path: root/src/macos.zig

//! libpardes — the static library the native macOS app links against.
//!
//! The split, which is the whole design: Zig keeps the core, the ptys, every
//! effect and the worker threads; Swift owns NSApplication, the window, input
//! translation and drawing. src/macos/pardes.h is the contract between them and
//! docs/macos.md argues for the shape.
//!
//! This is deliberately src/web.zig's boundary with the wasm removed. Both
//! hosts are the same animal — someone else owns the clock, feeds events in
//! through flat functions and reads one packed cell buffer out — and the
//! browser already proved the shape works. The one real divergence is that the
//! browser has no processes, so it forwards every effect to JavaScript, whereas
//! forkpty is right here and this file performs them.
//!
//! Everything below is main-thread only. The single exception is the `wakeup`
//! callback, which a pty reader task calls; the host's job is to hop to the
//! main thread and call pardes_tick.
//!
//! The Zig half is ordinary POSIX and builds/tests on Linux — see the dev-loop
//! section of docs/macos.md. Only the Swift app needs a Mac.

const std = @import("std");
const builtin = @import("builtin");
const posix = std.posix;
const libc = std.c;
const pardes = @import("pardes.zig");
const look = @import("look.zig");
const temp_file = @import("temp_file.zig");
const shell_bin = @import("shell_bin.zig");
const message = @import("message.zig");
const nested = @import("nested.zig");
/// The geometry types the pixel-attachment ABI carries. Behind the same
/// comptime gate the placements themselves are: a build without MuPDF emits no
/// attachments, so nothing here is analysed.
const image = if (pardes.pdf_enabled) @import("image.zig") else struct {};
const fonts = if (pardes.font_picker) @import("fonts.zig") else struct {
    pub const want: ?[]const u8 = null;
};
const user_config = @import("user_config.zig");

extern "c" fn forkpty(amaster: *c_int, name: ?[*:0]u8, termp: ?*const anyopaque, winp: ?*const posix.winsize) c_int;
extern "c" fn execv(path: [*:0]const u8, argv: [*:null]const ?[*:0]const u8) c_int;
extern "c" fn chdir(path: [*:0]const u8) c_int;
extern "c" fn _exit(status: c_int) noreturn;
extern "c" fn setenv(name: [*:0]const u8, value: [*:0]const u8, overwrite: c_int) c_int;

// TIOCSWINSZ: absent from std.c.T on darwin — _IOW('t', 103, winsize). Same
// constant the tty and gui shells spell for the same reason.
const TIOCSWINSZ: c_int = @bitCast(@as(u32, if (@hasDecl(posix.T, "IOCSWINSZ")) posix.T.IOCSWINSZ else 0x80087467));

// A library linked into an AppKit process has no terminal to garble, but it
// does share the app's stderr with Console.app. Same filter as src/main.zig:
// ghostty-vt narrates every unimplemented escape a child writes, and nobody
// wants that in a crash report. PARDES_LOG=1 gets the real logger back.
pub const std_options: std.Options = .{ .logFn = logFn };

fn logFn(
    comptime level: std.log.Level,
    comptime scope: @EnumLiteral(),
    comptime format: []const u8,
    args: anytype,
) void {
    if (scope != .macos and scope != .dump and std.c.getenv("PARDES_LOG") == null) return;
    std.log.defaultLog(level, scope, format, args);
}

const log = std.log.scoped(.macos);

// ---------------------------------------------------------------- boundary

/// Sync with: pardes_cell_s. The identical encoding is spelled a second time
/// for the browser as WebCell in src/web.zig.
///
/// ponytail: two copies of a fifteen-line pure encoder, not a shared module.
/// The web ABI is snapshot-tested through a headless Chrome that does not run
/// here, so extracting it would refactor a backend I cannot exercise to save
/// thirty lines. Merge them the day a third host wants the same bytes.
pub const Cell = extern struct {
    text: [8]u8,
    fg: u32,
    bg: u32,
    attrs: u16,
    len: u8,
    flags: u8,
};

/// Sync with: pardes_image_s. One rasterized attachment — a PDF page, or an
/// image pane's pixels — and where on the grid it goes.
///
/// Geometry travels in PHYSICAL PIXELS, because that is the space the core
/// already computed it in (pardes_resize hands it the physical cell). `cell_x`
/// and `cell_y` are the pane BODY's origin in cells and the only thing the
/// host has to multiply out; `dst` is relative to that origin, and `src` is
/// the crop of the raster to take. The core has already clipped both to the
/// viewport, which is what lets a host draw a continuous-scroll page without
/// inventing an overflow clip of its own.
pub const Image = extern struct {
    /// pane lifetime, page and raster generation: together the cache key. A
    /// host keeps its decoded texture while all three hold still, and `fit`,
    /// panning and scrolling deliberately do not move them.
    serial: u32,
    page: u32,
    revision: u32,
    cell_x: u16,
    cell_y: u16,
    /// the body this attachment may not paint outside of, in cells
    cell_w: u16,
    cell_h: u16,
    dst_x: u32,
    dst_y: u32,
    dst_w: u32,
    dst_h: u32,
    src_x: u32,
    src_y: u32,
    src_w: u32,
    src_h: u32,
    /// subpixel vertical displacement a proportional wheel kept
    offset_y: f32,
    iw: u32,
    ih: u32,
    /// iw * ih * 4 bytes, RGBA8. Borrowed until the next pardes_frame.
    rgba: [*]const u8,
};

/// Sync with: pardes_runtime_s. Two callbacks, because everything else the
/// core asks for it already does itself — it owns the ptys, and look.openLink
/// hands URLs to /usr/bin/open. Both are optional at the ABI level: a host that
/// passes null simply does without, rather than trapping inside the library.
pub const Runtime = extern struct {
    userdata: ?*anyopaque = null,
    wakeup: ?*const fn (?*anyopaque) callconv(.c) void = null,
    set_clipboard: ?*const fn (?*anyopaque, [*]const u8, usize) callconv(.c) void = null,
};

const color_default: u32 = 0x01000000;
const color_indexed: u32 = 0x02000000;
const cell_flag_default: u8 = 1;

// ---------------------------------------------------------------- state

/// One pty, and the task draining it. `gen` is the per-slot spawn generation:
/// the core reuses pane ids and has no close effect, so a respawned slot must
/// ignore the previous shell's late bytes rather than feed them to the new one.
const Pty = struct {
    file: std.Io.File,
    pid: posix.pid_t,
    gen: u32,
    reader: std.Io.Future(anyerror!void),
};

/// What a reader task hands the main thread. `gen` travels with the message so
/// a shell that was replaced while its read was in flight cannot have its
/// stragglers parsed into the pty that took its slot.
const Msg = union(enum) {
    output: struct { pane: u8, gen: u32, bytes: []u8 },
    eof: struct { pane: u8, gen: u32 },
    /// One `Look <path>` line from a pardes launched inside this one. Arrives
    /// on the listener thread; runs, like everything else, on the main one.
    command: []u8,

    fn free(m: Msg, gpa: std.mem.Allocator) void {
        switch (m) {
            .output => |o| gpa.free(o.bytes),
            .eof => {},
            .command => |c| gpa.free(c),
        }
    }
};

const inbox_capacity = 512;

const MessageBatch = struct {
    items: [inbox_capacity]Msg = undefined,
    len: usize = 0,

    fn slice(batch: *MessageBatch) []Msg {
        return batch.items[0..batch.len];
    }
};

const Inbox = struct {
    mutex: std.atomic.Mutex = .unlocked,
    items: [inbox_capacity]Msg = undefined,
    head: usize = 0,
    len: usize = 0,
    closed: bool = false,
    /// Set when a wakeup has been delivered and not yet answered by a tick.
    wake_pending: std.atomic.Value(bool) = .init(false),

    fn lock(q: *Inbox) void {
        // AppKit's main thread runs at a higher QoS than reader tasks, so yield
        // periodically rather than donating a full core to a preempted reader.
        var spins: u8 = 0;
        while (!q.mutex.tryLock()) {
            spins +%= 1;
            if (spins == 0) std.Thread.yield() catch {} else std.atomic.spinLoopHint();
        }
    }

    fn removeAt(q: *Inbox, offset: usize) Msg {
        const removed = q.items[(q.head + offset) % q.items.len];
        var i = offset;
        while (i + 1 < q.len) : (i += 1)
            q.items[(q.head + i) % q.items.len] = q.items[(q.head + i + 1) % q.items.len];
        q.len -= 1;
        return removed;
    }

