(u16, 2), p.fs.listeners);

    // A release names the NODE, not a handle: FUSE carries the nodeid on every
    // request, so there is no fid table to look one up in, and one release
    // answers for one open.
    _ = call(p, .{ .tag = 14, .op = .release, .node = event });
    try testing.expectEqual(@as(u16, 1), p.fs.panes[0].readers);
    try testing.expect(p.fs.scripted(0)); // the pane is STILL script-driven

    // a release with nothing left to release changes nothing and is not an
    // error, and neither is one naming a node that never counted
    _ = call(p, .{ .tag = 15, .op = .release, .node = Node.of(serialOf(p), .body) });
    try testing.expectEqual(@as(u16, 1), p.fs.listeners);

    _ = call(p, .{ .tag = 16, .op = .release, .node = event });
    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
    try testing.expect(!p.fs.scripted(0));
    _ = call(p, .{ .tag = 17, .op = .release, .node = event });
    try testing.expectEqual(@as(u16, 0), p.fs.listeners);
}

test "a pane deleted while its event file is open leaves no suppression behind" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "x\n");
    defer p.deinit();
    const serial = serialOf(p);
    const event = Node.of(serial, .event);
    // a second pane, so deleting the first does not quit the editor
    _ = look_up(p, @intFromEnum(TopFile.new), "body");

    const a = call(p, .{ .tag = 18, .op = .open, .node = event });
    const b = call(p, .{ .tag = 19, .op = .open, .node = event });
    try testing.expectEqual(@as(u16, 2), p.fs.listeners);

    _ = wr(p, Node.of(serial, .ctl), "delete\n");
    try testing.expect(p.paneBySerial(serial) == null);
    // the core's `State.forget` took BOTH readers out with the pane
    try testing.expectEqual(@as(u16, 0), p.fs.listeners);

    // ...and the two late releases must not underflow it back to 65535, which
    // would suppress every button action in the editor forever
    _ = call(p, .{ .tag = 20, .op = .release, .node = event, .handle = a.reply.handle });
    _ = call(p, .{ .tag = 21, .op = .release, .node = event, .handle = b.reply.handle });
    try testing.expectEqual(@as(u16, 0), p.fs.listeners);

    // every operation on the dead pane is ENOENT — acme's Edel
    try testing.expectEqual(E.NOENT, rd(p, event, 0, 64).errno());
    try testing.expectEqual(E.NOENT, rd(p, Node.of(serial, .body), 0, 64).errno());
    try testing.expectEqual(E.NOENT, wr(p, Node.of(serial, .ctl), "clean\n").errno());
    try testing.expectEqual(E.NOENT, call(p, .{ .tag = 22, .op = .open, .node = event }).errno());
}

test "writing an event record back performs the action it names" {
    const gpa = testing.allocator;
    const p = try withFile(gpa, "Msg fs-ran\n");
    defer p.deinit();
    const serial = serialOf(p);
    const event = Node.of(serial, .event);
    const pane = p.panes[0].?;

    // an `X` record over the body text `Msg fs-ran` is an Exec of it
    const w = wr(p, event, "FX0 10\n");
    try testing.expectEqual(Status.ok, w.reply.status);
    try testing.expectEqualStrings("fs-ran", pane.msg[0..pane.msg_len]);

    // several records in one write
    pane.msg_len = 0;
    try testing.expectEqual(Status.ok, wr(p, event, "FX0 10\nFX0 10\n").reply.status);
    try testing.expectEqualStrings("fs-ran", pane.msg[0..pane.msg_len]);

    // ...and nothing applies when any of it is malformed: acme's Ebadevent
    pane.msg_len = 0;
    for ([_][]const u8{
        "FX0 10\nFQ0 1\n", // unknown type character
        "FX0 999\n", // out of range
        "FX0 10", // no newline
        "FX5 1\n", // q0 > q1
        "FD0 3\n", // a report, not a request
        "F\n",
    }) |bad| {
        try testing.expectEqual(E.INVAL, wr(p, event, bad).errno());
        try testing.expectEqual(@as(usize, 0), pane.msg_len);
    }

    // The action is attributed to the FILESYSTEM (`F`), never to whatever the
    // writer put in the record's origin character — acme copies that byte
    // into `w->owner` and lets a script claim its Exec came from the
    // keyboard.
    _ = call(p, .{ .tag = 23, .op = .open, .node = event });
    p.fs.origin = 'K';
    _ = wr(p, event, "KX0 10\n");
    try testing.expectEqual(@as(u8, 'F'), p.fs.origin);
}