//! What a click on text MEANS. The acme "look" (right click / Enter): expand
//! the click to a file-ish word, then resolve it against the pane's directory.
//! How a word is SPELLED — the isfilec set, the `:LINE:COL` suffix, `@pN`, the
//! URL schemes, the image extensions — is config.zig; this file is only what
//! the spelling RESOLVES to.
//!
//! This is the one deliberately platform-divergent file — the divergence is a
//! comptime switch on pardes.platform, used the way the stdlib switches on
//! os.tag, so every platform's behavior sits in the same screenful:
//!   tty/gui  — the word resolves through the real filesystem (realpath,
//!              open(O_DIRECTORY)); dirs open shells, files open file panes.
//!   web      — tracked Pardes .zig sources form a build-generated read-only
//!              filesystem; URLs still open in a new tab.
const std = @import("std");
const builtin = @import("builtin");
const libc = std.c;
const pardes = @import("pardes.zig");
const config = @import("config.zig");
const pdf_enabled = @import("pardes_config").mupdf;
const embedded_sources = if (pardes.platform == .web) @import("embedded_sources") else struct {
    pub const Source = struct { path: []const u8, contents: []const u8 };
    pub const all = [_]Source{};
};

extern "c" fn realpath(path: [*:0]const u8, resolved: [*]u8) ?[*:0]u8;
extern "c" fn fork() c_int;
extern "c" fn execv(path: [*:0]const u8, argv: [*:null]const ?[*:0]const u8) c_int;
extern "c" fn _exit(status: c_int) noreturn;

// absolute opener path per OS: execv must not search PATH (no allocation
// between fork and exec), same rule as the shell spawn.
// ponytail: hardcoded path; a distro that puts xdg-open elsewhere (nix) needs
// a PATH search in the child, which is not fork-safe here.
const opener_path: ?[*:0]const u8 = switch (builtin.os.tag) {
    .linux => "/usr/bin/xdg-open",
    .macos => "/usr/bin/open",
    else => null,
};

/// Hand a URL to the desktop — the native half of the web backend's
/// window.open. Double fork: the opener is reparented to init, so the one
/// child we DO wait for exits immediately and nothing is left to reap.
pub fn openLink(url: []const u8) void {
    const opener = opener_path orelse return;
    var buf: [1024]u8 = undefined;
    const url_z = std.fmt.bufPrintSentinel(&buf, "{s}", .{url}, 0) catch return;
    const pid = fork();
    if (pid < 0) return;
    if (pid == 0) {
        if (fork() == 0) {
            const argv: [3:null]?[*:0]const u8 = .{ opener, url_z.ptr, null };
            _ = execv(opener, &argv);
        }
        _exit(0);
    }
    _ = libc.waitpid(pid, null, 0);
}

/// WHERE in a pane a look word points. A spot (`:LINE:COL`) — or a SPAN, when
/// the word carries a range (config.range_sep), which a look SELECTS instead
/// of merely parking on. Everything is 1-based and 0 means absent, so a bare
/// path is the all-zero Spot and `end_line == 0` is the question "is this a
/// range".
pub const Spot = struct {
    line: usize = 0,
    col: usize = 0,
    end_line: usize = 0,
    /// 0 with a live `end_line` is the whole-lines form: through the END of
    /// end_line, newline included, which is what helix's `x` selects.
    end_col: usize = 0,
};

/// digits at `i` and where they end; `end == i` means there were none. Four
/// numbers now come out of the same token, and spelling the scan four times
/// is how one of them ends up subtly different from the others.
fn num(tok: []const u8, i: usize) struct { v: usize, end: usize } {
    var v: usize = 0;
    var j = i;
    while (j < tok.len and std.ascii.isDigit(tok[j])) : (j += 1) v = v * 10 + (tok[j] - '0');
    return .{ .v = v, .end = j };
}

/// peel a trailing :LINE[:COL] spot, or one of the three range spellings, off
/// a look word (config.line_col_sep / config.range_sep own both characters):
///   main.zig:100         -> line 100
///   main.zig:100:7       -> line 100, col 7
///   main.zig:100:        -> line 100               grep -n's trailing delimiter
///   main.zig:100-104     -> lines 100..104 whole
///   main.zig:100:7-21    -> line 100, cols 7..21
///   main.zig:100:7-104:3 -> line 100 col 7 .. line 104 col 3
///
/// A tail that does not parse leaves the token a plain PATH, which is the rule
/// that keeps the dash safe: `a-b`, `build-2:3` and `x:1-y` are all paths (the
/// last one goes back to hunting for a later ':' and finds none), because a
/// range needs a number on both sides of its dash.
///
/// `end` is how far into `tok` the form actually REACHED. The read is lenient
/// by design — `main.zig:100:7x` is the file at line 100 and the mangled `:7x`
/// is simply dropped — so `end == tok.len` is the separate question "is the
/// whole token this target and nothing else", which is what a row-grained step
/// must ask before it selects a run of a line (lookableLineSpan).
pub fn parsePathLine(tok: []const u8) struct { path: []const u8, at: Spot, end: usize } {
    var sep: usize = 0;
    while (sep < tok.len) : (sep += 1) {
        if (tok[sep] != config.line_col_sep) continue;
        const l = num(tok, sep + 1);
        if (l.end == sep + 1) continue; // no digits after ':'
        const path = tok[0..sep];
        var i = l.end;
        // `:LINE-ENDLINE`: whole lines, no column anywhere in the form
        if (i < tok.len and tok[i] == config.range_sep) {
            const e = num(tok, i + 1);
            if (e.end == i + 1) continue; // a dash with no number is not a range
            if (e.end < tok.len and tok[e.end] != config.line_col_sep) continue; // junk after it
            return .{ .path = path, .at = .{ .line = l.v, .end_line = e.v }, .end = e.end };
        }
        if (i < tok.len and tok[i] != config.line_col_sep) continue; // junk after the number
        var at: Spot = .{ .line = l.v };
        if (i == tok.len) return .{ .path = path, .at = at, .end = i };
        // `:COL`. A column that does not parse is dropped and the LINE still
        // stands, which is how this has always read a half-mangled suffix — and
        // `end` stops at the last character that DID read, so the caller that
        // cares can tell the two apart.
        const c = num(tok, i + 1);
        if (c.end == i + 1) return .{ .path = path, .at = at, .end = i };
        if (c.end < tok.len and tok[c.end] != config.line_col_sep and tok[c.end] != config.range_sep)
            return .{ .path = path, .at = at, .end = i };
        at.col = c.v;
        i = c.end;
        if (i == tok.len or tok[i] != config.range_sep) return .{ .path = path, .at = at, .end = i };
        // `-ENDCOL` on this same line, unless a `:ENDCOL` follows — then that
        // first number was the end LINE all along. One lookahead, and it is
        // what lets the two-number and four-number forms share a spelling.
        const e = num(tok, i + 1);
        if (e.end == i + 1) return .{ .path = path, .at = at, .end = i };
        at.end_line = at.line;
        at.end_col = e.v;
        var end = e.end;
        if (e.end < tok.len and tok[e.end] == config.line_col_sep) {
            const e2 = num(tok, e.end + 1);
            if (e2.end > e.end + 1) {
                at.end_line = at.end_col;
                at.end_col = e2.v;
                end = e2.end;
            }
        }
        return .{ .path = path, .at = at, .end = end };
    }
    return .{ .path = tok, .at = .{}, .end = tok.len };
}

test "parsePathLine: spots, ranges, and the paths that merely look like them" {
    const cases = [_]struct { tok: []const u8, path: []const u8, at: Spot }{
        .{ .tok = "main.zig", .path = "main.zig", .at = .{} },
        .{ .tok = "main.zig:100", .path = "main.zig", .at = .{ .line = 100 } },
        .{ .tok = "main.zig:100:", .path = "main.zig", .at = .{ .line = 100 } },
        .{ .tok = "main.zig:100:7", .path = "main.zig", .at = .{ .line = 100, .col = 7 } },
        .{ .tok = "main.zig:100-104", .path = "main.zig", .at = .{ .line = 100, .end_line = 104 } },
        .{ .tok = "main.zig:100:7-21", .path = "main.zig", .at = .{ .line = 100, .col = 7, .end_line = 100, .end_col = 21 } },
        .{ .tok = "main.zig:100:7-104:3", .path = "main.zig", .at = .{ .line = 100, .col = 7, .end_line = 104, .end_col = 3 } },
        // the dash cases that must stay ORDINARY PATHS
        .{ .tok = "my-file.zig", .path = "my-file.zig", .at = .{} },
        .{ .tok = "my-file:10", .path = "my-file", .at = .{ .line = 10 } },
        .{ .tok = "x:1-y", .path = "x:1-y", .at = .{} },
        .{ .tok = "a-b-c", .path = "a-b-c", .at = .{} },
        .{ .tok = "2026-07-30", .path = "2026-07-30", .at = .{} },
        // a mangled tail still yields what parsed (unchanged behaviour)
        .{ .tok = "main.zig:100x", .path = "main.zig:100x", .at = .{} },
        .{ .tok = "main.zig:100:7x", .path = "main.zig", .at = .{ .line = 100 } },
    };
    for (cases) |c| {
        const got = parsePathLine(c.tok);
        try std.testing.expectEqualStrings(c.path, got.path);
        try std.testing.expectEqual(c.at, got.at);
    }
}