    /// Pty output is lossy under sustained backpressure. EOF is structural, and
    /// so is a nested `Look`: one is a reader that must be reaped, the other is
    /// a launch that already exited believing it was delivered. Admit both by
    /// evicting queued output. Every switch below is exhaustive on purpose — a
    /// new message kind has to say which of the two it is.
    fn push(q: *Inbox, gpa: std.mem.Allocator, m: Msg) void {
        q.lock();
        defer q.mutex.unlock();
        if (q.closed) {
            m.free(gpa);
            return;
        }
        if (q.len == q.items.len) {
            const lossy = switch (m) {
                .output => true,
                .eof, .command => false,
            };
            if (lossy) {
                m.free(gpa);
                return;
            }
            var offset: usize = 0;
            while (offset < q.len) : (offset += 1)
                if (switch (q.items[(q.head + offset) % q.items.len]) {
                    .output => true,
                    .eof, .command => false,
                }) break;
            if (offset == q.len) return;
            q.removeAt(offset).free(gpa);
        }
        q.items[(q.head + q.len) % q.items.len] = m;
        q.len += 1;
    }

    fn take(q: *Inbox) MessageBatch {
        q.lock();
        defer q.mutex.unlock();
        var batch: MessageBatch = .{};
        while (q.len > 0) {
            batch.items[batch.len] = q.items[q.head];
            batch.len += 1;
            q.head = (q.head + 1) % q.items.len;
            q.len -= 1;
        }
        q.head = 0;
        return batch;
    }

    fn close(q: *Inbox, gpa: std.mem.Allocator) void {
        q.lock();
        defer q.mutex.unlock();
        q.closed = true;
        while (q.len > 0) {
            q.items[q.head].free(gpa);
            q.head = (q.head + 1) % q.items.len;
            q.len -= 1;
        }
        q.head = 0;
    }
};

const State = struct {
    gpa: std.mem.Allocator,
    threaded: *std.Io.Threaded,
    io: std.Io,
    core: *pardes.Pardes,
    arena: std.heap.ArenaAllocator,
    runtime: Runtime,
    cells: []Cell = &.{},
    frame_len: usize = 0,
    /// The grid `cells` actually holds. Not read back off the core: a render
    /// can move screen_w/screen_h and then fail, and a host that sized its
    /// loops from those would walk off the buffer.
    frame_cols: u16 = 0,
    frame_rows: u16 = 0,
    /// This frame's pixel attachments, flattened out of Surface.images. Grown
    /// and reused like `cells`, and emptied by the same failure path — the
    /// accessors must never describe a different frame than the cell count.
    images: []Image = &.{},
    images_len: usize = 0,
    ptys: [pardes.MAX_PANES]?Pty = @splat(null),
    inbox: Inbox = .{},
    /// Per-slot spawn generation, owned by the main thread. A reader carries a
    /// copy in every message it posts; anything that no longer matches belongs
    /// to a shell this slot has already replaced.
    gens: [pardes.MAX_PANES]u32 = @splat(0),
    /// Sub-cell wheel distance the core has not been told about yet, one
    /// accumulator per axis. The core moves a whole row or column at a time,
    /// so fractional trackpad travel banks here and is spent as wheel presses
    /// — see pardes_scroll. Separate axes because a diagonal drift must not
    /// let one direction's residue push the other over a notch.
    scroll_lag: f32 = 0,
    scroll_lag_x: f32 = 0,
    /// Degrees of trackpad rotation not yet spent as a search step — the same
    /// accumulate-and-keep-the-remainder shape as scroll_lag, see pardes_rotate.
    rotate_lag: f32 = 0,
    /// The dial's angular velocity, in degrees per second. While fingers are
    /// down this is a running estimate off the event stream; when they lift it
    /// becomes the fling that `coasting` spends. Zero is a dial at rest.
    rotate_velocity: f32 = 0,
    /// When the last rotation event arrived, so the estimate above has a dt.
    rotate_last_ns: i128 = 0,
    /// Fingers are off and the dial is still turning. Separate from a nonzero
    /// velocity because during the gesture that velocity is a MEASUREMENT —
    /// spending it then would double every twist under the hand making it.
    rotate_coasting: bool = false,
    /// Panes whose shell has produced output since we last read its cwd.
    ///
    /// The cwd is wanted for pane tags and for resolving a relative Look, and
    /// asking libproc costs a syscall per pane. Polling it on a clock spends
    /// that forever to notice something that only ever changes when the shell
    /// runs a command — and a shell that ran a command always writes at least
    /// its next prompt. So the read is owed to output, not to time: mark here
    /// on the way past and settle it once at the end of the drain, however
    /// many chunks that burst arrived in.
    cwd_stale: [pardes.MAX_PANES]bool = @splat(false),
    /// The socket a pardes launched inside this app connects to (nested.zig),
    /// or -1 when it could not be bound and nested launches open their own
    /// window as they always did.
    sock_fd: c_int = -1,
    /// Owns the bytes of the user config, which Options only borrows.
    config_arena: std.heap.ArenaAllocator,
};

var state: ?State = null;

// ---------------------------------------------------------------- lifecycle

export fn pardes_init(runtime: ?*const Runtime, cols_arg: u16, rows_arg: u16) c_int {
    if (state != null) return 1; // already up; deinit first
    initCore(runtime, cols_arg, rows_arg) catch |err| {
        log.err("init failed: {t}", .{err});
        return 2;
    };
    return 0;
}

/// The body is split out purely so the cleanup below is real: `errdefer` fires
/// on an error return and nothing else, so writing this inside an export that
/// returns c_int would leave every one of these as dead code — and a half-built
/// init leaks an arena, leaves zstbi pointing at a dead allocator, and (because
/// Io.Threaded installs process-wide SIGIO/SIGPIPE handlers that only its
/// deinit restores) hands those handlers permanently to the host app.
fn initCore(runtime: ?*const Runtime, cols_arg: u16, rows_arg: u16) !void {
    const gpa = std.heap.smp_allocator;

    const allocs = pardes.allocators.init(gpa);
    errdefer pardes.allocators.deinit();

    const threaded = try gpa.create(std.Io.Threaded);
    errdefer gpa.destroy(threaded);
    threaded.* = .init(gpa, .{});
    errdefer threaded.deinit();
    const io = threaded.io();

    var config_arena: std.heap.ArenaAllocator = .init(gpa);
    errdefer config_arena.deinit();

    var opts: pardes.Options = .{
        .tty_only = true,
        .image_allocator = allocs.image,
        .pdf_allocator = allocs.pdf,
        .tree_sitter_allocator = allocs.tree_sitter,
    };
    // Native shells opt into the user config, and every builtin in it must have
    // run before the host can render a frame — so it is read here, before
    // Pardes.init, exactly as src/main.zig does it. The env map is rebuilt from
    // libc's environ because a library has no std.process.Init to inherit one.
    if (captureEnv(config_arena.allocator())) |*env| {
        const found = user_config.load(io, config_arena.allocator(), env);
        opts.startup_config = found.bytes;
        opts.startup_config_path = found.path;
    }

    pardes.image.start(io, allocs.image);
    errdefer pardes.image.stop();
    if (comptime pardes.pdf_enabled) pardes.pdf.start(allocs.pdf);
    errdefer if (comptime pardes.pdf_enabled) pardes.pdf.stop();
    pardes.syntax.start(allocs.tree_sitter);
    errdefer pardes.syntax.stop();

    const core = try pardes.Pardes.init(allocs.pardes, opts);
    errdefer core.deinit();
    // This host draws pixels. Without it the core assumes a terminal that
    // cannot, and a PDF pane degrades to counted page turns with nothing on
    // screen at all — which is exactly what it did. The SDL shell sets the
    // same flag; the tty one sets it from the terminal's kitty-graphics
    // capability, because there it is a question rather than a fact.
    core.native_images = true;

    // Shells emit OSC 133 prompt marks through these, which is what makes
    // prompt hiding and click-to-move work.
    writeFile(shell_bin.bash_rc_path, shell_bin.bash_rc);
    writeFile(shell_bin.fish_rc_path, shell_bin.fish_rc);
    // Apple's bash 3.2 prints the zsh-deprecation banner into every pane unless
    // this is in the environment BEFORE bash starts — the rc file is too late.
    if (comptime builtin.os.tag.isDarwin()) _ = setenv("BASH_SILENCE_DEPRECATION_WARNING", "1", 1);

    state = .{
        .gpa = gpa,
        .threaded = threaded,
        .io = io,
        .core = core,
        .arena = .init(allocs.frame),
        .config_arena = config_arena,
        .runtime = if (runtime) |r| r.* else .{},
    };
    const st = &state.?;

    // The real grid, delivered as an EVENT and not as Options.cols/rows: the
    // core defers each shell's greeting until it has seen a resize, and the
    // first forkpty below takes its winsize straight off the core.
    const cols = @max(1, cols_arg);
    const rows = @max(1, rows_arg);
    core.update(.{ .resize = .{ .cols = cols, .rows = rows } });

    // The initial spawns happen before any reader task exists, mirroring the
    // tty shell. Note the difference in what that buys: tty.zig runs from
    // main() and really is single-threaded there, whereas this is called from
    // applicationDidFinishLaunching, by which point AppKit and libdispatch
    // have long since spawned threads. What keeps the fork safe is the child
    // itself — chdir and execv, raw syscalls with nothing allocated between
    // fork and exec — not the thread count. Ordering it this way anyway keeps
    // the two backends readable side by side.
    _ = drainEffects(st, false);
    for (&st.ptys, 0..) |*slot, id| if (slot.*) |*pt| startReader(st, pt, @intCast(id));

