//! One editable text, acme's Text (editors/acme/dat.h:171-190, text.c): the
//! cursor, the selections and the modal state that edit it, and the undo of
//! an edit buffer. A pane's body is one, its tag is another, and so are the
//! answer typed into a pane's prompt and the tags of the columns and the
//! workspace. Where a body's characters come from --
//! a file's content, a terminal's edit overlay, a PDF's text -- is the
//! window's business (edit.zig's editText and flatSurface), the way acme's
//! Text reads its File; every other text owns its characters. Everything
//! here works on the flat text it is handed.
const std = @import("std");
const panes = @import("panes.zig");
const modal = @import("modal.zig");
const config = @import("config.zig");
const memory = @import("memory.zig");
const Pane = panes.Pane;
const File = panes.File;
const Terminal = panes.Terminal;
const Text = @This();

pub const Mode = enum { normal, insert, tty };

/// Which text of its window this is, acme's `what`. It says where the
/// characters live and whether moving the cursor scrolls a body.
pub const What = enum { body, tag, input, column, workspace };

/// A modal line selection (helix `x`): whole rows [r0, r1], absolute.
pub const LineSel = struct {
    active: bool = false,
    r0: i32 = 0,
    r1: i32 = 0,
};

pub const CharSel = struct {
    active: bool = false,
    row: i32 = 0,
    col: i32 = 0,
    explicit: bool = false,
};

pub const max_selections = memory.limits.selections;

pub const SelRange = struct {
    row: i32,
    col: i32,
    arow: i32,
    acol: i32,
    /// this range's own j/k goal column (helix Range::old_visual_position);
    /// the PRIMARY's copy is Text.sticky_col
    sticky: i32 = -1,
};

what: What = .body,
/// The characters of a text that holds its own (gpa-owned): everything but
/// a body, which reads its pane's. A tag nobody has changed has none yet and
/// shows the default its pane's kind starts with.
own: ?[]u8 = null,
mode: Mode = .normal,
cur_pinned: bool = false,
cur_row: i32 = 0,
cur_col: i32 = 0,
msel: LineSel = .{},
vsel: CharSel = .{},
sels: [max_selections - 1]SelRange = undefined,
nsel: u16 = 0,
select: bool = false,
/// sticky goal column for j/k runs (helix old_visual_position): any
/// non-vertical range write resets it to -1.
sticky_col: i32 = -1,
/// The insert session began with `a`: on Esc every range gives back the
/// one character it was stretched by (helix restore_cursor).
restore_cursor: bool = false,
/// Where the last edit ended, a deletion's start or an insertion's end, in
/// surface rows: for `g.` (helix last_edit_pos).
last_edit: ?struct { row: i32, col: i32 } = null,
/// insert `Ctrl-r` was pressed: the next key names the register to insert
reg_wait: bool = false,
normal: modal.Normal.State = .{},
/// last f/F/t/T motion, for Alt-. repeat
find_op: u8 = 0,
find_ch: u21 = 0,
ed_undo: [Terminal.history_max]Terminal.Snapshot = undefined,
ed_undo_len: usize = 0,
ed_redo: [Terminal.history_max]Terminal.Snapshot = undefined,
ed_redo_len: usize = 0,
/// Set when an edit would have changed characters this text does not own
/// (a tag's computed prefix) and was refused; the key that tried it puts
/// the cursor back.
refused: bool = false,

/// The window this text belongs to (acme's t->w). A column's or the
/// workspace's tag belongs to none, and the optional makes every caller
/// say what it does then.
pub fn pane(t: *Text) ?*Pane {
    return switch (t.what) {
        .body => @alignCast(@fieldParentPtr("body", t)),
        .tag => @alignCast(@fieldParentPtr("tag", t)),
        .input => @alignCast(@fieldParentPtr("input", t)),
        .column, .workspace => null,
    };
}

