path: root/src/file_pane.zig

//! File panes: everything a Pane does BECAUSE it has `file: ?State` set — the
//! disk read, the content swap undo/redo commits through, the tree-sitter
//! highlight window, and the two render passes only a file has (the line
//! number gutter and the syntax recolor). The rest of a file pane's behaviour
//! is the pane machinery in pardes.zig, which does not care what kind it is.
const std = @import("std");
const vaxis = @import("vaxis");
const pardes = @import("pardes.zig");
const config = @import("config.zig");
const Pardes = pardes.Pardes;
const Pane = pardes.Pane;
const modal = @import("modal.zig");
const look = @import("look.zig");
const output_pane = @import("output_pane.zig");
const syntax = @import("syntax.zig");
const tracy = @import("tracy.zig");
const term_pane = @import("term_pane.zig");
const dump = @import("dump.zig");

const SYNTAX_CONTEXT_AFTER_ROWS: usize = 2;
/// EDIT BOUNDARIES REMEMBERED PER FILE PANE. Every entry owns a gpa copy of
/// the WHOLE file, so this number multiplies heap, not just the pane: 256 of
/// them is not a bound a 384 KiB board could ever reach anyway. `pushHistory`
/// evicts and frees the oldest once full, so the smaller ring loses the
/// deepest undo steps and nothing else — no truncation, no dropped edit.
const undo_max = if (@import("pardes_config").platform == .p4) 16 else 256;

/// Content and primary selection at one file edit boundary. Keeping only the
/// primary avoids putting pardes.MAX_SELS ranges in every history entry.
pub const Snapshot = struct {
    content: []u8,
    cur_row: i32,
    cur_col: i32,
    vsel: pardes.CharSel,
};

/// A file pane's backing: owned content, its derived caches, and undo history.
pub const State = struct {
    path: []u8,
    content: []u8,
    /// Monotonic content identity for asynchronous edits. Every content swap
    /// goes through setContent, which bumps this; a pipe completion accepted
    /// against another revision would overwrite intervening work.
    revision: u32 = 0,
    /// Revision last known to match disk, after Save or an external reload.
    /// Equal means the screen matches disk.
    saved_revision: u32 = 0,
    /// Non-null for a generated output buffer rather than an on-disk file.
    output: ?output_pane.Output = null,
    scroll: usize = 0,
    /// `line_starts[i]` is line i's byte offset. Empty means not built yet.
    line_starts: []usize = &.{},
    /// One tree_sitter_gpa-owned syntax.Syn byte per highlighted source byte.
    highlights: []u8 = &.{},
    highlight_start: usize = 0,
    syntax_dirty: bool = true,
    undo: [undo_max]Snapshot = undefined,
    undo_len: usize = 0,
    redo: [undo_max]Snapshot = undefined,
    redo_len: usize = 0,
};

/// Serialize file-owned bytes and identity; output origin vocabulary is
/// supplied by output_pane at the core dispatch edge.
pub fn dumpPane(
    arena: std.mem.Allocator,
    pane: *const Pane,
    file: *const State,
    tag: []const u8,
    body: []const u8,
    scroll: usize,
    origin: []const u8,
    origin_arg: []const u8,
) !dump.Pane {
    return .{
        .kind = .file,
        .tag = tag,
        .body = body,
        .scroll = scroll,
        .cols = pane.cols,
        .rows = pane.rows,
        .vweight = pane.vweight,
        .file = .{
            .path = file.path,
            .content = file.content,
            .content_b64 = try dump.encodeBytes(arena, file.content),
            .origin = origin,
            .origin_arg = origin_arg,
        },
    };
}

pub fn graphemeDisplayWidth(grapheme: []const u8) usize {
    if (std.mem.eql(u8, grapheme, "\t")) return config.tab_width;
    // A one-byte printable ASCII grapheme is one cell, and saying so here rather than asking
    // `gwidth` costs a comparison instead of a Unicode table walk. `gwidth` was 6.9% of a profiled
    // keystroke at the P4's geometry, essentially all of it answering this question about `y`.
    // Bounded to 0x20..0x7e on purpose: DEL and the C0 controls are not one printable cell, and
    // `gwidth` is still the authority on them.
    if (grapheme.len == 1 and grapheme[0] >= 0x20 and grapheme[0] < 0x7f) return 1;
    return @max(1, @as(usize, vaxis.gwidth.gwidth(grapheme, .unicode)));
}

pub fn byteDisplayWidth(byte: u8) usize {
    return if (byte == '\t') config.tab_width else 1;
}

pub fn displayWidth(text: []const u8) usize {
    var width: usize = 0;
    var at: usize = 0;
    while (at < text.len) {
        const end = modal.nextGrapheme(text, at);
        width +|= graphemeDisplayWidth(text[at..end]);
        at = end;
    }
    return width;
}

/// Source byte at a zero-based display column. Every cell occupied by a tab
/// maps back to that one tab byte.
pub fn byteAtDisplay(text: []const u8, display_col: usize) usize {
    var col: usize = 0;
    var at: usize = 0;
    while (at < text.len) {
        const end = modal.nextGrapheme(text, at);
        const next = col +| graphemeDisplayWidth(text[at..end]);
        if (display_col < next) return at;
        col = next;
        at = end;
    }
    return text.len;
}