    // Last, because it is the one thing here that publishes this process to
    // the outside: nothing may connect before the core can answer. The shells
    // above are already forked, which is why the listener's fd is CLOEXEC —
    // an orphaned bash holding it would keep the socket bound after we quit.
    st.sock_fd = nested.listen();
    if (st.sock_fd >= 0) {
        const thread = std.Thread.spawn(.{}, lookServer, .{st}) catch |err| {
            // Bound but unattended would be worse than never bound: every
            // nested launch would connect, be believed, and vanish.
            log.warn("nested Look server did not start ({t})", .{err});
            nested.unlisten(st.sock_fd);
            st.sock_fd = -1;
            return;
        };
        thread.detach();
    }
}

/// Accept `Look <path>` lines from pardes instances launched inside this app
/// and post them where the main thread will run them.
///
/// A detached thread around a call that never returns, exactly like the tty
/// backend's: close(2) does not release a thread parked in accept(2), so this
/// dies with the process rather than with the socket. The window that leaves
/// is one connection accepted between the last tick and process exit posting
/// into an inbox nobody drains — the same bound the pty readers have, and a
/// self-pipe to close it would be more machinery than the window is worth.
fn lookServer(st: *State) void {
    var buf: [nested.max_line]u8 = undefined;
    while (nested.acceptLine(st.sock_fd, &buf)) |line| {
        const owned = st.gpa.dupe(u8, line) catch continue;
        st.inbox.push(st.gpa, .{ .command = owned });
        wake(st);
    }
}

export fn pardes_deinit() void {
    const st = &(state orelse return);
    // Before anything else: it is the only fd another process can reach us
    // through, and unlinking the file is what stops the next launch from
    // connecting to a session that is halfway through tearing itself down.
    // The thread parked in accept(2) is not released by this and dies with
    // the process, which is what its detach() already said.
    nested.unlisten(st.sock_fd);
    st.sock_fd = -1;
    // Every reader is joined here, before anything it touches is freed. The
    // runtime joins its tasks on exit, so a reader left parked in read(2) would
    // hang the process instead of the app quitting.
    for (0..pardes.MAX_PANES) |pane| reap(st, @intCast(pane));
    // Only now is the inbox quiet. Anything still queued owns gpa bytes and
    // would show up as a leak rather than as the shutdown it actually is.
    st.inbox.close(st.gpa);
    if (st.cells.len > 0) st.gpa.free(st.cells);
    if (st.images.len > 0) st.gpa.free(st.images);
    st.arena.deinit();
    st.core.deinit();
    pardes.image.stop();
    if (comptime pardes.pdf_enabled) pardes.pdf.stop();
    pardes.syntax.stop();
    st.config_arena.deinit();
    st.threaded.deinit();
    st.gpa.destroy(st.threaded);
    pardes.allocators.deinit();
    state = null;
}

export fn pardes_should_quit() bool {
    const st = &(state orelse return true);
    return st.core.quit;
}

/// Something on screen moves on its own and wants ~60 Hz ticks until it stops:
/// a theme transition fading, or the rotation dial coasting after a flick.
/// Both are spent by pardes_tick, so this is the host's only cue to keep
/// pumping — an idle pardes costs nothing precisely because it says false.
export fn pardes_animating() bool {
    const st = &(state orelse return false);
    return st.core.themeAnimationActive() or st.rotate_coasting;
}

/// The colour the host should paint everything the grid does not: the window
/// background behind the titlebar, and behind every pixel of a live resize the
/// view has not caught up with yet.
///
/// The theme's OWN background, not the chrome's, and so not animated — the
/// same split every other shell draws. Chrome (taglines, the move box, the
/// scrollbar) fades between themes over a handful of frames; document
/// backgrounds switch the instant the theme does, and this is one of those.
///
/// PARDES_COLOR_DEFAULT means the active theme declares NO background of its
/// own (`bg = null`: the curated `dark`, and every vendored `*_transparent`).
/// In a terminal that means "wear whatever the terminal is wearing"; a window
/// has nothing to wear, so the host lets its own backdrop through — see the
/// NSVisualEffectView in AppDelegate.
export fn pardes_theme_bg() u32 {
    // Before pardes_init there is no session, but there IS a theme: the ring's
    // first entry is what the core boots wearing, so answering with it keeps
    // the window from opening one colour and flipping to another a frame later.
    const th = if (state) |*st| st.core.theme() else &pardes.themes[0];
    const bg = th.bg orelse return color_default;
    return @as(u32, bg[0]) << 16 | @as(u32, bg[1]) << 8 | bg[2];
}

/// Re-read the cwd of every shell that just spoke, and only those.
///
/// A pane's tag shows this and a relative `Look` resolves against it, so it has
/// to follow the shell around rather than stay at the directory the pane was
/// spawned in. The tty and SDL hosts poll all of them every frame; here the
/// drain has just said exactly which shells produced bytes, and nothing else
/// can have changed one — a `cd` is a command, and a shell that ran a command
/// writes at least its next prompt. So an idle session costs nothing at all,
/// and a busy one costs one libproc call per pane per burst.
fn refreshCwds(st: *State) void {
    for (&st.cwd_stale, 0..) |*stale, id| {
        if (!stale.*) continue;
        stale.* = false;
        const pt = st.ptys[id] orelse continue;
        var buf: [1024]u8 = undefined;
        if (look.shellCwd(pt.pid, &buf)) |wd| st.core.setCwd(id, wd);
    }
}

/// Drain what the reader tasks collected into the core, then perform whatever
/// the core queued in response. Returns whether this tick did any IO.
///
/// NOT a repaint signal, however tempting: the core changes the grid on its own
/// for a cursor move, a selection, a mode change and a scroll, none of which
/// queue an effect or read a pty, so all four return false here. The macOS host
/// learned that the expensive way — see the comment on pump() in
/// src/macos/Sources/AppDelegate.swift.
export fn pardes_tick() bool {
    const st = &(state orelse return false);
    // Cleared before the drain: a reader that pushes during this tick must be
    // able to schedule the next one.
    st.inbox.wake_pending.store(false, .release);
    var batch = st.inbox.take();
    var changed = batch.len > 0;
    for (batch.slice()) |msg| {
        defer msg.free(st.gpa);
        switch (msg) {
            .output => |o| {
                if (st.gens[o.pane] != o.gen) continue;
                st.cwd_stale[o.pane] = true;
                st.core.update(.{ .output = .{ .pane = o.pane, .bytes = o.bytes } });
            },
            .eof => |e| {
                if (st.gens[e.pane] != e.gen) continue;
                // The shell is gone: join its reader (a completed future that
                // is never awaited leaks its allocation), close the master and
                // free the slot.
                reap(st, e.pane);
                st.core.update(.{ .eof = .{ .pane = e.pane } });
            },
            // Already filtered down to `Look ` by the accept side — this
            // socket may open things and that is all it may do.
            .command => |c| st.core.update(.{ .command = c }),
        }
    }
    refreshCwds(st);
    if (drainEffects(st, true)) changed = true;
    // ...and ADVANCE the transition, which is the whole reason the host keeps
    // ticking. The tty and SDL loops call `core.update(.tick)` on their own
    // clocks; this host has no loop of its own, so the pump IS the clock — and
    // without this the fade never moved: `chromeTheme()` stayed on the OLD
    // theme's chrome forever, so every tagline kept its previous colours until
    // the next launch, and `themeAnimationActive()` never went false, so the
    // 16 ms re-pump in AppDelegate.pump spun for the rest of the session.
    if (st.core.themeAnimationActive()) {
        st.core.update(.tick);
        changed = true;
    }
    // ...and the dial, for the same reason and off the same clock: one frame
    // of coast per tick, decayed, until it is slower than a notch a second.
    if (st.rotate_coasting) {
        spendRotation(st, st.rotate_velocity * rotation_fling_step);
        st.rotate_velocity *= rotation_fling_decay;
        if (@abs(st.rotate_velocity) < rotation_fling_stop) {
            st.rotate_velocity = 0;
            st.rotate_coasting = false;
            // The remainder dies with the gesture: a banked half-notch
            // surviving into the next twist is the hysteresis `rotate 0`
            // exists to clear.
            st.rotate_lag = 0;
        }
        changed = true;
    }
    return changed;
}

// ---------------------------------------------------------------- events in

export fn pardes_key(cp_arg: u32, text_ptr: ?[*]const u8, len: usize, mods: u32) void {
    const st = &(state orelse return);
    if (cp_arg > std.math.maxInt(u21)) return;
    const text: []const u8 = if (text_ptr) |p| p[0..len] else "";
    st.core.update(.{ .key = .{
        .cp = @intCast(cp_arg),
        .text = text,
        .ctrl = mods & 1 != 0,
        .alt = mods & 2 != 0,
        .shift = mods & 4 != 0,
    } });
}

export fn pardes_paste(text_ptr: ?[*]const u8, len: usize) void {
    const st = &(state orelse return);
    const text: []const u8 = if (text_ptr) |p| p[0..len] else "";
    st.core.update(.{ .paste = text });
}