test "parsePathLine: `end` separates a whole-token target from a lenient read" {
    // the whole token IS the target: every spelling the doc above lists
    for ([_][]const u8{
        "main.zig",         "main.zig:100",      "main.zig:100:7",
        "main.zig:100-104", "main.zig:100:7-21", "main.zig:100:7-104:3",
        "@p3:10:5",         "x:1-y",
    }) |tok| try std.testing.expectEqual(tok.len, parsePathLine(tok).end);
    // ...and the reads that DROP a tail: a result row with its matched text
    // still attached, which is exactly what a row-grained step must not select
    // whole (lookableLineSpan). Note where each one STOPS — a spot is only
    // taken once its whole form has read, so the `:7` of a `:100:7 text` row
    // is dropped along with the text and `end` says so.
    const partial = [_]struct { tok: []const u8, end: usize }{
        .{ .tok = "main.zig:100:", .end = "main.zig:100".len }, // trailing ':' is peeled, not parsed
        .{ .tok = "main.zig:100:7x", .end = "main.zig:100".len },
        .{ .tok = "main.zig:100:7 fn main() void {", .end = "main.zig:100".len },
        .{ .tok = "main.zig:100:7-21 const x = 1;", .end = "main.zig:100:7-21".len },
        .{ .tok = "@p3:10:5 /home/goblin", .end = "@p3:10".len },
    };
    for (partial) |c| try std.testing.expectEqual(c.end, parsePathLine(c.tok).end);
    // A form that breaks off mid-range is not a lenient read at all: the scan
    // goes back for a later ':', finds none, and the token is a plain PATH
    // whole — which resolves or does not on its own merits.
    const whole = "main.zig:100-104 whole lines";
    try std.testing.expectEqual(whole.len, parsePathLine(whole).end);
    try std.testing.expectEqualStrings(whole, parsePathLine(whole).path);
}

/// A file-like Look target has a rendering kind only in MuPDF builds. The
/// feature-off enum has no `pdf` tag at all, so `.pdf` is indistinguishable
/// from any other ordinary file before it reaches the core.
pub const FileKind = if (pdf_enabled) enum { text, pdf } else enum { text };

pub const FileTarget = struct {
    path: []const u8,
    at: Spot,
    kind: FileKind = .text,
};

pub const Target = union(enum) {
    none,
    dir: []const u8, // resolved absolute path, in caller's buf
    file: FileTarget,
    image: struct { path: []const u8 },
    url: []const u8,
    /// `@p7:10:5` — pane 7, line 10, column 5 (0 = unspecified). The one
    /// target that names a live pane instead of a path, because terminals and
    /// output buffers have no file for a location to point at.
    pane: struct { id: usize, at: Spot },
};

pub fn isImagePath(path: []const u8) bool {
    for (config.image_exts) |ext| {
        if (std.ascii.endsWithIgnoreCase(path, ext)) return true;
    }
    return false;
}

pub fn isPdfPath(path: []const u8) bool {
    if (comptime !pdf_enabled) return false;
    return std.ascii.endsWithIgnoreCase(path, ".pdf");
}

test "PDF file kinds exist only in MuPDF-enabled builds" {
    try std.testing.expectEqual(pdf_enabled, isPdfPath("manual.PDF"));
    try std.testing.expect(!isPdfPath("manual.pdf.txt"));
    try std.testing.expectEqual(
        pdf_enabled,
        std.meta.stringToEnum(FileKind, "pdf") != null,
    );
    try std.testing.expect(std.meta.stringToEnum(std.meta.Tag(Target), "pdf") == null);
}

test ".pdf Look paths are ordinary files when MuPDF is disabled" {
    if (!platform_has_fs) return;
    var realbuf: [4096]u8 = undefined;
    const target = resolve("docs/design.pdf", ".", &realbuf);
    switch (target) {
        .file => |file| {
            if (comptime pdf_enabled)
                try std.testing.expectEqual(FileKind.pdf, file.kind)
            else
                try std.testing.expectEqual(FileKind.text, file.kind);
        },
        else => return error.PdfDidNotResolveAsFile,
    }
}

/// Where a look-able word actually SITS inside a run of non-whitespace.
pub const Span = struct { start: usize, end: usize };

/// The punctuation a path wears in prose and never owns. Two sets, because
/// the two ends are not alike: a directory may legally END in `/`, and the
/// `:` that closes `grep -n`'s `main.zig:100:` is junk on the right and
/// meaningful nowhere on the left.
const lead_trim = "([{<\"'`*";
const trail_trim = ")]}>\"'`*,;:.!?";

/// The largest look-able span inside one whitespace-delimited `word`, or null
/// when nothing in it resolves. This is the WORD grain of n/N — split a row on
/// whitespace and take the biggest piece of each run Look can act on — which
/// is what a terminal, a file and a PDF step, because their lines are free
/// text and a line may hold several places (an `ls` row hops file to file).
/// A results buffer steps ROWS instead: lookableLineSpan.
///
/// TWO resolve attempts at most, which is what keeps a motion across a
/// screenful of prose from being a hundred realpaths: the run with every
/// wrapper character peeled off BOTH ends at once, then — only if that found
/// nothing — the run exactly as written.
///
/// PEELED FIRST, which is the ordering that matters. `resolve` is lenient
/// about a tail it cannot parse (`main.zig:12:3,` yields the FILE and drops
/// the position, by design), so asking it about the raw run first would
/// happily answer yes and swallow the comma along with the `:3`. Peeling
/// first hands it `main.zig:12:3` and the look lands on the column. The raw
/// run stays as the fallback for the file genuinely named `foo,` or `..`,
/// where the peel eats something real.
///
/// Deliberately NOT a search for the longest resolving substring: that costs
/// a syscall per prefix to find a path hiding inside a word nobody typed as
/// one. A run needing a cleverer peel is still one Enter away with the cursor
/// parked on it.
///
/// Direction-free on purpose: n and N ask this the same question about the
/// same run and get the same span back, which is what lets the two motions be
/// exact inverses of each other.
pub fn lookableSpan(word: []const u8, cwd: []const u8, realbuf: *[4096]u8) ?Span {
    if (word.len == 0) return null;
    var lo: usize = 0;
    var hi: usize = word.len;
    while (lo < hi and std.mem.indexOfScalar(u8, lead_trim, word[lo]) != null) lo += 1;
    while (hi > lo and std.mem.indexOfScalar(u8, trail_trim, word[hi - 1]) != null) hi -= 1;
    if (lo < hi and resolve(word[lo..hi], cwd, realbuf) != .none) return .{ .start = lo, .end = hi };
    // nothing came off, so the peeled attempt WAS the raw one
    if (lo == 0 and hi == word.len) return null;
    if (resolve(word, cwd, realbuf) == .none) return null;
    return .{ .start = 0, .end = word.len };
}

test "lookableSpan peels prose punctuation off a path, largest first" {
    if (!platform_has_fs) return;
    var realbuf: [4096]u8 = undefined;
    // the bare run resolves whole, wrappers and all left alone
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "src/look.zig".len }),
        lookableSpan("src/look.zig", ".", &realbuf),
    );
    // ...and a wrapped one gives back the span INSIDE the wrappers
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 1, .end = 1 + "src/look.zig".len }),
        lookableSpan("(src/look.zig),", ".", &realbuf),
    );
    // the `:LINE:COL` tail is part of the span: it is what a look READS
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "src/look.zig:12:3".len }),
        lookableSpan("src/look.zig:12:3,", ".", &realbuf),
    );
    // grep -n's trailing delimiter comes off, the line number stays
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "src/look.zig:12".len }),
        lookableSpan("src/look.zig:12:", ".", &realbuf),
    );
    try std.testing.expectEqual(@as(?Span, null), lookableSpan("nothing-here", ".", &realbuf));
    try std.testing.expectEqual(@as(?Span, null), lookableSpan("", ".", &realbuf));
    try std.testing.expectEqual(@as(?Span, null), lookableSpan("((()))", ".", &realbuf));
}