/// File cursor columns may live past EOL. Tabs expand before that boundary;
/// every virtual column after it remains one screen cell.
pub fn rawDisplayCol(line_text: []const u8, raw_col: usize) usize {
    const bounded = modal.graphemeStart(line_text, @min(raw_col, line_text.len));
    return displayWidth(line_text[0..bounded]) +| (raw_col -| line_text.len);
}

pub fn rawAtDisplay(line_text: []const u8, display_col: usize) usize {
    const width = displayWidth(line_text);
    if (display_col > width) return line_text.len +| (display_col - width);
    return byteAtDisplay(line_text, display_col);
}

pub fn byteAtDisplayFrom(line_text: []const u8, from_raw: usize, display_col: usize) usize {
    if (from_raw >= line_text.len) return from_raw +| display_col;
    const from = modal.graphemeStart(line_text, from_raw);
    return from +| rawAtDisplay(line_text[from..], display_col);
}

pub fn lineDisplayOffset(line_text: []const u8, from_raw: usize, to_raw: usize) i32 {
    const from_display = rawDisplayCol(line_text, from_raw);
    const to_display = rawDisplayCol(line_text, to_raw);
    if (to_display >= from_display) return @intCast(to_display - from_display);
    return -@as(i32, @intCast(from_display - to_display));
}

pub fn lineDisplayEndOffset(line_text: []const u8, from_raw: usize, at_raw: usize) i32 {
    const start = lineDisplayOffset(line_text, from_raw, at_raw);
    if (at_raw >= line_text.len) return start;
    const at = modal.graphemeStart(line_text, at_raw);
    const end = modal.nextGrapheme(line_text, at);
    return start + @as(i32, @intCast(graphemeDisplayWidth(line_text[at..end]))) - 1;
}

pub fn sourceLine(pane: *const Pane, row: i32) []const u8 {
    const f = pane.file orelse return "";
    if (row < 0) return "";
    return modal.lineSlice(f.content, @intCast(row));
}

pub fn displayOffset(pane: *const Pane, row: i32, from_raw: i32, to_raw: i32) i32 {
    const line_text = sourceLine(pane, row);
    const from: usize = @intCast(@max(0, from_raw));
    const to: usize = @intCast(@max(0, to_raw));
    return lineDisplayOffset(line_text, from, to);
}

pub fn displayEndOffset(pane: *const Pane, row: i32, from_raw: i32, at_raw: i32) i32 {
    const line_text = sourceLine(pane, row);
    return lineDisplayEndOffset(line_text, @intCast(@max(0, from_raw)), @intCast(@max(0, at_raw)));
}

pub fn byteAtRowDisplay(pane: *const Pane, row: i32, from_raw: i32, display_col: i32) i32 {
    const line_text = sourceLine(pane, row);
    return @intCast(byteAtDisplayFrom(line_text, @intCast(@max(0, from_raw)), @intCast(@max(0, display_col))));
}

/// Convert between rendered cells and UTF-8 byte columns. Tag rows always
/// need grapheme conversion; file body rows additionally skip PREFIX_W.
pub fn renderedLineByteCol(pane: *const Pane, row: i32, line_text: []const u8, display_col: usize) usize {
    if (row < pardes.BOX_H) return rawAtDisplay(line_text, display_col);
    if (pane.file == null) return rawAtDisplay(line_text, display_col);
    const prefix = @min(@as(usize, config.PREFIX_W), line_text.len);
    if (display_col <= prefix) return display_col;
    return prefix +| rawAtDisplay(line_text[prefix..], display_col - prefix);
}

pub fn renderedLineDisplayCol(pane: *const Pane, row: i32, line_text: []const u8, byte_col: usize) usize {
    if (row < pardes.BOX_H) return rawDisplayCol(line_text, byte_col);
    if (pane.file == null) return rawDisplayCol(line_text, byte_col);
    const prefix = @min(@as(usize, config.PREFIX_W), line_text.len);
    if (byte_col <= prefix) return byte_col;
    return prefix +| rawDisplayCol(line_text[prefix..], byte_col - prefix);
}

test "display columns map complete Unicode graphemes" {
    const text = "é界e\u{301}x";
    try std.testing.expectEqual(@as(usize, 5), displayWidth(text));
    try std.testing.expectEqual(@as(usize, 0), byteAtDisplay(text, 0));
    try std.testing.expectEqual(@as(usize, 2), byteAtDisplay(text, 1));
    try std.testing.expectEqual(@as(usize, 2), byteAtDisplay(text, 2));
    try std.testing.expectEqual(@as(usize, 5), byteAtDisplay(text, 3));
    try std.testing.expectEqual(@as(usize, 8), byteAtDisplay(text, 4));
    try std.testing.expectEqual(text.len, byteAtDisplay(text, 5));
    try std.testing.expectEqual(@as(usize, 3), rawDisplayCol(text, 5));
    try std.testing.expectEqual(@as(usize, 5), rawAtDisplay(text, 3));
    try std.testing.expectEqual(@as(usize, 2), graphemeDisplayWidth("👩\u{200d}🚀"));
}

fn fitEnd(text: []const u8, start: usize, width: usize) usize {
    var end = start;
    var used: usize = 0;
    // ASCII RUN. This is the loop a wrapped line pays per character, and it asks two function calls
    // to learn what arithmetic knows: `modal.nextGrapheme` and `graphemeDisplayWidth` each answer
    // ASCII in constant time, but they answer once per character and a 640-column line asks 640
    // times. A printable ASCII byte whose successor is also ASCII is a complete grapheme cluster one
    // column wide - the same guard, and the same reason, as `Surface.print` and `modal.nextGrapheme`
    // - so consume the run here and leave anything else to the general path below.
    while (used < width and end < text.len) {
        const b = text[end];
        if (b < 0x20 or b >= 0x7f) break;
        if (end + 1 < text.len and text[end + 1] >= 0x80) break;
        used += 1;
        end += 1;
    }
    while (end < text.len) {
        const next_end = modal.nextGrapheme(text, end);
        const next_used = used +| graphemeDisplayWidth(text[end..next_end]);
        if (next_used > width) return if (end == start) next_end else end;
        used = next_used;
        end = next_end;
    }
    return end;
}