/// Free what a text owns: its characters and the buffers its undo keeps.
pub fn deinit(t: *Text, gpa: std.mem.Allocator) void {
    if (t.own) |own| gpa.free(own);
    for (t.ed_undo[0..t.ed_undo_len]) |item| if (item.ovl) |overlay| gpa.free(overlay.text);
    for (t.ed_redo[0..t.ed_redo_len]) |item| if (item.ovl) |overlay| gpa.free(overlay.text);
    t.* = .{ .what = t.what };
}

/// Bring the cursor into view (acme's textshow). Only a body scrolls to it;
/// the tagline follows its caret where it is drawn, and a prompt's answer is
/// drawn whole.
pub fn show(t: *Text) void {
    if (t.what == .body) if (t.pane()) |w| w.ensureCursorVisible();
}

pub fn toModalCursor(t: *Text) modal.Cursor {
    return .{ .row = @intCast(@max(0, t.cur_row)), .col = @intCast(@max(0, t.cur_col)) };
}

pub fn fromModalCursor(t: *Text, c: modal.Cursor) void {
    t.cur_row = @as(i32, @intCast(c.row));
    t.cur_col = @intCast(c.col);
    t.cur_pinned = true;
}

pub fn insertVerticalCursor(lines: []const []const u8, c: modal.Cursor, down: bool) modal.Cursor {
    if (lines.len == 0) return c;
    const row = if (down) @min(c.row + 1, lines.len - 1) else c.row -| 1;
    const target = lines[row];
    if (target.len == 0) return .{ .row = row, .col = 0 };
    const source = if (c.row < lines.len) lines[c.row] else "";
    const goal = File.rawDisplayCol(source, c.col);
    const mapped = File.rawAtDisplay(target, goal);
    const last = modal.prevGrapheme(target, target.len);
    return .{ .row = row, .col = modal.graphemeStart(target, @min(mapped, last)) };
}

pub fn primaryRange(t: *Text, text: []const u8, row0: i32) modal.Selection {
    const c = File.textOffset(t, text, .{ .row = @intCast(@max(0, t.cur_row - row0)), .col = @intCast(@max(0, t.cur_col)) });
    if (t.msel.active) {
        // legacy line selection (file-search results highlight): linewise
        const r0: usize = @intCast(@max(0, @min(t.msel.r0, t.msel.r1) - row0));
        const r1: usize = @intCast(@max(0, @max(t.msel.r0, t.msel.r1) - row0));
        const s = modal.lineStartOffset(text, r0);
        const e = if (r1 + 1 >= modal.cursorLineCount(text)) text.len else modal.lineStartOffset(text, r1 + 1);
        return .{ .anchor = s, .head = @max(e, modal.nextGrapheme(text, c)) };
    }
    if (t.vsel.active) return cellRange(text, t.vsel.row - row0, t.vsel.col, t.cur_row - row0, t.cur_col);
    return .{ .anchor = c, .head = modal.nextGrapheme(text, c) };
}

pub fn cellRange(text: []const u8, arow: i32, acol: i32, hrow: i32, hcol: i32) modal.Selection {
    const a = modal.offsetAt(text, .{ .row = @intCast(@max(0, arow)), .col = @intCast(@max(0, acol)) });
    const c = modal.offsetAt(text, .{ .row = @intCast(@max(0, hrow)), .col = @intCast(@max(0, hcol)) });
    return cellOffRange(text, a, c);
}

/// the same, from the two cells' gap offsets
pub fn cellOffRange(text: []const u8, a: usize, c: usize) modal.Selection {
    if (a <= c) return .{ .anchor = a, .head = modal.nextGrapheme(text, c) };
    return .{ .anchor = modal.nextGrapheme(text, a), .head = c };
}

pub fn rangeCells(text: []const u8, r: modal.Selection) struct { cur: usize, anc: usize } {
    if (r.head > r.anchor) return .{ .cur = modal.prevGrapheme(text, r.head), .anc = r.anchor };
    if (r.head < r.anchor) return .{ .cur = r.head, .anc = modal.prevGrapheme(text, r.anchor) };
    return .{ .cur = r.head, .anc = r.head };
}