/// The largest look-able span ANCHORED at the start of `line`'s text, or null
/// when the row names no place at all. This is the ROW grain of n/N, and what
/// a results buffer steps: a row there IS one location — `path:LINE:COL text`
/// — and the words after the location are the MATCH, not a second place to
/// step to. One stop per row, always its head.
///
/// LARGEST, so the candidates are the run from the first non-blank cell out to
/// each whitespace boundary, tried LONGEST first: a path with a blank in it
/// (`old notes/plan.txt`) beats the word hiding inside it, which is the case
/// the word grain cannot express at all.
///
/// A candidate only counts when it is the target EXACTLY — parsePathLine
/// consuming every byte of it, after the same wrapper peel lookableSpan does.
/// That gate is what keeps longest-first from swallowing the whole row:
/// `resolve` is lenient by design and answers `src/x.zig:12:5 const y` with
/// the FILE, so without it every result row would select out to its right
/// margin and throw the `:5` away along with the text. A url is lenient the
/// same way in the other direction — it is recognised by its PREFIX, so a
/// longer run is not a longer link — and only the filesystem can vouch for a
/// span with a blank inside it, so only the filesystem is allowed to.
///
/// Cost is the word grain's: the exactness gate is pure parsing, so a row
/// spends at most one resolve per whitespace boundary and the ordinary result
/// row — whose head is its whole location — spends two.
pub fn lookableLineSpan(line: []const u8, cwd: []const u8, realbuf: *[4096]u8) ?Span {
    var lo: usize = 0;
    while (lo < line.len and (line[lo] == ' ' or line[lo] == '\t')) lo += 1;
    var hi = std.mem.trimEnd(u8, line, " \t\r").len;
    while (hi > lo) {
        var a = lo;
        var b = hi;
        while (a < b and std.mem.indexOfScalar(u8, lead_trim, line[a]) != null) a += 1;
        while (b > a and std.mem.indexOfScalar(u8, trail_trim, line[b - 1]) != null) b -= 1;
        const cand = line[a..b];
        if (cand.len > 0 and parsePathLine(cand).end == cand.len) switch (resolve(cand, cwd, realbuf)) {
            .dir, .file, .image => return .{ .start = a, .end = b },
            .url, .pane => if (std.mem.indexOfAny(u8, cand, " \t") == null)
                return .{ .start = a, .end = b },
            .none => {},
        };
        // ...else the same run one word shorter
        while (hi > lo and line[hi - 1] != ' ' and line[hi - 1] != '\t') hi -= 1;
        while (hi > lo and (line[hi - 1] == ' ' or line[hi - 1] == '\t')) hi -= 1;
    }
    return null;
}

test "lookableLineSpan takes the row's location and stops before its text" {
    if (!platform_has_fs) return;
    var realbuf: [4096]u8 = undefined;
    // a grep row: the location, and NOT the matched code after it — which
    // `resolve` would happily answer for, minus the column
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "src/look.zig:12:5-9".len }),
        lookableLineSpan("src/look.zig:12:5-9 const std = @import(\"std\");", ".", &realbuf),
    );
    // an lsp/jumplist row, whose column is followed by a blank rather than a
    // ':' — the form a lenient read drops on the floor
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "src/look.zig:12:5".len }),
        lookableLineSpan("src/look.zig:12:5 pub fn resolve", ".", &realbuf),
    );
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "@p3:10:5".len }),
        lookableLineSpan("@p3:10:5 /home/goblin", ".", &realbuf),
    );
    // a bare path row, wrappers peeled and blank indent skipped like anywhere
    // else — the anchor is the row's first non-blank cell, not column zero
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 3, .end = 3 + "src/look.zig".len }),
        lookableLineSpan("  (src/look.zig)", ".", &realbuf),
    );
    // a link row keeps its link and leaves the title alone: a longer run is
    // not a longer url
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "https://pardes.dev/a".len }),
        lookableLineSpan("https://pardes.dev/a  Chapter One", ".", &realbuf),
    );
    // ANCHORED: a place mentioned mid-row is not a stop, and a row with no
    // place at its head is no stop at all
    try std.testing.expectEqual(
        @as(?Span, null),
        lookableLineSpan("see also src/look.zig", ".", &realbuf),
    );
    try std.testing.expectEqual(@as(?Span, null), lookableLineSpan("   ", ".", &realbuf));
    try std.testing.expectEqual(@as(?Span, null), lookableLineSpan("", ".", &realbuf));
}

test "lookableLineSpan prefers the longest run, so a blank inside a path is one span" {
    if (!platform_has_fs) return;
    var realbuf: [4096]u8 = undefined;
    // A real path with a blank in it, under a directory whose own name is the
    // first word of the row: the word grain can only ever see `tmp`, and the
    // row grain sees the file, because it asks about the longest run first.
    const io = std.Io.Threaded.global_single_threaded.io();
    var tmp = try std.Io.Dir.cwd().openDir(io, "/tmp", .{});
    defer tmp.close(io);
    const name = "pardes look span.txt";
    try tmp.writeFile(io, .{ .sub_path = name, .data = "" });
    defer tmp.deleteFile(io, name) catch {};
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = ("tmp/" ++ name).len }),
        lookableLineSpan("tmp/" ++ name, "/", &realbuf),
    );
    // ...and the shrink still finds the shorter run when the long one is
    // prose. Candidates END at a blank, so the runs tried are whole words:
    // there is no hunt for a path hiding inside one (lookableSpan's rule).
    try std.testing.expectEqualDeep(
        @as(?Span, .{ .start = 0, .end = "tmp".len }),
        lookableLineSpan("tmp holds pardes look span.txt", "/", &realbuf),
    );
}

/// Resolve a looked-at word against the pane's directory. `realbuf` must
/// outlive the returned Target (native paths point into it; web paths are
/// process-lifetime slices in the embedded source archive).
pub fn resolve(word_raw: []const u8, cwd: []const u8, realbuf: *[4096]u8) Target {
    const trimmed = std.mem.trim(u8, word_raw, " \t\r\n");
    const pl = parsePathLine(trimmed);
    const word = pl.path;
    if (word.len == 0) return .none;

    // `@pN` addresses a pane, not a path: every platform, before the fs.
    if (word.len > config.pane_addr.len and std.mem.startsWith(u8, word, config.pane_addr)) {
        var id: usize = 0;
        for (word[config.pane_addr.len..]) |c| {
            if (!std.ascii.isDigit(c)) break;
            id = id * 10 + (c - '0');
        } else return .{ .pane = .{ .id = id, .at = pl.at } };
    }

    // a URL is a URL everywhere: no filesystem can answer it, so it leaves the
    // app (browser tab on web, xdg-open/open on the desktop).
    for (config.url_schemes) |scheme| {
        if (std.mem.startsWith(u8, trimmed, scheme)) return .{ .url = trimmed };
    }

    if (platform_has_fs) {
        var joinbuf: [2048]u8 = undefined;
        const joined: ?[:0]u8 = if (word[0] == '/')
            (std.fmt.bufPrintSentinel(&joinbuf, "{s}", .{word}, 0) catch null)
        else
            (std.fmt.bufPrintSentinel(&joinbuf, "{s}/{s}", .{ cwd, word }, 0) catch null);
        const jz = joined orelse return .none;
        const rp = realpath(jz.ptr, realbuf) orelse return .none;
        const resolved = std.mem.span(rp);
        if (isDir(rp)) return .{ .dir = resolved };
        if (comptime pdf_enabled) if (isPdfPath(resolved)) return .{ .file = .{
            .path = resolved,
            .at = pl.at,
            .kind = .pdf,
        } };
        if (isImagePath(resolved)) return .{ .image = .{ .path = resolved } };
        return .{ .file = .{ .path = resolved, .at = pl.at } };
    } else {
        // web: tracked Zig sources resolve inside the build-generated,
        // read-only source filesystem.
        if (resolveEmbedded(word, cwd, realbuf)) |source|
            return .{ .file = .{ .path = source.path, .at = pl.at } };
        return .none;
    }
}

/// Resolve a source path without teaching the core about a browser filesystem.
/// Cwd-relative and absolute dump paths are normalized, with printed archive
/// paths also accepted root-relative. The suffix match lets a dump made in
/// `/host/repo` address names that deliberately remain relative to the root.
fn resolveEmbedded(word: []const u8, cwd: []const u8, scratch: *[4096]u8) ?embedded_sources.Source {
    var wordbuf: [4096]u8 = undefined;
    const normalized_word = normalizeVirtualPath(word, &wordbuf) orelse return null;
    if (word.len > 0 and word[0] == '/') return findEmbeddedSource(normalized_word, true);

    var joined: [4096]u8 = undefined;
    if (std.fmt.bufPrint(&joined, "{s}/{s}", .{ cwd, word }) catch null) |candidate|
        if (normalizeVirtualPath(candidate, scratch)) |normalized|
            if (findEmbeddedSource(normalized, true)) |source| return source;
    // A printed archive path is root-relative even when its surrounding dump
    // pane came from some unrelated cwd.
    return findEmbeddedSource(normalized_word, false);
}

fn normalizeVirtualPath(path: []const u8, out: *[4096]u8) ?[]const u8 {
    var len: usize = 0;
    var parts = std.mem.tokenizeAny(u8, path, "/\\");
    while (parts.next()) |part| {
        if (std.mem.eql(u8, part, ".")) continue;
        if (std.mem.eql(u8, part, "..")) {
            while (len > 0 and out[len - 1] != '/') len -= 1;
            if (len > 0) len -= 1;
            continue;
        }
        const extra = part.len + @intFromBool(len != 0);
        if (len + extra > out.len) return null;
        if (len != 0) {
            out[len] = '/';
            len += 1;
        }
        @memcpy(out[len..][0..part.len], part);
        len += part.len;
    }
    if (len == 0) return null;
    return out[0..len];
}

fn findEmbeddedSource(path: []const u8, allow_root_suffix: bool) ?embedded_sources.Source {
    for (embedded_sources.all) |source|
        if (std.mem.eql(u8, source.path, path)) return source;
    if (!allow_root_suffix) return null;
    for (embedded_sources.all) |source| {
        if (path.len <= source.path.len or path[path.len - source.path.len - 1] != '/') continue;
        if (std.mem.endsWith(u8, path, source.path)) return source;
    }
    return null;
}

const platform_has_fs = switch (pardes.platform) {
    .tty, .gui, .macos => true,
    .web => false,
};