test "the ASCII run in fitEnd cuts where the grapheme walk would" {
    // fitEnd decides where a wrapped row BREAKS, so a fast path that is off by one column moves
    // text on screen. This pins it to the general walk it replaces rather than to a transcribed
    // expectation: same inputs, both routes, every width from 0 past the end of the string.
    const reference = struct {
        fn fitEnd(text: []const u8, start: usize, width: usize) usize {
            var end = start;
            var used: usize = 0;
            while (end < text.len) {
                const next_end = modal.nextGrapheme(text, end);
                const next_used = used +| graphemeDisplayWidth(text[end..next_end]);
                if (next_used > width) return if (end == start) next_end else end;
                used = next_used;
                end = next_end;
            }
            return end;
        }
    }.fitEnd;

    const cases = [_][]const u8{
        "",
        "hello world",
        // the fast path must hand over at the first non-ASCII byte, mid-run
        "abc\u{00e9}def",
        // a wide glyph is two columns, so a width boundary can land inside it
        "ab\u{4e16}\u{754c}cd",
        // a cluster the fast path must not split
        "a\u{0301}bc",
        // tabs and controls are excluded from the fast path by the range test
        "ab\tcd",
        "ab\rcd",
        // an ASCII byte followed by a continuation byte is NOT its own cluster
        "e\u{0301}x",
        "\u{1f1e6}\u{1f1e7}ok",
    };
    for (cases) |text| {
        var width: usize = 0;
        while (width <= text.len + 3) : (width += 1) {
            var start: usize = 0;
            while (start <= text.len) : (start += 1) {
                try std.testing.expectEqual(
                    reference(text, start, width),
                    fitEnd(text, start, width),
                );
            }
        }
    }
}

pub fn lineCount(content: []const u8) usize {
    return std.mem.count(u8, content, "\n") + 1;
}

/// THE LINE INDEX, built on demand: `line_starts[i]` is the byte offset where
/// line i begins and its length is the line count. Without it, every question
/// about lines is a scan from byte 0, and a file pane asks several of them per
/// keystroke — the scrollbar's total, the scroll clamp, the syntax window's
/// bounds, the body's first visible line. On a 300k-line file that was ~35% of
/// the whole frame, and it is what made a single `j` cost 25ms.
///
/// INVALIDATION — the part that rots if nobody says it out loud. The index is
/// dropped in EXACTLY ONE PLACE: setContent, immediately below, which is the
/// funnel every content swap in the editor already goes through (typing, undo,
/// redo, a save's normalisation, an output buffer refilling itself). A State
/// built by a struct literal starts with an empty index, and empty reads as
/// "not built yet" — a real index always has at least one entry, because a
/// file always has at least one line. So there is one and only one way to make
/// this wrong: assign `f.content` without going through setContent. Don't.
///
/// Fails only when the index could not be allocated. nlines and lineStart
/// swallow that and scan the old way, so OOM there is slow rather than wrong;
/// callers that need the whole table say `try` and drop the keystroke, which
/// is what they already did when their own arena ran out.
pub fn lineIndex(gpa: std.mem.Allocator, f: *State) ![]const usize {
    if (f.line_starts.len > 0) return f.line_starts;
    // Exact allocation: deinitPane frees `line_starts` itself, so the stored
    // slice must span the complete allocation rather than spare capacity.
    const starts = try gpa.alloc(usize, lineCount(f.content));
    starts[0] = 0;
    var i: usize = 1;
    var off: usize = 0;
    while (std.mem.indexOfScalarPos(u8, f.content, off, '\n')) |nl| {
        off = nl + 1;
        starts[i] = off;
        i += 1;
    }
    f.line_starts = starts;
    return starts;
}

/// line count, O(1) once the index is warm
pub fn nlines(gpa: std.mem.Allocator, f: *State) usize {
    const idx = lineIndex(gpa, f) catch return lineCount(f.content);
    return idx.len;
}

/// byte offset of line `row`, or content.len past the end — modal
/// .lineStartOffset's contract exactly, without its walk
pub fn lineStart(gpa: std.mem.Allocator, f: *State, row: usize) usize {
    const idx = lineIndex(gpa, f) catch return modal.lineStartOffset(f.content, row);
    return if (row >= idx.len) f.content.len else idx[row];
}

pub fn cursorLines(arena: std.mem.Allocator, pane: *Pane, f: *State) ![]const []const u8 {
    const index = try lineIndex(pane.gpa, f);
    const lines = try arena.alloc([]const u8, index.len);
    for (index, 0..) |start, i| {
        const end = if (i + 1 < index.len) index[i + 1] - 1 else f.content.len;
        lines[i] = f.content[start..end];
    }
    return lines;
}