/// Button and kind arrive as their boundary ordinals. An out-of-range value is
/// dropped rather than reaching an unchecked enum cast — same rule the browser
/// ABI keeps, for the same reason: the host is not part of this build.
export fn pardes_mouse(button_arg: c_int, kind_arg: c_int, col: u16, row: u16, mods: u32) void {
    const st = &(state orelse return);
    const button: pardes.Mouse.Button = switch (button_arg) {
        0 => .left,
        1 => .middle,
        2 => .right,
        3 => .wheel_up,
        4 => .wheel_down,
        5 => .wheel_left,
        6 => .wheel_right,
        7 => .none,
        else => return,
    };
    const kind: pardes.Mouse.Kind = switch (kind_arg) {
        0 => .press,
        1 => .release,
        2 => .motion,
        3 => .drag,
        else => return,
    };
    st.core.update(.{ .mouse = .{
        .button = button,
        .kind = kind,
        .col = col,
        .row = row,
        .ctrl = mods & 1 != 0,
    } });
}

export fn pardes_scroll(delta_rows: f32, delta_cols: f32, col: u16, row: u16) void {
    const st = &(state orelse return);
    var down_left = takeScrollTicks(&st.scroll_lag, delta_rows);
    while (down_left != 0) {
        const down = down_left > 0;
        down_left += if (down) -1 else 1;
        st.core.update(.{ .mouse = .{
            .button = if (down) .wheel_down else .wheel_up,
            .kind = .press,
            .col = col,
            .row = row,
        } });
    }
    // Horizontal after vertical, and through the same quantizer: the core's
    // own drift guard (config.wheelTick) is what decides whether a sideways
    // wobble during a vertical flick counts, so the shell must not second-guess
    // it by filtering here.
    var right_left = takeScrollTicks(&st.scroll_lag_x, delta_cols);
    while (right_left != 0) {
        const right = right_left > 0;
        right_left += if (right) -1 else 1;
        st.core.update(.{ .mouse = .{
            .button = if (right) .wheel_right else .wheel_left,
            .kind = .press,
            .col = col,
            .row = row,
        } });
    }
}

/// Spend a trackpad rotation as search steps. AppKit reports degrees since the
/// last event, counterclockwise positive; the core has no rotation, so the
/// dial is quantized into the keys a hand would otherwise press — clockwise is
/// `n` (forward through the matches), counterclockwise `N`.
export fn pardes_rotate(degrees: f32) void {
    const st = &(state orelse return);
    // A gesture beginning re-zeros the dial: leftover travel from the last
    // twist must not make the first degree of this one jump a match — and it
    // catches a fling still coasting, because a finger back down is how a hand
    // catches a dial.
    if (degrees == 0) {
        st.rotate_lag = 0;
        st.rotate_velocity = 0;
        st.rotate_coasting = false;
        st.rotate_last_ns = monotonicNs();
        return;
    }
    noteRotationVelocity(st, degrees);
    spendRotation(st, degrees);
}

/// The fingers lifted. What happens next is decided entirely by how fast they
/// were moving when they did: `rotationFling` subtracts the floor, so a slow
/// twist stops dead where it was put and a flick keeps going in proportion to
/// how hard it was thrown.
export fn pardes_rotate_end() void {
    const st = &(state orelse return);
    const last = st.rotate_last_ns;
    st.rotate_last_ns = 0;
    st.rotate_coasting = false;
    // A hand that turned the dial, STOPPED, and then lifted has released at
    // rest however fast it was moving before — and the last sample is still
    // sitting there saying otherwise. Without this the most deliberate twist
    // of all (turn, look at it, let go) is the one that flings.
    if (last == 0 or monotonicNs() - last > 90 * std.time.ns_per_ms) {
        st.rotate_velocity = 0;
        return;
    }
    st.rotate_velocity = rotationFling(st.rotate_velocity);
    st.rotate_coasting = st.rotate_velocity != 0;
}

/// Monotonic nanoseconds, the clock lsp_zls.zig already times with. Monotonic
/// and not REALTIME on purpose: a dial that flung because NTP stepped the wall
/// clock backwards would be a bug nobody ever reproduces.
///
/// Zero on failure, which is also the "no sample yet" sentinel — so a clock
/// that will not answer makes the dial refuse to fling rather than fling on a
/// garbage dt.
fn monotonicNs() i128 {
    var ts: libc.timespec = undefined;
    if (libc.clock_gettime(.MONOTONIC, &ts) != 0) return 0;
    return @as(i128, ts.sec) * std.time.ns_per_s + ts.nsec;
}

/// One event's contribution to the velocity estimate, in degrees per second.
/// Smoothed, because a single 120 Hz sample of a human wrist is mostly noise
/// and the fling would otherwise be decided by whichever one happened to land
/// last.
fn noteRotationVelocity(st: *State, degrees: f32) void {
    const now = monotonicNs();
    const last = st.rotate_last_ns;
    st.rotate_last_ns = now;
    st.rotate_coasting = false;
    if (last == 0 or now == 0) return;
    const dt_ns = now - last;
    // A gap this long is a gesture nobody announced the start of, not a slow
    // one: dividing by it would report a crawl and eat a real fling.
    if (dt_ns <= 0 or dt_ns > 200 * std.time.ns_per_ms) return;
    const seconds: f32 = @floatCast(@as(f64, @floatFromInt(dt_ns)) / @as(f64, std.time.ns_per_s));
    const sample = degrees / seconds;
    if (!std.math.isFinite(sample)) return;
    st.rotate_velocity = st.rotate_velocity * 0.35 + sample * 0.65;
}

/// Turn degrees into whole search steps, keeping the remainder. The one place
/// the dial reaches the core, so a hand-turned notch and a coasted one are the
/// same keystroke by construction.
fn spendRotation(st: *State, degrees: f32) void {
    var left = takeRotationNotches(&st.rotate_lag, degrees);
    while (left != 0) {
        const back = left > 0; // counterclockwise
        left += if (back) -1 else 1;
        st.core.update(.{ .key = .{ .cp = if (back) 'N' else 'n' } });
    }
}

export fn pardes_command(text_ptr: ?[*]const u8, len: usize) void {
    const st = &(state orelse return);
    const text: []const u8 = if (text_ptr) |p| p[0..len] else "";
    if (text.len == 0) return;
    st.core.update(.{ .command = text });
}

export fn pardes_resize(cols_arg: u16, rows_arg: u16, cell_w: u16, cell_h: u16) void {
    const st = &(state orelse return);
    const cols = @max(1, cols_arg);
    const rows = @max(1, rows_arg);
    st.core.update(.{ .resize = .{
        .cols = cols,
        .rows = rows,
        .cell_pixels = if (@hasField(pardes.CellPixels, "w"))
            .{ .w = @max(1, cell_w), .h = @max(1, cell_h) }
        else
            .{},
    } });
}

// ---------------------------------------------------------------- frame out

export fn pardes_frame() u32 {
    const st = &(state orelse return 0);
    _ = st.arena.reset(.retain_capacity);
    // The accessors below must never describe a different frame than the count
    // this returns, so a failure empties all of them together rather than
    // leaving last frame's buffer behind a fresh cols/rows.
    st.frame_len = 0;
    st.frame_cols = 0;
    st.frame_rows = 0;
    st.images_len = 0;
    const surface = st.core.render(st.arena.allocator()) catch |err| {
        log.err("render failed: {t}", .{err});
        return 0;
    };
    const count: usize = @as(usize, surface.cols) * surface.rows;
    if (count != st.cells.len) {
        if (count == 0) {
            if (st.cells.len > 0) st.gpa.free(st.cells);
            st.cells = &.{};
        } else {
            const resized = if (st.cells.len == 0)
                st.gpa.alloc(Cell, count)
            else
                st.gpa.realloc(st.cells, count);
            st.cells = resized catch return 0;
        }
    }
    st.frame_len = count;
    st.frame_cols = surface.cols;
    st.frame_rows = surface.rows;
    for (surface.cells, st.cells[0..count]) |cell, *out| {
        out.* = .{
            .text = @splat(0),
            .fg = encodeColor(cell.style.fg),
            .bg = encodeColor(cell.style.bg),
            .attrs = encodeAttrs(cell.style),
            .len = if (cell.default) 1 else cell.len,
            .flags = @intFromBool(cell.default),
        };
        if (cell.default) out.text[0] = ' ' else @memcpy(out.text[0..cell.len], cell.grapheme());
    }
    collectImages(st, surface);
    return @intCast(count);
}