// Find's safety rails. The core is SYNCHRONOUS — a Find at `/` runs inside the
// keystroke that asked for it — so the walk must end whatever it is pointed at.
// Three caps, because each alone leaks: hits bound the results buffer, depth
// bounds a deep tree, and steps bound a wide shallow one (a pattern that never
// matches would otherwise walk the whole disk without ever filling `hits`).
const find_max_hits = 512;
const find_max_depth = 16;
const find_max_steps = 100_000;
/// One search result buffer. A grep can visit one root per pane, each root can
/// contribute `find_max_hits`, and native paths are capped at 4096 bytes below.
/// Callers allocate this conservative ceiling once; a full buffer truncates at
/// the last complete row.
pub const search_max_output_bytes = pardes.MAX_PANES * find_max_hits * (4096 + 320);

/// Directories a source tree has no answers in, skipped whole. fd reads
/// .gitignore for this; pardes has no ignore parser, and every one of these
/// costs a real search: agave's `target/` alone is 456_000 of its 460_000
/// entries and holds 1_200 of the 1_242 paths matching "bank", so a Find for
/// `bank` burned the whole 512-hit budget on build artifacts and never
/// reached `runtime/src/bank.rs`. That looked like a broken matcher.
const find_skip = [_][]const u8{
    ".git",  ".jj",         "target",     "node_modules",
    ".venv", "__pycache__", ".zig-cache", "zig-out",
};

/// `fd`, in-core: every path under `dir` whose NAME contains `pat` (plain
/// case-insensitive substring — fd's default is a regex and pardes has no
/// regex engine to spend on one), one path per line into `out`, RELATIVE to
/// `dir` — the results buffer is itself named `dir/+Search`, so every row
/// resolves against the same directory the walk started in and reads as the
/// short name the searcher was looking for. Only real directories are
/// entered, so a symlink can never close a cycle.
/// Filesystem setup and traversal errors are returned to the UI boundary.
pub fn find(arena: std.mem.Allocator, dir: []const u8, pat: []const u8, out: []u8) !usize {
    var hits: [find_max_hits][]const u8 = undefined;
    var hits_len: usize = 0;
    if (platform_has_fs) {
        // Zig 0.16 moved the filesystem behind std.Io; the blocking
        // single-threaded implementation (the one std.debug itself holds) IS
        // the synchronous walk a sans-IO core wants — no pool, no cancelation.
        const io = std.Io.Threaded.global_single_threaded.io();
        var root = try std.Io.Dir.cwd().openDir(io, dir, .{ .iterate = true });
        defer root.close(io);
        // walkSelectively, not walk: descending is opt-in, which is the only
        // way to express the depth cap and find_skip at all.
        var w = try root.walkSelectively(arena);
        defer w.deinit();
        var steps: usize = 0;
        walk: while (steps < find_max_steps and hits_len < hits.len) {
            steps += 1; // an unreadable dir burns a step too, so it cannot spin
            const e = (try w.next(io)) orelse break;
            if (std.ascii.indexOfIgnoreCase(e.basename, pat) != null) {
                // e.path points into the walker's own buffer, dead at next()
                hits[hits_len] = try arena.dupe(u8, e.path);
                hits_len += 1;
            }
            if (e.kind != .directory or e.depth() >= find_max_depth) continue;
            for (find_skip) |s| if (std.mem.eql(u8, e.basename, s)) continue :walk;
            try w.enter(io, e);
        }
    } else {
        // web: the build-generated source archive IS the filesystem, and it is
        // already a flat list of paths — the whole walk is the match.
        for (embedded_sources.all) |s| {
            if (hits_len >= hits.len) break;
            if (std.ascii.indexOfIgnoreCase(std.fs.path.basename(s.path), pat) != null) {
                hits[hits_len] = s.path;
                hits_len += 1;
            }
        }
    }
    // readdir order is undefined; sort so the same tree gives the same buffer
    // twice running and n/N walks it in a sane order.
    std.mem.sort([]const u8, hits[0..hits_len], {}, struct {
        fn lt(_: void, a: []const u8, b: []const u8) bool {
            return std.mem.lessThan(u8, a, b);
        }
    }.lt);
    var written: usize = 0;
    for (hits[0..hits_len]) |h| {
        if (h.len + 1 > out.len - written) break;
        @memcpy(out[written..][0..h.len], h);
        written += h.len;
        out[written] = '\n';
        written += 1;
    }
    return written;
}

/// how much of one file Grep reads. The core is synchronous, so a tree with a
/// core dump in it must not stall the keystroke: past this the tail of the file
/// is simply not searched (`grep -R` would read it all).
const grep_max_bytes = 256 * 1024;
const grep_max_files = 20_000;

/// every line of `text` holding `pat`, as `path:LINE:COL-ENDCOL text` rows —
/// the shared half of grep(), and the shape every result row in pardes has:
/// the leading word is a look target, so n/N walk the hits. The row names the
/// MATCH's span and not just its first cell, so stepping onto one selects the
/// text that matched (config.range_sep). Returns the rows written, at most
/// `budget`.
const GrepResult = struct { bytes: usize, hits: usize };

fn grepText(path: []const u8, text: []const u8, pat: []const u8, out: []u8, budget: usize) GrepResult {
    var result: GrepResult = .{ .bytes = 0, .hits = 0 };
    var line: usize = 0;
    var it = std.mem.splitScalar(u8, text, '\n');
    while (it.next()) |raw| {
        line += 1;
        if (result.hits >= budget) break;
        const at = std.ascii.indexOfIgnoreCase(raw, pat) orelse continue;
        // one minified line can be the whole file: cut it, but never mid
        // codepoint — a partial UTF-8 sequence reaches the renderer as a hit
        // row and there is nothing sane for it to draw.
        const ln = std.mem.trimEnd(u8, raw, " \t\r");
        var cut = @min(ln.len, 200);
        while (cut > 0 and cut < ln.len and ln[cut] & 0xc0 == 0x80) cut -= 1;
        const row = std.fmt.bufPrint(out[result.bytes..], "{s}:{d}:{d}{c}{d} {s}\n", .{
            path, line, at + 1, config.range_sep, at + pat.len, ln[0..cut],
        }) catch break;
        result.bytes += row.len;
        result.hits += 1;
    }
    return result;
}

/// `grep -R`, in-core: every LINE of every file under `dir` containing `pat`
/// (plain case-insensitive substring, like every other search here), one row
/// per hit into `out`. A row's path is RELATIVE to `base` — the directory of
/// the pane that asked, which is also the one its results buffer is named in,
/// so a row reads as the short name that pane would have typed and still looks
/// up. A hit `base` does not contain (another pane's tree) keeps its absolute
/// path, which resolves from anywhere. Same walk, same skip list and same three
/// caps as find(), plus grep_max_bytes and a NUL sniff so a binary never lands
/// in the results.
/// Filesystem setup, traversal, and read errors are returned to the UI boundary.
pub fn grep(arena: std.mem.Allocator, gpa: std.mem.Allocator, dir: []const u8, base: []const u8, pat: []const u8, out: []u8) !usize {
    var hits: usize = 0;
    var written: usize = 0;
    if (!platform_has_fs) {
        // web: the build-generated source archive IS the filesystem
        for (embedded_sources.all) |s| {
            if (hits >= find_max_hits or written == out.len) break;
            const result = grepText(s.path, s.contents, pat, out[written..], find_max_hits - hits);
            hits += result.hits;
            written += result.bytes;
        }
        return written;
    }
    const root_path = std.mem.trimEnd(u8, dir, "/");
    const home = std.mem.trimEnd(u8, base, "/");
    // The walk collects into one bounded allocation, then the read scans in
    // sorted order. e.path dies at the next next(), so these are copies.
    const files = try arena.alloc([]const u8, grep_max_files);
    var files_len: usize = 0;
    {
        const io = std.Io.Threaded.global_single_threaded.io();
        var root = try std.Io.Dir.cwd().openDir(io, dir, .{ .iterate = true });
        defer root.close(io);
        var w = try root.walkSelectively(arena);
        defer w.deinit();
        var steps: usize = 0;
        walk: while (steps < find_max_steps and files_len < files.len) {
            steps += 1;
            const e = (try w.next(io)) orelse break;
            if (e.kind == .directory) {
                if (e.depth() >= find_max_depth) continue;
                for (find_skip) |s| if (std.mem.eql(u8, e.basename, s)) continue :walk;
                try w.enter(io, e);
                continue;
            }
            if (e.kind != .file) continue;
            files[files_len] = try std.fmt.allocPrint(arena, "{s}/{s}", .{ root_path, e.path });
            files_len += 1;
        }
    }
    std.mem.sort([]const u8, files[0..files_len], {}, struct {
        fn lt(_: void, a: []const u8, b: []const u8) bool {
            return std.mem.lessThan(u8, a, b);
        }
    }.lt);
    // ONE bounded buffer reused for every file: a synchronous search must not
    // swallow a file it cannot afford to hold.
    const buf = try gpa.alloc(u8, grep_max_bytes);
    defer gpa.free(buf);
    for (files[0..files_len]) |path| {
        if (hits >= find_max_hits or written == out.len) break;
        var pathbuf: [4096]u8 = undefined;
        const path_z = std.fmt.bufPrintSentinel(&pathbuf, "{s}", .{path}, 0) catch return error.PathTooLong;
        const fd = libc.open(path_z, .{ .ACCMODE = .RDONLY, .CLOEXEC = true });
        if (fd < 0) return error.OpenFailed;
        var len: usize = 0;
        while (len < buf.len) {
            const n = libc.read(fd, buf[len..].ptr, buf.len - len);
            if (n < 0) {
                if (libc.errno(n) == .INTR) continue;
                _ = libc.close(fd);
                return error.ReadFailed;
            }
            if (n == 0) break;
            len += @intCast(n);
        }
        _ = libc.close(fd);
        const text = buf[0..len];
        if (std.mem.indexOfScalar(u8, text[0..@min(len, 1024)], 0) != null) continue; // binary
        // per PATH, not per root: one root can straddle the asking pane's
        // directory (a shell at `/a` searching for a file pane at `/a/b`), and
        // the rows inside it are the ones worth shortening
        const shown = if (path.len > home.len and std.mem.startsWith(u8, path, home) and path[home.len] == '/')
            path[home.len + 1 ..]
        else
            path;
        const result = grepText(shown, text, pat, out[written..], find_max_hits - hits);
        hits += result.hits;
        written += result.bytes;
    }
    return written;
}