/// Use the file's line index only when `text` is its complete live content.
/// Edit-buffer fragments and other temporary text retain modal's scan path.
fn contentIndex(pane: *Pane, text: []const u8) ?[]const usize {
    const f = if (pane.file) |*file| file else return null;
    if (text.ptr != f.content.ptr or text.len != f.content.len) return null;
    return lineIndex(pane.gpa, f) catch null;
}

pub fn textOffset(pane: *Pane, text: []const u8, cursor: modal.Cursor) usize {
    const index = contentIndex(pane, text) orelse return modal.hxOff(text, cursor);
    const row = @min(cursor.row, index.len - 1);
    const start = index[row];
    const end = if (row + 1 < index.len) index[row + 1] - 1 else text.len;
    return start + modal.graphemeStart(text[start..end], @min(cursor.col, end - start));
}

pub fn textLineStart(pane: *Pane, text: []const u8, row: usize) usize {
    const index = contentIndex(pane, text) orelse return modal.lineStartOffset(text, row);
    return if (row >= index.len) text.len else index[row];
}

pub fn textLineCount(pane: *Pane, text: []const u8) usize {
    const index = contentIndex(pane, text) orelse return modal.hxLineCount(text);
    return index.len;
}

pub fn textPosition(pane: *Pane, text: []const u8, offset: usize) modal.Cursor {
    const index = contentIndex(pane, text) orelse return modal.hxPos(text, offset);
    const bounded = @min(offset, text.len);
    const row = std.sort.upperBound(usize, index, bounded, struct {
        fn cmp(key: usize, item: usize) std.math.Order {
            return std.math.order(key, item);
        }
    }.cmp) - 1;
    const start = index[row];
    const end = if (row + 1 < index.len) index[row + 1] - 1 else text.len;
    return .{ .row = row, .col = modal.graphemeStart(text[start..end], @min(bounded - start, end - start)) };
}

pub fn open(p: *Pardes, id: usize, path: []const u8, line: usize) !*Pane {
    const content = try look.readFile(p.gpa, path);
    errdefer p.gpa.free(content);
    const path_copy = try p.gpa.dupe(u8, path);
    errdefer p.gpa.free(path_copy);
    const pane = try p.newDocPane(id);
    const total = lineCount(content);
    const scroll: usize = if (line > 0 and line <= total) line - 1 else 0;
    pane.file = .{ .path = path_copy, .content = content, .scroll = scroll };
    pane.cur_pinned = true;
    pane.cur_row = @intCast(scroll);
    // watches follow pane lifetime: this is the only place a real file is read
    // off disk, and deinitPane is the only place one goes away
    p.emit(.{ .watch = .{ .pane = @intCast(id), .on = true } });
    return pane;
}

/// Rebuild a dumped file or output buffer. Byte ownership, output identity,
/// cursor projection, and file watching are all properties of this payload;
/// column registration and custom tag restoration remain core invariants.
pub fn restore(p: *Pardes, id: usize, src: dump.Pane) !*Pane {
    const saved = src.file.?;
    const content: []u8 = if (saved.content_b64.len > 0)
        try dump.decodeBytes(p.gpa, saved.content_b64)
    else
        try p.gpa.dupe(u8, saved.content);
    errdefer p.gpa.free(content);
    const path = try p.gpa.dupe(u8, saved.path);
    errdefer p.gpa.free(path);

    const output: ?output_pane.Output = if (output_pane.fromWord(saved.origin)) |origin| blk: {
        var value: output_pane.Output = .{ .from = origin };
        try output_pane.setArg(&value, saved.origin_arg);
        break :blk value;
    } else null;

    const pane = try p.newDocPane(id);
    pane.file = .{ .path = path, .content = content, .output = output, .scroll = src.scroll };
    pane.cur_pinned = true;
    pane.cur_row = @intCast(src.scroll);
    pane.cols = @max(1, src.cols);
    pane.rows = @max(1, src.rows);
    // A restored file is watched exactly like one opened from disk. Its dump
    // bytes may differ from disk; the first external write reconciles them and
    // leaves the restored version one undo away. Output buffers have no file.
    if (output == null) p.emit(.{ .watch = .{ .pane = @intCast(id), .on = true } });
    return pane;
}

/// Release the complete file payload while the owning pane is still installed
/// (the slot is needed to identify a disappearing file watch). Common pane
/// overlays and the shared terminal stub remain the core's responsibility.
pub fn deinit(p: *Pardes, pane: *Pane, file: *State) void {
    if (file.output == null) for (p.panes, 0..) |slot, id| {
        if (slot == pane) p.emit(.{ .watch = .{ .pane = @intCast(id), .on = false } });
    };
    p.gpa.free(file.path);
    p.gpa.free(file.content);
    if (file.line_starts.len > 0) p.gpa.free(file.line_starts);
    if (file.highlights.len > 0) p.tree_sitter_gpa.free(file.highlights);
    for (file.undo[0..file.undo_len]) |snap| p.gpa.free(snap.content);
    for (file.redo[0..file.redo_len]) |snap| p.gpa.free(snap.content);
}

/// TELL A SCRIPT WHAT CHANGED, when one is listening.
///
/// acme reports edits from the two places that make them — `textinsert` and
/// `textdelete`, which already know their range — so a replacement arrives as
/// a `D` record and then an `I`. pardes has no such pair: every edit lands
/// here as a whole new buffer, so the range is recovered by DIFFING, and
/// `acmefs.noteReplace` owns both the diff and the D-then-I order.
///
/// The cost is two vectorised scans of the content, and it is paid only while
/// a script holds an `event` file open (`p.fs.listeners`); the editor nobody
/// is scripting does one branch. The pane lookup is a walk of at most
/// MAX_PANES slots comparing the FILE pointer — a file pane's state is stored
/// inline in its pane, so that identifies the pane exactly.
fn reportEdit(p: *Pardes, f: *State, new: []const u8) void {
    if (p.fs.listeners == 0) return;
    const id = for (p.panes, 0..) |slot, i| {
        const pane = slot orelse continue;
        if (pane.file) |*state| if (state == f) break i;
    } else return;
    pardes.acmefs.noteReplace(p, id, false, f.content, new);
}