/// Flatten Surface.images into the flat C array the host walks.
///
/// A dropped attachment is a page that does not draw, never a wrong one, so
/// every failure here just stops collecting: the frame is still valid, it
/// simply has fewer pictures in it than the core offered.
fn collectImages(st: *State, surface: *const pardes.Surface) void {
    if (comptime !pardes.pdf_enabled) return;
    if (surface.nimages == 0) return;
    if (st.images.len < surface.nimages) {
        const resized = if (st.images.len == 0)
            st.gpa.alloc(Image, surface.nimages)
        else
            st.gpa.realloc(st.images, surface.nimages);
        st.images = resized catch return;
    }
    for (surface.images[0..surface.nimages]) |maybe| {
        const place = maybe orelse continue;
        if (place.iw == 0 or place.ih == 0 or place.rgba.len == 0) continue;
        // Continuous documents hand over geometry the core already clipped to
        // the viewport. Anything else (a static image pane) is the whole
        // raster scaled into the whole body, which is the same two rectangles
        // spelled without a crop.
        const geometry = place.native.geometry orelse image.NativeGeometry{
            .src = .{ .x = 0, .y = 0, .w = @intCast(place.iw), .h = @intCast(place.ih) },
            .dst = .{
                .x = 0,
                .y = 0,
                .w = @as(u32, place.w) * st.core.cell_pixels.w,
                .h = @as(u32, place.h) * st.core.cell_pixels.h,
            },
        };
        if (geometry.dst.w == 0 or geometry.dst.h == 0) continue;
        if (geometry.src.w == 0 or geometry.src.h == 0) continue;
        st.images[st.images_len] = .{
            .serial = place.serial,
            .page = place.native.page,
            .revision = place.native.revision,
            .cell_x = place.x,
            .cell_y = place.y,
            .cell_w = place.w,
            .cell_h = place.h,
            .dst_x = geometry.dst.x,
            .dst_y = geometry.dst.y,
            .dst_w = geometry.dst.w,
            .dst_h = geometry.dst.h,
            .src_x = geometry.src.x,
            .src_y = geometry.src.y,
            .src_w = geometry.src.w,
            .src_h = geometry.src.h,
            .offset_y = place.native.pixel_offset_y,
            .iw = @intCast(place.iw),
            .ih = @intCast(place.ih),
            .rgba = place.rgba.ptr,
        };
        st.images_len += 1;
    }
}

export fn pardes_frame_images() u32 {
    const st = &(state orelse return 0);
    return @intCast(st.images_len);
}

export fn pardes_frame_image_list() ?[*]const Image {
    const st = &(state orelse return null);
    return if (st.images_len == 0) null else st.images.ptr;
}

export fn pardes_frame_cells() ?[*]const Cell {
    const st = &(state orelse return null);
    return if (st.frame_len == 0) null else st.cells.ptr;
}

export fn pardes_frame_cols() u16 {
    const st = &(state orelse return 0);
    return st.frame_cols;
}

export fn pardes_frame_rows() u16 {
    const st = &(state orelse return 0);
    return st.frame_rows;
}

export fn pardes_cursor_x() i32 {
    const st = &(state orelse return -1);
    return if (st.core.surface.cursor) |c| c.x else -1;
}

export fn pardes_cursor_y() i32 {
    const st = &(state orelse return -1);
    return if (st.core.surface.cursor) |c| c.y else -1;
}

export fn pardes_cursor_bar() bool {
    const st = &(state orelse return false);
    return if (st.core.surface.cursor) |c| c.bar else false;
}

/// The acme verb the core last performed, and clears it. Ordinals, not the
/// enum: the host is not part of this build, so the boundary speaks integers
/// and the ABI guard asserts they are the ones the header names.
export fn pardes_take_haptic() c_int {
    const st = &(state orelse return 0);
    return switch (st.core.takeHaptic()) {
        .none => 0,
        .exec => 1,
        .look => 2,
    };
}

/// The file the `Font` builtin asked for, and clears it — the same take-once
/// shape as the haptic above, and the same one the SDL shell uses on this
/// exact variable.
///
/// A copy rather than the borrowed slice: `fonts.want` is a length and no
/// terminator, and C wants a string. One static buffer because there is one
/// core and the header promises the value only until the next call.
var font_path_z: [4096:0]u8 = undefined;

export fn pardes_font_take() ?[*:0]const u8 {
    _ = state orelse return null;
    if (comptime !pardes.font_picker) return null;
    const want = fonts.want orelse return null;
    fonts.want = null;
    if (want.len >= font_path_z.len) return null;
    @memcpy(font_path_z[0..want.len], want);
    font_path_z[want.len] = 0;
    return &font_path_z;
}

/// The FILE behind the focused pane, or null when there is none — a terminal,
/// an output buffer (`+Search` names a directory, not a document), or nothing
/// focused at all. A PDF and an image both count: they are real paths on disk,
/// and the titlebar's proxy icon is about the file, not about who can edit it.
///
/// A copy into a static buffer for the reason pardes_font_take keeps one: the
/// core owns a length and no terminator, C wants a string, and there is one
/// core. Valid until the next call.
var active_path_z: [4096:0]u8 = undefined;

export fn pardes_active_path() ?[*:0]const u8 {
    const st = &(state orelse return null);
    const path = activeFilePath(st) orelse return null;
    if (path.len == 0 or path.len >= active_path_z.len) return null;
    @memcpy(active_path_z[0..path.len], path);
    active_path_z[path.len] = 0;
    return &active_path_z;
}

/// Does the focused pane hold edits that are not on disk? False for everything
/// that cannot be saved in the first place, which is the same set
/// pardes_active_path answers null for minus the PDFs and images — those have
/// a path but no buffer, so they are never dirty.
export fn pardes_active_dirty() bool {
    const st = &(state orelse return false);
    const pane = st.core.panes[st.core.active] orelse return false;
    const f = if (pane.file) |*x| x else return false;
    if (f.output != null) return false;
    return f.revision != f.saved_revision;
}

fn activeFilePath(st: *State) ?[]const u8 {
    const pane = st.core.panes[st.core.active] orelse return null;
    if (pane.file) |*f| return if (f.output == null) f.path else null;
    if (comptime pardes.pdf_enabled) if (pane.pdfPath()) |path| return path;
    if (pane.image) |*iv| return iv.path;
    return null;
}

// ---------------------------------------------------------------- effects

/// Perform the IO the core queued. `threads_ok` is false for the one drain
/// inside pardes_init, which runs before any reader task exists.
///
/// ponytail: the lsp, pipe and watch effects do no work here. Each wants real
/// machinery — a worker plus a snapshot of the pane's file for lsp
/// (src/tty/tty.zig:919), a job copy for pipe, and FSEvents for watch, since
/// inotify is Linux-only. Lift tty.zig's implementations when the app is past
/// first light. Pipe and watch may simply be dropped; lsp may NOT, see below.
fn drainEffects(st: *State, threads_ok: bool) bool {
    const core = st.core;
    var did = false;
    while (core.nextEffect()) |effect| {
        did = true;
        switch (effect) {
            .spawn => |sp| {
                // The core reuses pane ids and has no close effect, so a
                // deleted pane's shell lives in its slot until a respawn lands
                // here. Reap it: cancel joins the reader, and the generation
                // bump makes its late bytes and eof unreadable.
                reap(st, sp.pane);
                st.gens[sp.pane] +%= 1;
                const gen = st.gens[sp.pane];

                const cwd = sp.cwd.slice();
                var cwd_buf: [256:0]u8 = undefined;
                var cwd_z: ?[*:0]const u8 = null;
                // <= because writing the sentinel slot of a [N:0]u8 is legal,
                // and Effect's cwd buffer is exactly 256: `<` would silently
                // drop a maximal path and start the shell wherever the app
                // bundle was launched from instead.
                if (cwd.len > 0 and cwd.len <= cwd_buf.len) {
                    @memcpy(cwd_buf[0..cwd.len], cwd);
                    cwd_buf[cwd.len] = 0;
                    cwd_z = @ptrCast(&cwd_buf);
                }
                const child = forkShell(core.shellBin(), cwd_z, core.screen_h, core.screen_w);
                st.ptys[sp.pane] = .{
                    .file = child.file,
                    .pid = child.pid,
                    .gen = gen,
                    .reader = .{ .any_future = null, .result = {} },
                };
                // Report the pane's starting directory back to the core (tags).
                var lbuf: [1024]u8 = undefined;
                if (look.shellCwd(child.pid, &lbuf)) |wd| core.setCwd(sp.pane, wd);
                if (threads_ok) if (st.ptys[sp.pane]) |*pt| startReader(st, pt, sp.pane);
            },
            .write => |w| {
                if (st.ptys[w.pane]) |pt| writeFd(pt.file.handle, w.bytes.slice());
            },
            .resize_pty => |rs| {
                if (st.ptys[rs.pane]) |pt| {
                    const ws: posix.winsize = .{ .row = rs.rows, .col = rs.cols, .xpixel = 0, .ypixel = 0 };
                    _ = posix.system.ioctl(pt.file.handle, TIOCSWINSZ, @intFromPtr(&ws));
                }
            },
            .open_link => |url| look.openLink(url.slice()),
            .save_file => |sf| {
                const pane = core.panes[sf.pane] orelse continue;
                const f = pane.file orelse continue;
                var pathbuf: [4096:0]u8 = undefined;
                if (f.path.len >= pathbuf.len) continue;
                @memcpy(pathbuf[0..f.path.len], f.path);
                pathbuf[f.path.len] = 0;
                const fd = libc.open(pathbuf[0..f.path.len :0], .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, @as(libc.mode_t, 0o644));
                if (fd < 0) continue;
                writeFd(fd, f.content);
                _ = libc.close(fd);
                // After the write, not beside it: every `continue` above is a
                // save that did not happen and must not be reported as one.
                var mbuf: [256]u8 = undefined;
                core.setMessage(sf.pane, message.stamp(&mbuf, "saved", f.path));
            },
            .new_file => |request| {
                var path_buf: [4096:0]u8 = undefined;
                const made = temp_file.create(&path_buf) orelse continue;
                if (core.openNewFile(request.pane, request.serial, made.path))
                    made.adopt()
                else
                    made.discard();
            },
            .write_dump => {
                const out = core.dump_out orelse continue;
                var pbuf: [1024:0]u8 = undefined;
                const path = pardes.dump.outPath(&pbuf) orelse continue;
                const fd = libc.open(path, .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, @as(libc.mode_t, 0o644));
                if (fd < 0) continue;
                writeFd(fd, out);
                _ = libc.close(fd);
                core.setLastDump(path);
            },
            .set_clipboard => {
                const cb = st.runtime.set_clipboard orelse continue;
                const y = core.yank orelse continue;
                cb(st.runtime.userdata, y.ptr, y.len);
            },
            // An empty answer, immediately: the honest reply from a shell with
            // no worker, and the only safe one. Tab after a `.` DIVERTS to the
            // backend instead of indenting and indents late, when the answer
            // comes back empty (lspResponse); drop the effect and lsp_wait
            // stays armed, the retroactive indent never fires, and every
            // dot-Tab for the rest of the session does nothing at all.
            .lsp => |q| core.update(.{ .lsp_resp = .{ .id = q.id, .rows = "" } }),
            .pipe, .watch => {},
            .quit => {},
        }
    }
    return did;
}