/// true if `path` exists and is a directory (open(O_DIRECTORY), no stat needed)
fn isDir(path: [*:0]const u8) bool {
    const fd = libc.open(path, .{ .ACCMODE = .RDONLY, .DIRECTORY = true, .CLOEXEC = true });
    if (fd < 0) return false;
    _ = libc.close(fd);
    return true;
}

/// Read a whole file (gpa-owned) — the look side of opening a file pane. Web
/// reads from the generated source archive; native shells read the real fs.
const read_file_max_bytes = 256 * 1024 * 1024;
const read_stream_max_bytes = 4 * 1024 * 1024;

/// Read a whole file with one size-bounded allocation. A file that grows after
/// fstat is read as the snapshot size; zero-size virtual files get a separate
/// bounded stream read. Files over either applicable cap are rejected.
pub fn readFile(gpa: std.mem.Allocator, path: []const u8) ![]u8 {
    if (!platform_has_fs) {
        var normalized_buf: [4096]u8 = undefined;
        const normalized = normalizeVirtualPath(path, &normalized_buf) orelse return error.OpenFailed;
        const source = findEmbeddedSource(normalized, true) orelse return error.OpenFailed;
        if (source.contents.len > read_file_max_bytes) return error.FileTooLarge;
        return gpa.dupe(u8, source.contents);
    }
    var pathbuf: [4096]u8 = undefined;
    const path_z = std.fmt.bufPrintSentinel(&pathbuf, "{s}", .{path}, 0) catch return error.PathTooLong;
    const fd = libc.open(path_z, .{ .ACCMODE = .RDONLY });
    if (fd < 0) return error.OpenFailed;
    defer _ = libc.close(fd);
    const end = libc.lseek(fd, 0, libc.SEEK.END);
    const size: usize = if (end < 0) 0 else @intCast(end);
    if (end >= 0 and libc.lseek(fd, 0, libc.SEEK.SET) < 0) return error.ReadFailed;
    if (size == 0) {
        // procfs and similar virtual files report zero size. Probe once so a
        // genuinely empty file remains an exact zero-byte allocation, then use
        // one conservative bounded allocation for a non-empty stream.
        var first: [16 * 1024]u8 = undefined;
        var first_len: usize = 0;
        while (true) {
            const n = libc.read(fd, &first, first.len);
            if (n < 0) {
                if (libc.errno(n) == .INTR) continue;
                return error.ReadFailed;
            }
            first_len = @intCast(n);
            break;
        }
        if (first_len == 0) return gpa.alloc(u8, 0);
        var stream = try gpa.alloc(u8, read_stream_max_bytes);
        errdefer gpa.free(stream);
        @memcpy(stream[0..first_len], first[0..first_len]);
        var stream_len = first_len;
        while (stream_len < stream.len) {
            const n = libc.read(fd, stream[stream_len..].ptr, stream.len - stream_len);
            if (n < 0) {
                if (libc.errno(n) == .INTR) continue;
                return error.ReadFailed;
            }
            if (n == 0) break;
            stream_len += @intCast(n);
        }
        if (stream_len == stream.len) {
            var extra: [1]u8 = undefined;
            while (true) {
                const n = libc.read(fd, &extra, 1);
                if (n < 0 and libc.errno(n) == .INTR) continue;
                if (n < 0) return error.ReadFailed;
                if (n > 0) return error.FileTooLarge;
                break;
            }
        }
        if (stream_len != stream.len) stream = try gpa.realloc(stream, stream_len);
        return stream;
    }
    if (size > read_file_max_bytes) return error.FileTooLarge;
    var buf = try gpa.alloc(u8, size);
    errdefer gpa.free(buf);
    var len: usize = 0;
    while (len < buf.len) {
        const n = libc.read(fd, buf[len..].ptr, buf.len - len);
        if (n < 0) {
            if (libc.errno(n) == .INTR) continue;
            return error.ReadFailed;
        }
        if (n == 0) break;
        len += @intCast(n);
    }
    if (len != buf.len) buf = try gpa.realloc(buf, len);
    return buf;
}

// ---- shell cwd: what directory a pane's looks resolve against ----

// macOS has no /proc; libproc's proc_pidinfo(PROC_PIDVNODEPATHINFO) yields the
// cwd vnode path. Not in std.c — layout from xnu's sys/proc_info.h.
const vnode_info_path = extern struct {
    vi: [152]u8 align(8), // struct vnode_info: vinfo_stat + type + pad + fsid
    path: [1024]u8, // MAXPATHLEN
};
const proc_vnodepathinfo = extern struct {
    cdir: vnode_info_path,
    rdir: vnode_info_path,
};
const PROC_PIDVNODEPATHINFO: c_int = 9;
extern "c" fn proc_pidinfo(pid: c_int, flavor: c_int, arg: u64, buffer: *anyopaque, buffersize: c_int) c_int;

/// Live cwd of a shell process (pane tags, look resolution). linux reads
/// /proc/<pid>/cwd, darwin asks libproc; other POSIX systems have no cheap
/// answer — return null and panes keep their spawn-time cwd (callers already
/// tolerate failure: dead shells have no cwd either).
pub fn shellCwd(pid: libc.pid_t, buf: *[1024]u8) ?[]const u8 {
    switch (builtin.os.tag) {
        .linux => {
            var pbuf: [64]u8 = undefined;
            const path = std.fmt.bufPrintSentinel(&pbuf, "/proc/{d}/cwd", .{pid}, 0) catch return null;
            const n = libc.readlink(path, buf, buf.len);
            if (n <= 0) return null;
            return buf[0..@intCast(n)];
        },
        .macos, .ios, .tvos, .watchos, .visionos => {
            var info: proc_vnodepathinfo = undefined;
            const n = proc_pidinfo(pid, PROC_PIDVNODEPATHINFO, 0, &info, @sizeOf(proc_vnodepathinfo));
            if (n < @as(c_int, @sizeOf(proc_vnodepathinfo))) return null;
            const path = std.mem.sliceTo(&info.cdir.path, 0);
            if (path.len == 0) return null;
            @memcpy(buf[0..path.len], path);
            return buf[0..path.len];
        },
        else => return null,
    }
}

// ---- tty occupancy: is a pane's terminal still the prompt pardes forked? ----

// Linux answers TIOCGPGRP asked of the pty MASTER with the SLAVE side's
// foreground process group — the number the kernel would deliver ^C to. Not in
// std.c, and the master is the only end pardes holds.
extern "c" fn tcgetpgrp(fd: c_int) libc.pid_t;

/// How far the descendant walk goes before it stops trusting itself. A shell
/// sitting at its prompt has no descendants at all and a foreground job is one
/// hop, so these are not a budget, they are a fuse: the walk is driven by
/// numbers read out of the kernel and must not be able to spin on a surprising
/// one (the same reason nested.outer() caps its hops). Hitting either bound
/// answers OCCUPIED — a tree we did not finish reading may hide the foreground
/// job, and typing a command line into vim is worse than declining to type it
/// into a shell that really was idle under 32 background jobs.
const occ_max_depth: u8 = 8;
const occ_max_visited: usize = 32;

/// Scratch for one `ttyTaken` answer: the walk's helpers share it rather than
/// each declaring its own copy of a path buffer. Lives in the probe's own
/// frame — there is no polling loop to hoist it out of any more, because the
/// core asks this question only where it is about to type a command line.
const TtyProbe = struct {
    /// the forked shell's own executable, read once per probe
    self_exe: [std.fs.max_path_bytes]u8 = undefined,
    /// ...and one descendant's, to compare against it
    exe: [std.fs.max_path_bytes]u8 = undefined,
    /// one small /proc text at a time: a children list, a stat line, a status
    /// blob. Each is consumed (parsed to numbers) before the next read.
    blob: [4096]u8 = undefined,
    /// the DFS worklist, bounded by the same fuse as the visit count
    pending: [occ_max_visited]Node = undefined,

    const Node = struct { pid: libc.pid_t, depth: u8 };
};