/// The ONE content swap. Everything that edits a file pane lands here, which
/// is what lets the line index above have a single invalidation point — and
/// is why one diff HERE is every body edit a script can be told about.
pub fn setContent(p: *Pardes, f: *State, new: []u8) void {
    reportEdit(p, f, new);
    p.gpa.free(f.content);
    f.content = new;
    f.revision +%= 1;
    if (f.line_starts.len > 0) p.gpa.free(f.line_starts);
    f.line_starts = &.{};
    // the highlights go too, and not just because they are stale: their byte
    // range is what refreshHighlights tests a scroll against, and a range
    // measured on the OLD content would let it skip a re-parse it needs
    if (f.highlights.len > 0) p.tree_sitter_gpa.free(f.highlights);
    f.highlights = &.{};
    f.highlight_start = 0;
    f.syntax_dirty = true;
}

/// undo/redo restores the selection recorded with the snapshot (helix keeps
/// selections in its history transactions), clamped: the content it was taken
/// against may be shorter than the one it is being restored onto.
pub fn restoreSnap(pane: *Pane, f: *State, snap: Snapshot) void {
    const n = nlines(pane.gpa, f);
    const row: usize = @min(@as(usize, @intCast(@max(0, snap.cur_row))), n - 1);
    const llen = modal.lineSlice(f.content, row).len;
    pane.cur_row = @intCast(row);
    pane.cur_col = @intCast(@min(@as(usize, @intCast(@max(0, snap.cur_col))), llen));
    pane.vsel = snap.vsel;
    pane.msel.active = false;
    pane.cur_pinned = true;
    pane.sticky_col = -1;
    pane.ensureCursorVisible();
}

fn pushHistory(gpa: std.mem.Allocator, slots: []Snapshot, len: *usize, snap: Snapshot) void {
    if (len.* == slots.len) {
        gpa.free(slots[0].content);
        std.mem.copyForwards(Snapshot, slots[0 .. slots.len - 1], slots[1..]);
        len.* -= 1;
    }
    slots[len.*] = snap;
    len.* += 1;
}

pub fn pushUndo(p: *Pardes, pane: *Pane) void {
    const f = if (pane.file) |*file| file else return;
    if (f.undo_len > 0 and std.mem.eql(u8, f.undo[f.undo_len - 1].content, f.content)) return;
    const snap: Snapshot = .{
        .content = p.gpa.dupe(u8, f.content) catch return,
        .cur_row = pane.cur_row,
        .cur_col = pane.cur_col,
        .vsel = pane.vsel,
    };
    pushHistory(p.gpa, &f.undo, &f.undo_len, snap);
    for (f.redo[0..f.redo_len]) |item| p.gpa.free(item.content);
    f.redo_len = 0;
}

pub fn undo(p: *Pardes, pane: *Pane) void {
    const f = if (pane.file) |*file| file else return;
    if (f.undo_len == 0) return;
    const current: Snapshot = .{
        .content = p.gpa.dupe(u8, f.content) catch return,
        .cur_row = pane.cur_row,
        .cur_col = pane.cur_col,
        .vsel = pane.vsel,
    };
    pushHistory(p.gpa, &f.redo, &f.redo_len, current);
    f.undo_len -= 1;
    const previous = f.undo[f.undo_len];
    setContent(p, f, previous.content);
    restoreSnap(pane, f, previous);
}

pub fn redo(p: *Pardes, pane: *Pane) void {
    const f = if (pane.file) |*file| file else return;
    if (f.redo_len == 0) return;
    const current: Snapshot = .{
        .content = p.gpa.dupe(u8, f.content) catch return,
        .cur_row = pane.cur_row,
        .cur_col = pane.cur_col,
        .vsel = pane.vsel,
    };
    pushHistory(p.gpa, &f.undo, &f.undo_len, current);
    f.redo_len -= 1;
    const next = f.redo[f.redo_len];
    setContent(p, f, next.content);
    restoreSnap(pane, f, next);
}

/// Commit an externally rewritten file onto the same undo history as typed
/// edits. Unsaved work remains one `u` away; there is no third merge state.
pub fn changed(p: *Pardes, id: u8, bytes: []const u8) void {
    const pane = p.panes[id] orelse return;
    const f = if (pane.file) |*file| file else return;
    if (std.mem.eql(u8, f.content, bytes)) return;
    const new = p.gpa.dupe(u8, bytes) catch return;
    pushUndo(p, pane);
    setContent(p, f, new);
    // These bytes came from the watched path, so the new on-screen revision
    // is already saved. Undoing back to displaced local work bumps revision
    // again and makes that restored edit dirty, as it should.
    f.saved_revision = f.revision;
    // restoreSnap only consumes cursor/selection from this synthetic snapshot.
    restoreSnap(pane, f, .{
        .content = undefined,
        .cur_row = pane.cur_row,
        .cur_col = pane.cur_col,
        .vsel = pane.vsel,
    });
}