// ---------------------------------------------------------------- workers

fn startReader(st: *State, pt: *Pty, id: u8) void {
    pt.reader = st.io.concurrent(readPty, .{ st, st.io, pt.file, id, pt.gen }) catch |err| {
        // No reader means the shell fills its pty buffer, blocks in write(2)
        // and the pane silently freezes. Nothing recovers it, so at least say
        // so — this is what PARDES_LOG exists for.
        log.err("pane {d} has no reader ({t}); it will not show output", .{ id, err });
        return;
    };
}

/// Release one pane's shell: join the reader, close the master, reap the child.
/// Order matters — cancel is what unblocks a task parked in read(2), and the fd
/// must not be closed under a live reader. Called on eof and again on a spawn
/// into the same slot, so it has to tolerate an empty slot.
fn reap(st: *State, pane: u8) void {
    var pt = st.ptys[pane] orelse return;
    st.ptys[pane] = null;
    pt.reader.cancel(st.io) catch {};
    _ = libc.close(pt.file.handle);
    // A library inside an app that runs for hours cannot leave these: the tty
    // shell gets away with never reaping because the process exits seconds
    // later, but here it would be one zombie per shell ever opened. NOHANG
    // because the child may still be dying and the UI thread must not wait for
    // it; the next reap or process exit collects whatever is left.
    _ = libc.waitpid(pt.pid, null, posix.W.NOHANG);
}

/// Drain one pty into its inbox and wake the host. The same shape as the tty
/// shell's reader, with the vaxis event queue replaced by a mutex and one
/// callback: do the blocking thing away from the loop, hand the bytes over,
/// leave the core a state machine that never waits.
fn readPty(st: *State, io: std.Io, pty: std.Io.File, id: u8, gen: u32) anyerror!void {
    var read_buf: [0x10000]u8 = undefined;
    var reader = pty.readerStreaming(io, &read_buf);
    while (true) {
        var buf: [0x10000]u8 = undefined;
        var vec = [_][]u8{&buf};
        const n = reader.interface.readVec(&vec) catch break;
        if (n == 0) break;
        // Duped outside the lock on purpose — see Inbox.
        const bytes = st.gpa.dupe(u8, buf[0..n]) catch break;
        st.inbox.push(st.gpa, .{ .output = .{ .pane = id, .gen = gen, .bytes = bytes } });
        wake(st);
    }
    st.inbox.push(st.gpa, .{ .eof = .{ .pane = id, .gen = gen } });
    wake(st);
}

/// Ask the host for a tick, at most once per tick. `pardes_tick` clears the
/// flag before it drains, so a push that lands mid-drain still wakes and no
/// message can be left sitting in the inbox with nobody scheduled to read it.
fn wake(st: *State) void {
    const cb = st.runtime.wakeup orelse return;
    if (st.inbox.wake_pending.swap(true, .acq_rel)) return;
    cb(st.runtime.userdata);
}

// ---------------------------------------------------------------- helpers

fn forkShell(bin: []const u8, cwd: ?[*:0]const u8, rows: u16, cols: u16) struct { file: std.Io.File, pid: posix.pid_t } {
    var master: c_int = undefined;
    // Resolved BEFORE the fork, into this frame, which the child inherits:
    // nothing between fork and exec may allocate, so a PATH search cannot
    // happen there.
    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
    const spawn = shell_bin.resolve(bin, &path_buf);
    const ws = posix.winsize{ .row = rows, .col = cols, .xpixel = 0, .ypixel = 0 };
    const pid = forkpty(&master, null, null, &ws);
    if (pid == 0) {
        if (cwd) |c| _ = chdir(c);
        _ = execv(spawn.path, &spawn.argv);
        _exit(127);
    }
    return .{ .file = .{ .handle = master, .flags = .{ .nonblocking = false } }, .pid = pid };
}

fn writeFd(fd: c_int, data: []const u8) void {
    var off: usize = 0;
    while (off < data.len) {
        const n = libc.write(fd, data[off..].ptr, data.len - off);
        if (n < 0) {
            if (libc.errno(n) == .INTR) continue;
            return;
        }
        // A zero-byte write makes no progress; looping on it would spin the
        // main thread forever, which here means a beachball rather than the
        // tty shell's hung terminal.
        if (n == 0) return;
        off += @intCast(n);
    }
}

fn writeFile(path: [*:0]const u8, contents: []const u8) void {
    const fd = libc.open(path, .{ .ACCMODE = .WRONLY, .CREAT = true, .TRUNC = true }, @as(libc.mode_t, 0o644));
    if (fd < 0) return;
    defer _ = libc.close(fd);
    writeFd(fd, contents);
}

/// Rebuild the process environment as a Map, because a library never sees the
/// std.process.Init that main() gets one from. Only the config-path lookup
/// reads it, and the arena owns the copies for the life of the process.
fn captureEnv(arena: std.mem.Allocator) ?std.process.Environ.Map {
    var map: std.process.Environ.Map = .init(arena);
    const environ = std.c.environ;
    var i: usize = 0;
    while (environ[i]) |entry| : (i += 1) {
        const line = std.mem.span(entry);
        const eq = std.mem.indexOfScalar(u8, line, '=') orelse continue;
        map.put(line[0..eq], line[eq + 1 ..]) catch return null;
    }
    return map;
}

fn encodeColor(color: pardes.Color) u32 {
    return switch (color) {
        .default => color_default,
        .index => |index| color_indexed | @as(u32, index),
        .rgb => |rgb| (@as(u32, rgb[0]) << 16) | (@as(u32, rgb[1]) << 8) | rgb[2],
    };
}

fn encodeAttrs(style: pardes.CellStyle) u16 {
    var attrs: u16 = 0;
    attrs |= @as(u16, @intFromBool(style.bold)) << 0;
    attrs |= @as(u16, @intFromBool(style.dim)) << 1;
    attrs |= @as(u16, @intFromBool(style.italic)) << 2;
    attrs |= @as(u16, @intFromBool(style.blink)) << 3;
    attrs |= @as(u16, @intFromBool(style.reverse)) << 4;
    attrs |= @as(u16, @intFromBool(style.invisible)) << 5;
    attrs |= @as(u16, @intFromBool(style.strikethrough)) << 6;
    attrs |= @as(u16, @intFromEnum(style.ul)) << 8;
    return attrs;
}

/// Spend accumulated sub-row travel as whole wheel notches, keeping the
/// remainder. The core has no fractional scroll — both other shells do this
/// too — and the clamp is so that an absurd delta (a momentum-phase kinetic
/// fling reported in points, a NaN) cannot spin the emit loop.
fn takeScrollTicks(lag: *f32, delta_rows: f32) i32 {
    if (!std.math.isFinite(delta_rows)) return 0;
    const next = std.math.clamp(lag.* + delta_rows, -256, 256);
    if (!std.math.isFinite(next)) return 0;
    const whole: i32 = @intFromFloat(@trunc(next));
    lag.* = next - @as(f32, @floatFromInt(whole));
    return whole;
}

/// One search step per this many degrees of twist. Every notch is a jump to
/// another match, so it stays coarse enough that a thumb resettling cannot
/// walk the cursor across the file — but 20 degrees was more than a wrist
/// gives without thinking about it, and the dial felt stuck. Ten is still a
/// deliberate twist, and 36 steps to a full turn.
const rotation_notch_degrees: f32 = 10;