/// Is something OTHER than the shell prompt pardes forked sitting on this
/// pane's tty — vim, less, an agent, a build? An Exec must never type a command
/// line into such a program (it would land as vim keystrokes), so a taken
/// terminal is treated exactly like no terminal at all: the core routes the
/// command to another shell.
///
/// The predicate, and the false answer each clause exists to prevent:
///
///   fg = tcgetpgrp(master)          the tty's foreground pgrp, from the kernel
///   fg < 0                 -> free  no answer at all (not a tty, a host that
///                                   does not allow the ioctl): behave as before
///   self = exe(shell_pid)           the binary of the terminal we spawned,
///                                   straight out of /proc, so no spawn path has
///                                   to be plumbed through three frontends' Pty
///                                   structs and kept in step with shell_bin
///   self == null           -> free  the shell is gone; the pane's EOF is about
///                                   to remove it anyway
///   walk descendants of shell_pid:
///     exe unreadable       -> occupied, unless the child is a zombie (or has
///                                   already vanished), which is provably not on
///                                   the tty. Unreadable-but-alive is a setuid
///                                   program — `sudo` waiting for a password is
///                                   the case that must NOT be typed into.
///     exe != self          -> occupied iff its pgrp is fg. The pgrp filter is
///                                   what keeps `sleep 30 &` from looking
///                                   occupied: a background job is a child of an
///                                   idle prompt, and its pgrp is not the tty's.
///     exe == self          -> recurse. A nested shell prompt is still a usable
///                                   prompt, so `bash` inside `bash` stays
///                                   Exec-able; and the leaf is the answer, which
///                                   is what catches `bash -c 'sleep 30'` — there
///                                   the foreground pgrp LEADER's exe is our own
///                                   shell binary while the tty really belongs to
///                                   `sleep`.
///   no visited process in pgrp fg, and fg != shell_pid
///                          -> occupied  the tty belongs to a group we could not
///                                   attribute to anything we forked (a
///                                   foreground leader that died or re-parented);
///                                   never type into it.
///   otherwise              -> free
pub fn ttyTaken(shell_pid: libc.pid_t, master_fd: c_int) bool {
    switch (builtin.os.tag) {
        .linux => {
            var probe: TtyProbe = undefined;
            const fg = tcgetpgrp(master_fd);
            if (fg < 0) return false;
            const self_exe = procExe(shell_pid, &probe.self_exe) orelse return false;

            // The shell's own pgrp is normally the tty's when it is at its
            // prompt (forkpty made it the session and group leader), so the
            // idle answer is reached without reading its stat at all — the
            // whole fast path is tcgetpgrp, one readlink, and an empty
            // children file.
            var saw_fg = fg == shell_pid;
            var pending: usize = 0;
            var visited: usize = 0;
            switch (pushChildren(&probe, &pending, shell_pid, 1)) {
                .pushed => {},
                // No children file: a kernel without CONFIG_PROC_CHILDREN
                // cannot answer this question at all, so answer free and leave
                // behaviour exactly as it was before this probe existed.
                .unreadable => return false,
                .full => return true,
            }

            while (pending > 0) {
                pending -= 1;
                const node = probe.pending[pending];
                visited += 1;
                if (visited > occ_max_visited) return true;

                // One stat read carries the group; note it before anything can
                // return, because the final clause is about every process we
                // looked at, not only the ones that decided the answer.
                const pgrp = procPgrp(node.pid, &probe.blob);
                if (pgrp) |g| {
                    if (g == fg) saw_fg = true;
                }

                const exe = procExe(node.pid, &probe.exe) orelse {
                    if (offTty(node.pid, &probe.blob)) continue;
                    return true;
                };
                if (!std.mem.eql(u8, exe, self_exe)) {
                    if (pgrp) |g| if (g == fg) return true;
                    continue;
                }
                if (node.depth >= occ_max_depth) return true;
                switch (pushChildren(&probe, &pending, node.pid, node.depth + 1)) {
                    .pushed => {},
                    // This one exited while we walked (or the kernel stopped
                    // answering for it); its own pgrp was already counted and
                    // there is nothing below it to learn.
                    .unreadable => {},
                    .full => return true,
                }
            }
            return !saw_fg;
        },
        // A darwin implementation is tcgetpgrp (which xnu also allows on the
        // master) plus a descendant walk built from proc_listchildpids, with
        // proc_pidpath for the exe and proc_bsdinfo's pbi_pgid for the group —
        // there is no /proc to read. Until then macOS behaves as it did before
        // this probe existed: every terminal is a prompt.
        else => return false,
    }
}

/// Read a small /proc text in one go. These files are generated on read and
/// answer completely in a single call at these sizes; a short read would only
/// truncate a field, which every parser below treats as "no answer".
fn readProc(path: [*:0]const u8, buf: []u8) ?[]const u8 {
    const fd = libc.open(path, .{ .ACCMODE = .RDONLY });
    if (fd < 0) return null;
    defer _ = libc.close(fd);
    const got = libc.read(fd, buf.ptr, buf.len);
    if (got <= 0) return null;
    return buf[0..@intCast(got)];
}

/// The binary behind a pid, as the kernel spells it. Fails for a zombie (no mm
/// to point at) and for a process we may not inspect — the two cases `ttyTaken`
/// has to tell apart.
fn procExe(pid: libc.pid_t, buf: *[std.fs.max_path_bytes]u8) ?[]const u8 {
    var name: [64:0]u8 = undefined;
    const link = std.fmt.bufPrintSentinel(&name, "/proc/{d}/exe", .{@as(u32, @intCast(pid))}, 0) catch return null;
    const n = libc.readlink(link, buf, buf.len);
    if (n <= 0) return null;
    return buf[0..@intCast(n)];
}

/// A pid's process group.
fn procPgrp(pid: libc.pid_t, buf: *[4096]u8) ?libc.pid_t {
    var name: [64:0]u8 = undefined;
    const path = std.fmt.bufPrintSentinel(&name, "/proc/{d}/stat", .{@as(u32, @intCast(pid))}, 0) catch return null;
    return parsePgrp(readProc(path, buf) orelse return null);
}

/// Field 5 of /proc/<pid>/stat, found by scanning back from the LAST ')'
/// rather than counting fields from the start: field 2 is `comm` in
/// parentheses, and a comm may contain spaces AND parentheses, so a process
/// named `sh (a b)` shifts everything after it and a positional parse silently
/// reads some other number as the group. Same trap nested.parsePPid documents;
/// the kernel puts comm's closing paren last precisely so this scan works.
fn parsePgrp(stat: []const u8) ?libc.pid_t {
    const close = std.mem.lastIndexOfScalar(u8, stat, ')') orelse return null;
    var fields = std.mem.tokenizeAny(u8, stat[close + 1 ..], " \t\n");
    _ = fields.next() orelse return null; // 3: state
    _ = fields.next() orelse return null; // 4: ppid
    const pgrp = fields.next() orelse return null; // 5: pgrp
    return std.fmt.parseInt(libc.pid_t, pgrp, 10) catch null;
}

/// Is this pid provably NOT holding the tty even though its exe is unreadable:
/// a zombie (dead, waiting to be reaped) or already gone. Everything else that
/// hides its exe — a setuid program — is alive and on the terminal.
fn offTty(pid: libc.pid_t, buf: *[4096]u8) bool {
    var name: [64:0]u8 = undefined;
    const path = std.fmt.bufPrintSentinel(&name, "/proc/{d}/status", .{@as(u32, @intCast(pid))}, 0) catch return false;
    // No status at all: the pid died between the children read and here. A
    // process that no longer exists cannot be typed into.
    const status = readProc(path, buf) orelse return true;
    return parseZombie(status);
}

/// The `State:` field of a /proc/<pid>/status blob, and only Z. Line-anchored,
/// so a comm that spells `State: Z` inside the `Name:` line cannot answer.
fn parseZombie(status: []const u8) bool {
    var lines = std.mem.splitScalar(u8, status, '\n');
    while (lines.next()) |line| {
        if (!std.mem.startsWith(u8, line, "State:")) continue;
        const state = std.mem.trim(u8, line["State:".len..], " \t\r");
        return state.len > 0 and state[0] == 'Z';
    }
    return false;
}

const Pushed = enum { pushed, unreadable, full };

/// Put a pid's direct children on the worklist. The children file is the whole
/// reason this walk is cheap: an idle shell's is empty, so the fast path reads
/// one empty file instead of scanning /proc.
///
/// Spelled out rather than routed through `readProc` precisely because of that
/// empty file: readProc treats a zero-byte answer as no answer, which is right
/// for a stat line and exactly wrong here — "this process has no children" is
/// the most informative reply the walk ever gets, and calling it unreadable
/// would make the whole probe give up on every idle shell.
fn pushChildren(probe: *TtyProbe, pending: *usize, pid: libc.pid_t, depth: u8) Pushed {
    var name: [96:0]u8 = undefined;
    const path = std.fmt.bufPrintSentinel(&name, "/proc/{d}/task/{d}/children", .{
        @as(u32, @intCast(pid)), @as(u32, @intCast(pid)),
    }, 0) catch return .unreadable;
    const fd = libc.open(path, .{ .ACCMODE = .RDONLY });
    if (fd < 0) return .unreadable;
    defer _ = libc.close(fd);
    const got = libc.read(fd, &probe.blob, probe.blob.len);
    if (got < 0) return .unreadable;

    var kids: [occ_max_visited]libc.pid_t = undefined;
    const total = parseChildren(probe.blob[0..@intCast(got)], &kids);
    if (total > kids.len or pending.* + total > probe.pending.len) return .full;
    for (kids[0..total]) |kid| {
        probe.pending[pending.*] = .{ .pid = kid, .depth = depth };
        pending.* += 1;
    }
    return .pushed;
}