/// re-highlight the visible window of any file whose syntax went stale
/// (edit, scroll, load) — visible-range-first so big files stay snappy
pub fn refreshHighlights(p: *Pardes) void {
    const tz = tracy.zone(@src(), "refreshHighlights");
    defer tz.end();
    for (p.panes) |slot| {
        const pane = slot orelse continue;
        if (pane.file == null) continue;
        const f = &pane.file.?;
        if (!f.syntax_dirty) continue;
        if (!p.settings.colors) {
            if (f.highlights.len > 0) p.tree_sitter_gpa.free(f.highlights);
            f.highlights = &.{};
            f.highlight_start = 0;
            f.syntax_dirty = false;
            continue;
        }
        // What the screen needs coloured right now. If the last parse still
        // covers it, this scroll is free — and that is the whole point of the
        // slack below. Highlights only ever survive while the CONTENT does:
        // setContent throws them away, so these byte offsets cannot be stale.
        const need_start = lineStart(p.gpa, f, f.scroll);
        const need_end = @max(need_start, lineStart(p.gpa, f, f.scroll + pane.rows + SYNTAX_CONTEXT_AFTER_ROWS));
        if (f.highlights.len > 0 and need_start >= f.highlight_start and
            need_end <= f.highlight_start + f.highlights.len)
        {
            f.syntax_dirty = false;
            continue;
        }
        // How much MORE than the screen to parse. An edit or a fresh open has
        // no previous parse to widen (setContent throws the highlights away),
        // and slack would be pure loss there: every keystroke of typing pays
        // this parse and none of it is amortised over anything. A SCROLL that
        // outran the covered range is the opposite case — take a screenful
        // above and below and the next ~pane.rows rows cost nothing at all.
        // Scrolling used to re-parse the visible window on every single row,
        // which on a file with 8000-column lines is a third of a megabyte per
        // keypress. Three screens once beats one screen forty times.
        //
        // The slack also means those lines are parsed with real context above
        // them, so a construct that opens off-screen now colours correctly —
        // a fidelity gain, and one that cannot reach a file shown from the top
        // (scroll 0 clamps the window to exactly what it always was).
        const slack: usize = if (f.highlights.len == 0) 0 else pane.rows;
        const start = lineStart(p.gpa, f, f.scroll -| slack);
        const end = @max(start, lineStart(p.gpa, f, f.scroll + pane.rows + SYNTAX_CONTEXT_AFTER_ROWS + slack));
        const new_highlights = (switch (pane.colorAlgo()) {
            .diff => syntax.highlightDiff(p.tree_sitter_gpa, f.content, start, end),
            else => syntax.highlightFileRange(p.tree_sitter_gpa, f.path, f.content, start, end),
        }) catch {
            f.syntax_dirty = false;
            continue;
        };
        if (f.highlights.len > 0) p.tree_sitter_gpa.free(f.highlights);
        f.highlights = new_highlights;
        f.highlight_start = if (f.highlights.len > 0) start else 0;
        f.syntax_dirty = false;
    }
}

/// The width a wrapped row of THIS pane holds, in cells, or 0 when the pane is
/// not wrapping — the render decision, named once so motion cannot disagree
/// with paint. One column is left for the break marker: a row that filled its
/// last cell would have nowhere to say it continues. A pane taller than the
/// map refuses to wrap rather than record part of itself (see Pane.wrap_line).
pub fn wrapWidth(pane: *const Pane, wrap: bool) usize {
    if (!wrap or pane.rows > pane.wrap_line.len) return 0;
    return @max(1, @as(usize, pane.cols -| config.PREFIX_W) -| 1);
}

pub const VisualRow = struct { start: usize, end: usize };

/// The visual row of `line` holding byte `col`: `[start, end)`, where `end` is
/// where the next visual row of the same line begins and equals `line.len` on
/// the last one. This is the same walk `fillBody` renders with, so `gj`/`gk`
/// step exactly the breaks a reader sees. `width == 0` (not wrapping) makes
/// the whole line one visual row, which is what collapses visual motion onto
/// textual motion instead of special-casing it upstream.
pub fn visualRow(line: []const u8, col: usize, width: usize) VisualRow {
    if (width == 0) return .{ .start = 0, .end = line.len };
    var start: usize = 0;
    while (true) {
        const end = fitEnd(line, start, width);
        if (col < end or end >= line.len) return .{ .start = start, .end = end };
        start = end;
    }
}

test "visual rows partition a line at the breaks the body renders" {
    const line = "abcdefgh";
    try std.testing.expectEqual(VisualRow{ .start = 0, .end = line.len }, visualRow(line, 5, 0));
    try std.testing.expectEqual(VisualRow{ .start = 0, .end = 3 }, visualRow(line, 0, 3));
    try std.testing.expectEqual(VisualRow{ .start = 0, .end = 3 }, visualRow(line, 2, 3));
    try std.testing.expectEqual(VisualRow{ .start = 3, .end = 6 }, visualRow(line, 3, 3));
    // Past the end (a cursor on the newline) names the LAST row, and a short
    // line is one row however narrow the pane is.
    try std.testing.expectEqual(VisualRow{ .start = 6, .end = 8 }, visualRow(line, line.len, 3));
    try std.testing.expectEqual(VisualRow{ .start = 0, .end = 0 }, visualRow("", 0, 3));
    // A grapheme wider than the row still occupies exactly one row.
    try std.testing.expectEqual(VisualRow{ .start = 0, .end = 4 }, visualRow("👩x", 0, 1));
}

/// the body a file pane renders: `pane.rows` SCREEN rows from the scroll
/// offset, each behind its right-aligned line number, then cut by hscroll.
///
/// With `wrap` on a line too long for the pane takes several rows instead of
/// running off the right edge, and this is where that happens — it is a render
/// property, and the one thing the rest of the editor reads back is the map:
/// which line each row showed and at which byte column it began, recorded into
/// pane.wrap_line/wrap_col as the rows are built. `wrap_n` stays 0 for an
/// unwrapped body, and that is the value the readers treat as "rows are
/// lines", so the off path never consults an array.
pub fn bodyText(arena: std.mem.Allocator, pane: *Pane, f: *State, wrap: bool) ![]const u8 {
    const width = wrapWidth(pane, wrap);
    pane.wrap_n = 0;

    // Count the exact rendered bytes first. Unwrapped source lines are not
    // bounded by the pane width, so a rows*cols buffer would either truncate
    // them or quietly restore a growable builder under another name.
    const len = fillBody(null, pane, f, width, false);
    const out = try arena.alloc(u8, len);
    const filled = fillBody(out, pane, f, width, true);
    std.debug.assert(filled == out.len);
    return out;
}