/// Where momentum STARTS, in degrees per second — and it starts at zero.
///
/// The fling is the release speed MINUS this, so a slow twist coasts not a
/// little but not at all, and the faster the flick the more there is. A plain
/// threshold would hand out two free notches the instant it was crossed, which
/// is the one thing a dial must not do: the same gesture, a hair quicker,
/// jumping twice as far is how a control stops feeling like a control.
const rotation_fling_floor: f32 = 70;
/// ...and the ceiling on what is left after that subtraction. AppKit reports a
/// thousand degrees a second for one frame of a twitch, and this cap is what
/// decides how far the hardest possible flick throws the list: 400 deg/s is
/// about 111 degrees of coast, so eleven matches. Twenty read as the list
/// getting away from you.
const rotation_fling_max: f32 = 400;
/// One pump of coasting. Fixed rather than measured: the host re-pumps at
/// ~60 Hz for exactly as long as pardes_animating says to, and a fixed step
/// makes one fling spend the same travel every time — which is what lets a
/// golden assert it instead of asserting the machine's timer jitter.
const rotation_fling_step: f32 = 1.0 / 60.0;
/// Per-step decay. 0.94 at 60 Hz is a little over half a second of coast, the
/// same order as the trackpad's own inertial scrolling.
const rotation_fling_decay: f32 = 0.94;
/// Below this the dial is at rest: one notch a second is not momentum, it is a
/// list still stepping long after the hand has moved on.
const rotation_fling_stop: f32 = 18;

/// The velocity a release at `speed` degrees/second actually coasts at, after
/// the floor is subtracted and the remainder capped. Zero means the twist was
/// a placement, not a throw — which is most of them.
///
/// Total travel follows from it and the decay as a geometric series:
/// `v * step / (1 - decay)`, i.e. about 0.28 degrees per degree/second. A
/// 200 deg/s release therefore coasts ~36 degrees, three or four notches.
fn rotationFling(speed: f32) f32 {
    const excess = @min(@abs(speed) - rotation_fling_floor, rotation_fling_max);
    if (excess < rotation_fling_stop) return 0;
    return std.math.copysign(excess, speed);
}

/// Spend accumulated rotation as whole search steps, keeping the remainder.
/// Same contract as takeScrollTicks, including the clamp: an absurd delta
/// spends a bounded number of notches instead of spinning the emit loop.
fn takeRotationNotches(lag: *f32, degrees: f32) i32 {
    if (!std.math.isFinite(degrees)) return 0;
    const limit = rotation_notch_degrees * 64;
    const next = std.math.clamp(lag.* + degrees, -limit, limit);
    if (!std.math.isFinite(next)) return 0;
    const whole: i32 = @intFromFloat(@trunc(next / rotation_notch_degrees));
    lag.* = next - @as(f32, @floatFromInt(whole)) * rotation_notch_degrees;
    return whole;
}

// ---------------------------------------------------------------- ABI guard

// The header is hand-written, so nothing but a test keeps it honest. build.zig
// translate-C's src/macos/pardes.h into this test build and every constant and
// layout below is asserted against the Zig side — ghostty's trick, and the
// cheapest possible insurance against a silent ABI skew.
/// Compare one declaration's arity and scalar widths against the header's.
/// Not a type equality — translate-C spells pointers `[*c]` and mints its own
/// struct types, so nothing here would ever match exactly. Arity and width are
/// what actually break: a parameter added on one side only (which is how the
/// Swift host first got pardes_scroll wrong), or a u16 that became a u32.
fn expectSameAbi(comptime C: type, comptime Z: type) !void {
    const c_fn = @typeInfo(C).@"fn";
    const z_fn = @typeInfo(Z).@"fn";
    try std.testing.expectEqual(c_fn.params.len, z_fn.params.len);
    inline for (c_fn.params, z_fn.params) |cp, zp|
        try std.testing.expectEqual(@sizeOf(cp.type.?), @sizeOf(zp.type.?));
    try std.testing.expectEqual(@sizeOf(c_fn.return_type.?), @sizeOf(z_fn.return_type.?));
}

test "pardes.h declares every export the way it is defined" {
    const c = @import("pardes.h");
    try expectSameAbi(@TypeOf(c.pardes_init), @TypeOf(pardes_init));
    try expectSameAbi(@TypeOf(c.pardes_deinit), @TypeOf(pardes_deinit));
    try expectSameAbi(@TypeOf(c.pardes_tick), @TypeOf(pardes_tick));
    try expectSameAbi(@TypeOf(c.pardes_should_quit), @TypeOf(pardes_should_quit));
    try expectSameAbi(@TypeOf(c.pardes_animating), @TypeOf(pardes_animating));
    try expectSameAbi(@TypeOf(c.pardes_key), @TypeOf(pardes_key));
    try expectSameAbi(@TypeOf(c.pardes_paste), @TypeOf(pardes_paste));
    try expectSameAbi(@TypeOf(c.pardes_mouse), @TypeOf(pardes_mouse));
    try expectSameAbi(@TypeOf(c.pardes_scroll), @TypeOf(pardes_scroll));
    try expectSameAbi(@TypeOf(c.pardes_rotate), @TypeOf(pardes_rotate));
    try expectSameAbi(@TypeOf(c.pardes_rotate_end), @TypeOf(pardes_rotate_end));
    try expectSameAbi(@TypeOf(c.pardes_command), @TypeOf(pardes_command));
    try expectSameAbi(@TypeOf(c.pardes_resize), @TypeOf(pardes_resize));
    try expectSameAbi(@TypeOf(c.pardes_frame), @TypeOf(pardes_frame));
    try expectSameAbi(@TypeOf(c.pardes_frame_cells), @TypeOf(pardes_frame_cells));
    try expectSameAbi(@TypeOf(c.pardes_frame_cols), @TypeOf(pardes_frame_cols));
    try expectSameAbi(@TypeOf(c.pardes_frame_rows), @TypeOf(pardes_frame_rows));
    try expectSameAbi(@TypeOf(c.pardes_frame_images), @TypeOf(pardes_frame_images));
    try expectSameAbi(@TypeOf(c.pardes_frame_image_list), @TypeOf(pardes_frame_image_list));
    try expectSameAbi(@TypeOf(c.pardes_cursor_x), @TypeOf(pardes_cursor_x));
    try expectSameAbi(@TypeOf(c.pardes_cursor_y), @TypeOf(pardes_cursor_y));
    try expectSameAbi(@TypeOf(c.pardes_cursor_bar), @TypeOf(pardes_cursor_bar));
    try expectSameAbi(@TypeOf(c.pardes_take_haptic), @TypeOf(pardes_take_haptic));
    try expectSameAbi(@TypeOf(c.pardes_font_take), @TypeOf(pardes_font_take));
    try expectSameAbi(@TypeOf(c.pardes_active_path), @TypeOf(pardes_active_path));
    try expectSameAbi(@TypeOf(c.pardes_active_dirty), @TypeOf(pardes_active_dirty));
    try expectSameAbi(@TypeOf(c.pardes_theme_bg), @TypeOf(pardes_theme_bg));
}

test "pardes.h matches the Zig boundary" {
    const c = @import("pardes.h");
    const expectEqual = std.testing.expectEqual;

    try expectEqual(@sizeOf(c.pardes_cell_s), @sizeOf(Cell));
    try expectEqual(@offsetOf(c.pardes_cell_s, "text"), @offsetOf(Cell, "text"));
    try expectEqual(@offsetOf(c.pardes_cell_s, "fg"), @offsetOf(Cell, "fg"));
    try expectEqual(@offsetOf(c.pardes_cell_s, "bg"), @offsetOf(Cell, "bg"));
    try expectEqual(@offsetOf(c.pardes_cell_s, "attrs"), @offsetOf(Cell, "attrs"));
    try expectEqual(@offsetOf(c.pardes_cell_s, "len"), @offsetOf(Cell, "len"));
    try expectEqual(@offsetOf(c.pardes_cell_s, "flags"), @offsetOf(Cell, "flags"));
    // The attachment struct is a wide one and every field is read by hand on
    // the Swift side, so its layout is checked at both ends rather than at the
    // two that happen to be easy.
    try expectEqual(@sizeOf(c.pardes_image_s), @sizeOf(Image));
    inline for (@typeInfo(Image).@"struct".fields) |field|
        try expectEqual(@offsetOf(c.pardes_image_s, field.name), @offsetOf(Image, field.name));
    try expectEqual(@sizeOf(c.pardes_runtime_s), @sizeOf(Runtime));

    try expectEqual(@as(u32, c.PARDES_COLOR_DEFAULT), color_default);
    try expectEqual(@as(u32, c.PARDES_COLOR_INDEXED), color_indexed);
    try expectEqual(@as(u8, c.PARDES_CELL_DEFAULT), cell_flag_default);

    // Every key the host has a name for must be the codepoint the core reads.
    try expectEqual(@as(u21, c.PARDES_KEY_ENTER), pardes.Key.enter);
    try expectEqual(@as(u21, c.PARDES_KEY_ESCAPE), pardes.Key.escape);
    try expectEqual(@as(u21, c.PARDES_KEY_TAB), pardes.Key.tab);
    try expectEqual(@as(u21, c.PARDES_KEY_BACKSPACE), pardes.Key.backspace);
    try expectEqual(@as(u21, c.PARDES_KEY_UP), pardes.Key.up);
    try expectEqual(@as(u21, c.PARDES_KEY_DOWN), pardes.Key.down);
    try expectEqual(@as(u21, c.PARDES_KEY_LEFT), pardes.Key.left);
    try expectEqual(@as(u21, c.PARDES_KEY_RIGHT), pardes.Key.right);
    try expectEqual(@as(u21, c.PARDES_KEY_HOME), pardes.Key.home);
    try expectEqual(@as(u21, c.PARDES_KEY_END), pardes.Key.end);
    try expectEqual(@as(u21, c.PARDES_KEY_PAGE_UP), pardes.Key.page_up);
    try expectEqual(@as(u21, c.PARDES_KEY_PAGE_DOWN), pardes.Key.page_down);
    try expectEqual(@as(u21, c.PARDES_KEY_DELETE), pardes.Key.delete);