/// The pids in a /proc/<pid>/task/<tid>/children blob: space separated, with a
/// trailing space, and empty for the overwhelmingly common idle shell. Returns
/// how many valid pids the blob HAS, having written the first `out.len` of them
/// — a total past `out.len` is the caller's overflow signal. A token that is
/// not strictly digits is skipped rather than answered wrong: this drives who
/// gets walked, and parseInt alone would take `-1` and `+7`.
fn parseChildren(text: []const u8, out: []libc.pid_t) usize {
    var total: usize = 0;
    var it = std.mem.tokenizeAny(u8, text, " \t\n\r");
    while (it.next()) |tok| {
        if (std.mem.indexOfNone(u8, tok, "0123456789") != null) continue;
        const kid = std.fmt.parseInt(libc.pid_t, tok, 10) catch continue;
        if (total < out.len) out[total] = kid;
        total += 1;
    }
    return total;
}

test "the children blob parses to pids, and a garbage token never becomes one" {
    var out: [8]libc.pid_t = undefined;
    // the idle shell, which is the case the whole fast path is shaped around
    try std.testing.expectEqual(@as(usize, 0), parseChildren("", &out));
    try std.testing.expectEqual(@as(usize, 0), parseChildren(" ", &out));
    // one child — the kernel writes a TRAILING space and no newline
    try std.testing.expectEqual(@as(usize, 1), parseChildren("991 ", &out));
    try std.testing.expectEqual(@as(libc.pid_t, 991), out[0]);
    // several, with and without the trailing separator
    try std.testing.expectEqual(@as(usize, 3), parseChildren("7 8 9 ", &out));
    try std.testing.expectEqualSlices(libc.pid_t, &.{ 7, 8, 9 }, out[0..3]);
    try std.testing.expectEqual(@as(usize, 2), parseChildren("11 12", &out));
    try std.testing.expectEqual(@as(usize, 2), parseChildren("11 12\n", &out));
    // garbage: this list decides whose /proc entries get read, and parseInt
    // alone would take every one of these. The pids AROUND the junk still
    // answer — dropping the tree because one token was odd would silently turn
    // a busy terminal into a free one.
    try std.testing.expectEqual(@as(usize, 2), parseChildren("5 -1 +7 0x3 abc 6 ", &out));
    try std.testing.expectEqualSlices(libc.pid_t, &.{ 5, 6 }, out[0..2]);
    // overflow is REPORTED, not silently truncated: the total is what the blob
    // HAS, so the caller can answer "occupied" instead of walking a tree it
    // only partly read
    var two: [2]libc.pid_t = undefined;
    try std.testing.expectEqual(@as(usize, 4), parseChildren("1 2 3 4 ", &two));
    try std.testing.expectEqualSlices(libc.pid_t, &.{ 1, 2 }, two[0..2]);
}

test "the process group comes off the last ')', not a comm-shifted stat field" {
    // the comm here contains a space AND parentheses — the exact shape that
    // breaks `field 5 of /proc/<pid>/stat` (see nested.parsePPid). Counting
    // from the left answers `b))` for the state and `S` for the group.
    const shifted = "1234 (sh (a b)) S 991 992 993 34816 992 4194560 " ++
        "1729 0 0 0 1 0 0 0 20 0 1 0 8244630 9887744 1131";
    try std.testing.expectEqual(@as(libc.pid_t, 992), parsePgrp(shifted).?);
    // ...and the ordinary shape still reads the same field
    try std.testing.expectEqual(@as(libc.pid_t, 7), parsePgrp("42 (bash) S 1 7 7 34816 7 4194304").?);
    // a group of its own, which is what a background job has
    try std.testing.expectEqual(@as(libc.pid_t, 42), parsePgrp("42 (sleep) S 7 42 7 0 -1").?);
    // a truncated read must not answer from a half line, and a blob that is
    // not a stat line at all must not answer at all
    try std.testing.expect(parsePgrp("") == null);
    try std.testing.expect(parsePgrp("1234 (bash) S 991") == null);
    try std.testing.expect(parsePgrp("1234 (bash) S 991 notanumber") == null);
    try std.testing.expect(parsePgrp("no parens here at all") == null);
}

test "the zombie state comes off its own status line" {
    // a reaped-but-not-yet-collected child: no exe to read, and provably not
    // holding the tty, so the walk must skip it instead of answering occupied
    try std.testing.expect(parseZombie("Name:\tsh (a b)\nUmask:\t0022\nState:\tZ (zombie)\nTgid:\t1234\n"));
    try std.testing.expect(parseZombie("State:\tZ (zombie)\n"));
    // every other state is a live process, and an unreadable exe then means
    // setuid (sudo asking for a password) — the one thing never to type into
    try std.testing.expect(!parseZombie("Name:\tsh\nState:\tS (sleeping)\n"));
    try std.testing.expect(!parseZombie("Name:\tvim\nState:\tR (running)\n"));
    try std.testing.expect(!parseZombie("Name:\tvim\nState:\tT (stopped)\n"));
    // a comm that spells the field cannot answer for it: the scan is anchored
    // to the start of a line, and `Name:` is where a comm lives
    try std.testing.expect(!parseZombie("Name:\tsh (State: Z)\nState:\tS (sleeping)\n"));
    // a truncated read is not a zombie (and so stays conservative)
    try std.testing.expect(!parseZombie("Name:\tsh\nSta"));
    try std.testing.expect(!parseZombie("State:\t"));
}

// ---- tests: the predicate against real processes on a real pty ----
//
// The parsers above cannot see any of what follows: whether Linux answers
// TIOCGPGRP on the MASTER at all, whether bash really puts a background job in
// its own group, and whether `bash -c` leaves our own binary as the foreground
// leader are all facts about the system, and every one of them decides an
// answer. So these fork a real bash on a real pty — the way
// test/e2e_harness.zig forks the whole app — and drive it.
extern "c" fn forkpty(
    amaster: *c_int,
    name: ?[*:0]u8,
    termp: ?*const anyopaque,
    winp: ?*const std.posix.winsize,
) c_int;

const test_shell = "/bin/bash";
const test_prompt = "PZX> ";

/// A real interactive bash on a pty of our own, plus the polling the cases need.
/// Nothing here sleeps for a fixed time waiting for the shell: every step polls
/// to a deadline, and every poll DRAINS the master — a shell whose output is
/// never read blocks on a full pty buffer and then nothing else happens either.
const TestShell = struct {
    master: c_int,
    pid: libc.pid_t,
    /// a rolling window of what the shell has written, so a case can wait for
    /// the prompt (or a job-control notice) instead of guessing a duration
    tail: [8192]u8 = undefined,
    tail_len: usize = 0,

    fn start() ?TestShell {
        if (!haveFile(test_shell)) return null;
        var master: c_int = undefined;
        const ws = std.posix.winsize{ .row = 24, .col = 80, .xpixel = 0, .ypixel = 0 };
        const pid = forkpty(&master, null, null, &ws);
        if (pid < 0) return null;
        if (pid == 0) {
            // --norc: the developer's own bashrc must not decide what these
            // tests see. -i: job control, which is what puts a background job
            // in a group of its own and is half of what is under test.
            const argv: [3:null]?[*:0]const u8 = .{ test_shell, "--norc", "-i" };
            _ = execv(test_shell, &argv);
            _exit(127);
        }
        var sh: TestShell = .{ .master = master, .pid = pid };
        // A prompt of our own — EXPORTED, so a nested bash shows the same one —
        // spelled with a '' seam, so the echo of the command that sets it
        // cannot be mistaken for the prompt it produces.
        sh.send("export PS1='PZ''X> '\n");
        if (!sh.waitText(test_prompt, 10_000)) {
            sh.stop();
            return null;
        }
        sh.forget();
        return sh;
    }

    fn send(sh: *TestShell, bytes: []const u8) void {
        _ = libc.write(sh.master, bytes.ptr, bytes.len);
    }

    fn forget(sh: *TestShell) void {
        sh.tail_len = 0;
    }

    /// Read everything the shell has produced so far, without blocking.
    fn drain(sh: *TestShell) void {
        while (true) {
            var fds = [1]libc.pollfd{.{ .fd = sh.master, .events = libc.POLL.IN, .revents = 0 }};
            if (libc.poll(&fds, 1, 0) <= 0) return;
            if (fds[0].revents & libc.POLL.IN == 0) return;
            var chunk: [4096]u8 = undefined;
            const n = libc.read(sh.master, &chunk, chunk.len);
            if (n <= 0) return;
            sh.append(chunk[0..@intCast(n)]);
        }
    }

    fn append(sh: *TestShell, bytes: []const u8) void {
        if (bytes.len >= sh.tail.len) {
            @memcpy(&sh.tail, bytes[bytes.len - sh.tail.len ..]);
            sh.tail_len = sh.tail.len;
            return;
        }
        const room = sh.tail.len - sh.tail_len;
        if (bytes.len > room) {
            const drop = bytes.len - room;
            std.mem.copyForwards(u8, sh.tail[0 .. sh.tail_len - drop], sh.tail[drop..sh.tail_len]);
            sh.tail_len -= drop;
        }
        @memcpy(sh.tail[sh.tail_len..][0..bytes.len], bytes);
        sh.tail_len += bytes.len;
    }

    fn waitText(sh: *TestShell, needle: []const u8, ms: i64) bool {
        const deadline = nowMs() + ms;
        while (true) {
            sh.drain();
            if (std.mem.indexOf(u8, sh.tail[0..sh.tail_len], needle) != null) return true;
            if (nowMs() >= deadline) return false;
            sleepMs(5);
        }
    }

    fn taken(sh: *TestShell) bool {
        sh.drain();
        return ttyTaken(sh.pid, sh.master);
    }