/// Run the file-body row walk. With no destination it is the exact sizing
/// pass; with one it fills that allocation and records the wrapping map.
fn fillBody(dst: ?[]u8, pane: *Pane, f: *State, width: usize, record_wrap: bool) usize {
    if (record_wrap) pane.wrap_n = 0;
    // start ON the first visible line instead of walking the file to it: this
    // walk was O(f.scroll) and recolorSyntax below ran the identical one again
    var flines = std.mem.splitScalar(u8, f.content[lineStart(pane.gpa, f, f.scroll)..], '\n');
    // scrolled past EOF (an edit shortened the file under a stale scroll): the
    // old walk left the iterator dry, so drop the one empty line a slice split
    // still yields, or the body grows a phantom numbered row
    if (f.scroll >= nlines(pane.gpa, f)) _ = flines.next();
    // the line the NEXT row comes from and the byte column of it that row
    // starts at — the two the map records, walked forward by the loop
    var abs: i32 = @intCast(f.scroll);
    var at: usize = 0;
    var cur = flines.next();
    var written: usize = 0;
    for (0..pane.rows) |i| {
        if (i > 0) {
            if (dst) |out| out[written] = '\n';
            written += 1;
        }
        if (width > 0 and record_wrap) {
            pane.wrap_line[i] = abs;
            pane.wrap_col[i] = @intCast(at);
            pane.wrap_n = @intCast(i + 1);
        }
        if (cur) |text| {
            var lbuf: [16]u8 = undefined;
            // unsigned: {d} prints a leading '+' for signed ints
            const lineno: usize = @intCast(abs + 1);
            // the number belongs to the LINE, so only its first row carries
            // one — repeated down a wrapped line it would read as several
            // lines, which is exactly what this is not
            const prefix = if (at > 0)
                "     "
            else
                std.fmt.bufPrint(&lbuf, "{d: >4} ", .{lineno}) catch "     ";
            if (dst) |out| @memcpy(out[written..][0..prefix.len], prefix);
            written += prefix.len;

            // Wrap and horizontal-scroll cuts are always grapheme boundaries.
            // Source columns remain byte offsets, while widths are terminal
            // cells; keeping the conversion here prevents a view operation
            // from manufacturing malformed UTF-8.
            const end = if (width == 0) text.len else fitEnd(text, at, width);
            const take = end - at;
            const cut = if (pane.hscroll > 0 and width == 0)
                modal.graphemeStart(text[at..end], @min(@as(usize, @intCast(pane.hscroll)), take))
            else
                0;
            const shown = text[at + cut .. end];
            if (dst) |out| @memcpy(out[written..][0..shown.len], shown);
            written += shown.len;
            if (width > 0 and end < text.len) {
                at = end;
            } else {
                abs += 1;
                at = 0;
                cur = flines.next();
            }
        } else abs += 1;
    }
    return written;
}

/// line-number gutter: mute the first PREFIX_W columns. Cheap chrome, not
/// gated on settings.colors; selection/cursor passes still win. The cursor row's
/// number takes the tag style (same row math as renderPane's cursor pass) so
/// the eye finds the current line.
pub fn drawGutter(p: *Pardes, pane: *Pane, r: pardes.Rect, tx: u16, tw: u16, body_h: u16, active: bool) void {
    const s = &p.surface;
    const ch = p.chromeTheme();
    const goff = pane.scroll();
    const gcur = term_pane.gridCursor(pane);
    const gcrow = if (pane.cur_pinned) pane.cur_row else @as(i32, gcur.y) + goff;
    // the cursor's LINE, not its row: wrapped, one line owns a run of rows and
    // the number sits on the first of them, so the whole run lights up — the
    // gutter is naming the line you are on, and that is still one line
    const cur_line: i32 = if (active and !pane.tag_edit) gcrow else std.math.minInt(i32);
    // the body's first row, the way renderPane derives it (Tagbottom)
    const body_y = if (p.settings.tag_bottom) r.y else r.y + pardes.BOX_H;
    var vr: u16 = 0;
    while (vr < body_h) : (vr += 1) {
        const row_line: i32 = if (pane.wrap_n == 0)
            goff + @as(i32, vr)
        else if (vr < pane.wrap_n) pane.wrap_line[vr] else std.math.maxInt(i32);
        const on_cursor = row_line == cur_line;
        var c: u16 = 0;
        while (c < config.PREFIX_W and c < tw) : (c += 1) {
            const cell = s.at(tx + c, body_y + vr);
            cell.default = false; // paints blank gutter rows too
            if (on_cursor) {
                cell.style.fg = .{ .rgb = ch.tag_fg };
                cell.style.bg = .{ .rgb = ch.tag_bg };
            } else cell.style.fg = .{ .rgb = ch.lineno };
        }
    }
}

const SynStyle = struct { fg: [3]u8, bold: bool };

fn synStyle(p: *Pardes, sy: syntax.Syn) ?SynStyle {
    return switch (sy) {
        .none => null,
        .keyword => .{ .fg = p.theme().kw, .bold = true },
        .string => .{ .fg = p.theme().str, .bold = false },
        .number => .{ .fg = p.theme().num, .bold = false },
        .comment => .{ .fg = p.theme().comment, .bold = true },
    };
}