    // The mouse ordinals the switch in pardes_mouse decodes are the enum's own
    // declaration order; a reorder there is a silent remap of acme's buttons.
    try expectEqual(c.PARDES_MOUSE_LEFT, @intFromEnum(pardes.Mouse.Button.left));
    try expectEqual(c.PARDES_MOUSE_MIDDLE, @intFromEnum(pardes.Mouse.Button.middle));
    try expectEqual(c.PARDES_MOUSE_RIGHT, @intFromEnum(pardes.Mouse.Button.right));
    try expectEqual(c.PARDES_MOUSE_WHEEL_UP, @intFromEnum(pardes.Mouse.Button.wheel_up));
    try expectEqual(c.PARDES_MOUSE_WHEEL_DOWN, @intFromEnum(pardes.Mouse.Button.wheel_down));
    try expectEqual(c.PARDES_MOUSE_WHEEL_LEFT, @intFromEnum(pardes.Mouse.Button.wheel_left));
    try expectEqual(c.PARDES_MOUSE_WHEEL_RIGHT, @intFromEnum(pardes.Mouse.Button.wheel_right));
    try expectEqual(c.PARDES_MOUSE_NONE, @intFromEnum(pardes.Mouse.Button.none));
    try expectEqual(c.PARDES_MOUSE_PRESS, @intFromEnum(pardes.Mouse.Kind.press));
    try expectEqual(c.PARDES_MOUSE_RELEASE, @intFromEnum(pardes.Mouse.Kind.release));
    try expectEqual(c.PARDES_MOUSE_MOTION, @intFromEnum(pardes.Mouse.Kind.motion));
    try expectEqual(c.PARDES_MOUSE_DRAG, @intFromEnum(pardes.Mouse.Kind.drag));

    // The haptic ordinals pardes_take_haptic returns, against the header's
    // names and the core's enum. Three places, checked as one.
    try expectEqual(c.PARDES_HAPTIC_NONE, @intFromEnum(pardes.Haptic.none));
    try expectEqual(c.PARDES_HAPTIC_EXEC, @intFromEnum(pardes.Haptic.exec));
    try expectEqual(c.PARDES_HAPTIC_LOOK, @intFromEnum(pardes.Haptic.look));

    // The attribute bits the host decodes, against the encoder that writes them.
    try expectEqual(@as(u16, c.PARDES_ATTR_BOLD), encodeAttrs(.{ .bold = true }));
    try expectEqual(@as(u16, c.PARDES_ATTR_DIM), encodeAttrs(.{ .dim = true }));
    try expectEqual(@as(u16, c.PARDES_ATTR_ITALIC), encodeAttrs(.{ .italic = true }));
    try expectEqual(@as(u16, c.PARDES_ATTR_BLINK), encodeAttrs(.{ .blink = true }));
    try expectEqual(@as(u16, c.PARDES_ATTR_REVERSE), encodeAttrs(.{ .reverse = true }));
    try expectEqual(@as(u16, c.PARDES_ATTR_INVISIBLE), encodeAttrs(.{ .invisible = true }));
    try expectEqual(@as(u16, c.PARDES_ATTR_STRIKETHROUGH), encodeAttrs(.{ .strikethrough = true }));
    try expectEqual(
        @as(u16, c.PARDES_UL_CURLY) << c.PARDES_ATTR_UL_SHIFT,
        encodeAttrs(.{ .ul = .curly }),
    );
}

test "colors encode to the three tags the host decodes" {
    const expectEqual = std.testing.expectEqual;
    try expectEqual(@as(u32, 0x01000000), encodeColor(.default));
    try expectEqual(@as(u32, 0x02000021), encodeColor(.{ .index = 33 }));
    try expectEqual(@as(u32, 0x00112233), encodeColor(.{ .rgb = .{ 0x11, 0x22, 0x33 } }));
}

test "sub-row scroll spends whole notches and keeps the remainder" {
    const expectEqual = std.testing.expectEqual;
    var lag: f32 = 0;
    // Four quarter-row flicks are one row, and not before the fourth.
    try expectEqual(@as(i32, 0), takeScrollTicks(&lag, 0.25));
    try expectEqual(@as(i32, 0), takeScrollTicks(&lag, 0.25));
    try expectEqual(@as(i32, 0), takeScrollTicks(&lag, 0.25));
    try expectEqual(@as(i32, 1), takeScrollTicks(&lag, 0.25));
    try expectEqual(@as(f32, 0), lag);

    // Direction reverses without the accumulated travel leaking across it.
    try expectEqual(@as(i32, -2), takeScrollTicks(&lag, -2.5));
    try expectEqual(@as(i32, 0), takeScrollTicks(&lag, 0.25));

    // Garbage moves nothing and leaves the accumulator usable; a fling far
    // past the clamp spends at most one screen and does not spin the caller.
    lag = 0;
    try expectEqual(@as(i32, 0), takeScrollTicks(&lag, std.math.nan(f32)));
    try expectEqual(@as(i32, 0), takeScrollTicks(&lag, std.math.inf(f32)));
    try expectEqual(@as(f32, 0), lag);
    try expectEqual(@as(i32, 256), takeScrollTicks(&lag, 1e9));
}

test "trackpad rotation spends whole search steps and keeps the remainder" {
    const expectEqual = std.testing.expectEqual;
    var lag: f32 = 0;
    // A twist under one notch moves nothing; crossing it moves exactly one,
    // and the overshoot is credited to the next.
    try expectEqual(@as(i32, 0), takeRotationNotches(&lag, 7));
    try expectEqual(@as(i32, 1), takeRotationNotches(&lag, 5));
    try expectEqual(@as(f32, 2), lag);

    // Reversing spends the residue first, so a twist back is not amplified by
    // travel the other direction already banked.
    try expectEqual(@as(i32, -1), takeRotationNotches(&lag, -12));
    try expectEqual(@as(f32, 0), lag);

    // One deliberate half-turn is several matches, not a hundred.
    lag = 0;
    try expectEqual(@as(i32, 18), takeRotationNotches(&lag, 180));

    // Garbage moves nothing and leaves the dial usable; an absurd delta is
    // clamped rather than spinning the emit loop.
    lag = 0;
    try expectEqual(@as(i32, 0), takeRotationNotches(&lag, std.math.nan(f32)));
    try expectEqual(@as(i32, 0), takeRotationNotches(&lag, -std.math.inf(f32)));
    try expectEqual(@as(f32, 0), lag);
    try expectEqual(@as(i32, 64), takeRotationNotches(&lag, 1e9));
}

test "the dial flings in proportion to the release, and not at all when placed" {
    // The whole point of the curve: momentum ramps UP FROM ZERO at the floor
    // rather than switching on at it, so no release speed exists where the
    // same gesture a hair quicker suddenly jumps several matches further.
    try std.testing.expectEqual(@as(f32, 0), rotationFling(0));
    try std.testing.expectEqual(@as(f32, 0), rotationFling(40));
    try std.testing.expectEqual(@as(f32, 0), rotationFling(rotation_fling_floor));
    // Just over the floor is still nothing: what is left has to beat the
    // at-rest threshold before it is worth waking the pump for.
    try std.testing.expectEqual(@as(f32, 0), rotationFling(rotation_fling_floor + 5));

    // ...and past that it is linear in the release speed, both ways.
    try std.testing.expectEqual(@as(f32, 130), rotationFling(200));
    try std.testing.expectEqual(@as(f32, -130), rotationFling(-200));

    // A twitch is capped rather than emptying the list.
    try std.testing.expectEqual(rotation_fling_max, rotationFling(100_000));
    try std.testing.expectEqual(-rotation_fling_max, rotationFling(-100_000));

    // What that buys, in the units a hand feels: total coast is the geometric
    // series v*step/(1-decay), so a brisk 200 deg/s release is a few matches
    // and the hardest flick the cap allows is bounded well short of a hundred.
    const travel = struct {
        fn of(speed: f32) f32 {
            return @abs(rotationFling(speed)) * rotation_fling_step / (1 - rotation_fling_decay);
        }
    }.of;
    try std.testing.expect(travel(200) / rotation_notch_degrees < 5);
    try std.testing.expect(travel(200) / rotation_notch_degrees >= 3);
    // ...and the hardest flick a trackpad can report is bounded at about a
    // dozen matches. This is the number to change if the dial ever feels like
    // it is getting away from the hand.
    try std.testing.expect(travel(100_000) / rotation_notch_degrees < 12);
    try std.testing.expect(travel(100_000) / rotation_notch_degrees > 8);
}