    /// Poll until the verdict is `want` — the answer changes when the SHELL
    /// gets around to forking or reaping, not when we sent the line.
    fn waitTaken(sh: *TestShell, want: bool, ms: i64) bool {
        const deadline = nowMs() + ms;
        while (true) {
            if (sh.taken() == want) return true;
            if (nowMs() >= deadline) return false;
            sleepMs(5);
        }
    }

    /// ...and the other direction: the verdict STAYS `want` for a window. What
    /// a false positive looks like is a probe that flickers to occupied while
    /// the shell sits at its prompt with a background job, and a single sample
    /// can miss it.
    fn holdsTaken(sh: *TestShell, want: bool, ms: i64) bool {
        const deadline = nowMs() + ms;
        while (nowMs() < deadline) {
            if (sh.taken() != want) return false;
            sleepMs(5);
        }
        return true;
    }

    /// Kill the shell AND everything under it, then reap and close. The tree
    /// has to be collected BEFORE the shell dies: a foreground job lives in its
    /// own process group, so killing bash alone leaves `sleep 30` running,
    /// re-parented to init — a stray that outlives the test binary.
    fn stop(sh: *TestShell) void {
        var probe: TtyProbe = undefined;
        var pending: usize = 0;
        var doomed: [occ_max_visited]libc.pid_t = undefined;
        var n: usize = 0;
        _ = pushChildren(&probe, &pending, sh.pid, 1);
        while (pending > 0) {
            pending -= 1;
            const node = probe.pending[pending];
            if (n == doomed.len) break;
            doomed[n] = node.pid;
            n += 1;
            if (node.depth < occ_max_depth) _ = pushChildren(&probe, &pending, node.pid, node.depth + 1);
        }
        _ = libc.kill(sh.pid, libc.SIG.KILL);
        for (doomed[0..n]) |kid| {
            _ = libc.kill(kid, libc.SIG.KILL);
            // ...and its group, for a program that forked helpers of its own
            _ = libc.kill(-kid, libc.SIG.KILL);
        }
        _ = libc.waitpid(sh.pid, null, 0);
        _ = libc.close(sh.master);
    }
};

fn haveFile(path: [*:0]const u8) bool {
    const fd = libc.open(path, .{ .ACCMODE = .RDONLY });
    if (fd < 0) return false;
    _ = libc.close(fd);
    return true;
}

fn nowMs() i64 {
    var ts: libc.timespec = undefined;
    _ = libc.clock_gettime(.MONOTONIC, &ts);
    return @as(i64, @intCast(ts.sec)) * 1000 + @divFloor(@as(i64, @intCast(ts.nsec)), 1_000_000);
}

fn sleepMs(ms: i64) void {
    const ts = libc.timespec{
        .sec = @intCast(@divFloor(ms, 1000)),
        .nsec = @intCast(@mod(ms, 1000) * 1_000_000),
    };
    _ = libc.nanosleep(&ts, null);
}

test "an idle prompt is free, a foreground job takes the tty, and Ctrl-C hands it back" {
    if (comptime builtin.os.tag != .linux) return error.SkipZigTest;
    var sh = TestShell.start() orelse return error.SkipZigTest;
    defer sh.stop();

    // The whole point of the default: a shell sitting at its prompt is usable,
    // and stays usable across samples.
    try std.testing.expect(sh.holdsTaken(false, 200));

    // A foreground job IS the terminal now — this is the answer an Exec needs,
    // and typing a command line here would be typing it at `sleep`.
    sh.send("sleep 30\n");
    try std.testing.expect(sh.waitTaken(true, 10_000));

    // ^C, and the tty is the prompt's again. Nothing is cached: the next poll
    // simply finds no children, which is why recovery needs no event.
    sh.forget();
    sh.send("\x03");
    try std.testing.expect(sh.waitTaken(false, 10_000));
    try std.testing.expect(sh.waitText(test_prompt, 10_000));
}

test "a background job is not the tty's owner" {
    if (comptime builtin.os.tag != .linux) return error.SkipZigTest;
    var sh = TestShell.start() orelse return error.SkipZigTest;
    defer sh.stop();

    // The false positive the pgrp filter exists for. Waiting for the job
    // notice first matters: the verdict has to be taken while the child is
    // genuinely alive, or this test would pass with no probe at all.
    sh.send("sleep 30 &\n");
    try std.testing.expect(sh.waitText("[1]", 10_000));
    try std.testing.expect(sh.holdsTaken(false, 300));

    // ...and it is still free once the job is gone, which also means the
    // zombie between `kill` and bash's reap is not read as an occupant.
    sh.send("kill %1\n");
    try std.testing.expect(sh.holdsTaken(false, 300));
}

test "a nested interactive shell is still a prompt" {
    if (comptime builtin.os.tag != .linux) return error.SkipZigTest;
    var sh = TestShell.start() orelse return error.SkipZigTest;
    defer sh.stop();

    // `bash` inside `bash`: the leaf matches the binary we spawned, so it is a
    // prompt like any other and Exec must keep working. This is the case the
    // recursion is FOR, and the reason "any child at all" would be wrong.
    sh.forget();
    sh.send("bash --norc -i\n");
    try std.testing.expect(sh.waitText(test_prompt, 10_000));
    try std.testing.expect(sh.holdsTaken(false, 300));

    // ...and one level deeper still
    sh.forget();
    sh.send("bash --norc -i\n");
    try std.testing.expect(sh.waitText(test_prompt, 10_000));
    try std.testing.expect(sh.holdsTaken(false, 300));

    // a job inside the INNER shell is still the tty's owner
    sh.send("sleep 30\n");
    try std.testing.expect(sh.waitTaken(true, 10_000));
    sh.send("\x03");
    try std.testing.expect(sh.waitTaken(false, 10_000));
}

test "the walk reaches the leaf: bash -c 'sleep 30' takes the tty" {
    if (comptime builtin.os.tag != .linux) return error.SkipZigTest;
    var sh = TestShell.start() orelse return error.SkipZigTest;
    defer sh.stop();

    sh.send("bash --norc -c 'sleep 30'\n");
    try std.testing.expect(sh.waitTaken(true, 10_000));
    sh.send("\x03");
    try std.testing.expect(sh.waitTaken(false, 10_000));

    // The same shape where bash provably CANNOT exec the command in place (two
    // commands, so the wrapper has to stay around and fork): the foreground
    // group's leader is then our own shell binary while the tty really belongs
    // to `sleep`. A predicate that stopped at the leader would call this free.
    sh.send("bash --norc -c 'sleep 30; :'\n");
    try std.testing.expect(sh.waitTaken(true, 10_000));

    // ...and that is the shape asserted, not assumed: the shell's only child
    // runs the same binary the shell does.
    var probe: TtyProbe = undefined;
    var pending: usize = 0;
    try std.testing.expectEqual(Pushed.pushed, pushChildren(&probe, &pending, sh.pid, 1));
    try std.testing.expectEqual(@as(usize, 1), pending);
    var wrapper_buf: [std.fs.max_path_bytes]u8 = undefined;
    var shell_buf: [std.fs.max_path_bytes]u8 = undefined;
    try std.testing.expectEqualStrings(
        procExe(sh.pid, &shell_buf).?,
        procExe(probe.pending[0].pid, &wrapper_buf).?,
    );

    sh.send("\x03");
    try std.testing.expect(sh.waitTaken(false, 10_000));
}

test "a full-screen program takes the tty until it quits" {
    if (comptime builtin.os.tag != .linux) return error.SkipZigTest;
    // The two shapes a human actually loses a terminal to: an editor that takes
    // the alternate screen, and a pager that does not. Both are skipped rather
    // than failed where they are not installed.
    const cases = [_]struct { bin: [*:0]const u8, run: []const u8, quit: []const u8 }{
        // -u NONE -i NONE: no vimrc, no viminfo — this must not touch the
        // developer's own files, and an rc that starts a plugin would change
        // the process tree under test.
        .{ .bin = "/usr/bin/vim", .run = "vim -u NONE -i NONE\n", .quit = "\x1b:q!\r" },
        // LESS= so a developer's own -F (quit if one screen) cannot make the
        // pager exit before it is asked to
        .{ .bin = "/usr/bin/less", .run = "env LESS= less /etc/hosts\n", .quit = "q" },
    };
    var ran: usize = 0;
    for (cases) |c| {
        if (!haveFile(c.bin)) continue;
        var sh = TestShell.start() orelse return error.SkipZigTest;
        defer sh.stop();
        sh.send(c.run);
        try std.testing.expect(sh.waitTaken(true, 10_000));
        sh.forget();
        sh.send(c.quit);
        try std.testing.expect(sh.waitTaken(false, 10_000));
        try std.testing.expect(sh.waitText(test_prompt, 10_000));
        ran += 1;
    }
    if (ran == 0) return error.SkipZigTest;
}