/// syntax colors: recolor each content cell from its tree-sitter style byte;
/// content starts after the lineno gutter
pub fn recolorSyntax(p: *Pardes, pane: *Pane, f: *State, r: pardes.Rect, tx: u16, tw: u16, body_h: u16) void {
    if (f.highlights.len == 0) return;
    const s = &p.surface;
    const tz_recolor = tracy.zone(@src(), "synRecolor");
    defer tz_recolor.end();
    // indexed start, same as bodyText — an empty tail simply paints nothing
    var flines = std.mem.splitScalar(u8, f.content[lineStart(p.gpa, f, f.scroll)..], '\n');
    const total = nlines(p.gpa, f);
    const body_y = if (p.settings.tag_bottom) r.y else r.y + pardes.BOX_H;
    var vr: u16 = 0;
    while (vr < body_h) : (vr += 1) {
        // A colour has to land on the byte it belongs to, so this walk reads
        // the same map the body was built from: wrapped, the screen row names
        // its own line and the byte column it began at, and it ends where the
        // NEXT row of that line begins. Unwrapped the rows ARE the lines in
        // order and the split iterator is the cheaper walk.
        var base: usize = undefined;
        var line: []const u8 = undefined;
        var hs: usize = @intCast(@max(0, pane.hscroll));
        var limit: usize = undefined;
        if (pane.wrap_n == 0) {
            line = flines.next() orelse break;
            base = @intFromPtr(line.ptr) - @intFromPtr(f.content.ptr);
            limit = line.len;
        } else {
            if (vr >= pane.wrap_n) break;
            const lrow: usize = @intCast(@max(0, pane.wrap_line[vr]));
            if (lrow >= total) break;
            base = lineStart(p.gpa, f, lrow);
            const lend = if (lrow + 1 < total) lineStart(p.gpa, f, lrow + 1) -| 1 else f.content.len;
            line = f.content[base..lend];
            hs = @intCast(pane.wrap_col[vr]);
            limit = if (vr + 1 < pane.wrap_n and pane.wrap_line[vr + 1] == pane.wrap_line[vr])
                @min(line.len, @as(usize, @intCast(pane.wrap_col[vr + 1])))
            else
                line.len;
        }
        hs = modal.graphemeStart(line, @min(hs, line.len));
        var c: usize = 0;
        var screen_c: usize = 0;
        while (hs + c < limit and config.PREFIX_W + screen_c < tw) {
            const grapheme_end = @min(limit, modal.nextGrapheme(line, hs + c));
            const cells = graphemeDisplayWidth(line[hs + c .. grapheme_end]);
            const idx = base + hs + c;
            if (idx >= f.highlight_start) {
                const hidx = idx - f.highlight_start;
                if (hidx < f.highlights.len) {
                    if (synStyle(p, @enumFromInt(f.highlights[hidx]))) |ss| {
                        var fill: usize = 0;
                        while (fill < cells and config.PREFIX_W + screen_c + fill < tw) : (fill += 1) {
                            const cell = s.at(tx + @as(u16, @intCast(config.PREFIX_W + screen_c + fill)), body_y + vr);
                            if (cell.default) continue;
                            cell.style.fg = .{ .rgb = ss.fg };
                            cell.style.bold = ss.bold;
                        }
                    }
                }
            }
            screen_c += cells;
            c = grapheme_end - hs;
        }
    }
}

/// Mark every visible wrapped row which continues onto the next screen row.
/// This is file chrome: it follows syntax recoloring and precedes the shared
/// selection passes, so neither source ink nor the marker can win over a user
/// selection.
pub fn drawWrapMarkers(
    p: *Pardes,
    pane: *const Pane,
    r: pardes.Rect,
    tx: u16,
    tw: u16,
    body_h: u16,
    pane_bg: pardes.Color,
) void {
    if (tw <= config.PREFIX_W + 1) return;
    const body_y = if (p.settings.tag_bottom) r.y else r.y + pardes.BOX_H;
    const marker_fg = p.chromeTheme().lineno;
    var row: u16 = 0;
    while (row + 1 < pane.wrap_n and row + 1 < body_h) : (row += 1) {
        if (pane.wrap_line[row + 1] != pane.wrap_line[row]) continue;
        p.surface.set(tx + tw - 1, body_y + row, config.wrap_marker, .{
            .fg = .{ .rgb = marker_fg },
            .bg = pane_bg,
        });
    }
}

/// Paint one logical file word through the last frame's wrap map. Unlike a
/// rectangular mouse selection, a path may cross continuation rows without
/// highlighting unrelated cells between its endpoints.
pub fn paintWordSelection(
    p: *Pardes,
    pane: *Pane,
    r: pardes.Rect,
    row: i32,
    word_lo: i32,
    word_hi: i32,
    bg: [3]u8,
) void {
    const tx = r.x + config.GUTTER;
    const tw = r.w - config.GUTTER;
    const body_y = if (p.settings.tag_bottom) r.y else r.y + pardes.BOX_H;
    var vr: i32 = 0;
    while (vr + @as(i32, pardes.BOX_H) < @as(i32, r.h)) : (vr += 1) {
        const here = pane.wrapAt(vr);
        if (here.line != row) continue;
        var hi = word_hi;
        const next = pane.wrapAt(vr + 1);
        if (next.line == row) hi = @min(hi, next.at);
        const lo = @max(word_lo, here.at);
        if (hi <= lo) continue;
        const c0 = @as(i32, config.PREFIX_W) + displayOffset(pane, row, here.at, lo);
        const c1 = @as(i32, config.PREFIX_W) + displayEndOffset(pane, row, here.at, hi - 1);
        var col = @max(@as(i32, config.PREFIX_W), c0);
        while (col <= c1 and col < @as(i32, tw)) : (col += 1) {
            const cell = p.surface.at(tx + @as(u16, @intCast(col)), body_y + @as(u16, @intCast(vr)));
            cell.default = false;
            cell.style.bg = .{ .rgb = bg };
        }
    }
}