pub fn setRange(t: *Text, text: []const u8, row0: i32, r0: modal.Selection, explicit: bool) void {
    var r = r0;
    if (r.anchor == r.head) r.head = modal.nextGrapheme(text, r.head); // min_width_1
    const off = rangeCells(text, r);
    const cc = File.textPosition(t, text, off.cur);
    // a bare block cursor has both cells on the same offset — the common
    // case by far — and this conversion is not free even indexed
    const ac = if (off.anc == off.cur) cc else File.textPosition(t, text, off.anc);
    t.cur_row = @as(i32, @intCast(cc.row)) + row0;
    t.cur_col = @intCast(cc.col);
    t.vsel = .{
        .active = off.anc != off.cur or t.select,
        .row = @as(i32, @intCast(ac.row)) + row0,
        .col = @intCast(ac.col),
        .explicit = explicit or t.select,
    };
    t.msel.active = false;
    t.nsel = 0; // writing ONE range means the selection IS that range
    t.cur_pinned = true;
    t.sticky_col = -1;
    t.normal.clear();
    t.show();
}

pub fn ranges(t: *Text, text: []const u8, row0: i32, out: *[max_selections]modal.Selection) struct { n: usize, pri: usize } {
    const pr = primaryRange(t, text, row0);
    var n: usize = 0;
    var pri: usize = 0;
    var placed = false;
    for (t.sels[0..t.nsel]) |s| {
        const r = cellRange(text, s.arow - row0, s.acol, s.row - row0, s.col);
        if (!placed and @min(pr.anchor, pr.head) <= @min(r.anchor, r.head)) {
            pri = n;
            out[n] = pr;
            n += 1;
            placed = true;
        }
        out[n] = r;
        n += 1;
    }
    if (!placed) {
        pri = n;
        out[n] = pr;
        n += 1;
    }
    return .{ .n = n, .pri = pri };
}

pub fn setRanges(t: *Text, text: []const u8, in: []const modal.Selection, sticky: []const i32, pri0: usize, explicit: bool) void {
    if (in.len == 0) return; // helix asserts non-empty; here it just means "no change"
    var r: [max_selections]modal.Selection = undefined;
    var st: [max_selections]i32 = undefined;
    var n: usize = @min(in.len, max_selections);
    var pri: usize = @min(pri0, n - 1);
    for (in[0..n], 0..) |x, i| {
        r[i] = x;
        if (r[i].anchor == r[i].head) r[i].head = modal.nextGrapheme(text, r[i].head);
        st[i] = if (i < sticky.len) sticky[i] else -1;
    }
    // insertion sort by start — n is tiny and usually already ordered
    var i: usize = 1;
    while (i < n) : (i += 1) {
        var j = i;
        while (j > 0 and @min(r[j].anchor, r[j].head) < @min(r[j - 1].anchor, r[j - 1].head)) : (j -= 1) {
            std.mem.swap(modal.Selection, &r[j], &r[j - 1]);
            std.mem.swap(i32, &st[j], &st[j - 1]);
            if (pri == j) pri = j - 1 else if (pri == j - 1) pri = j;
        }
    }
    var k: usize = 0;
    i = 1;
    while (i < n) : (i += 1) {
        const a = r[k];
        const b = r[i];
        const af = @min(a.anchor, a.head);
        const at = @max(a.anchor, a.head);
        const bf = @min(b.anchor, b.head);
        const bt = @max(b.anchor, b.head);
        if (af == bf or (at > bf and bt > af)) {
            r[k] = if (a.anchor > a.head and b.anchor > b.head)
                .{ .anchor = @max(a.anchor, b.anchor), .head = @min(a.head, b.head) }
            else
                .{ .anchor = @min(af, bf), .head = @max(at, bt) };
            if (pri == i) pri = k;
            if (st[k] < 0) st[k] = st[i];
            continue;
        }
        k += 1;
        r[k] = b;
        st[k] = st[i];
        if (pri == i) pri = k;
    }
    n = k + 1;
    setRange(t, text, 0, r[pri], explicit);
    t.sticky_col = st[pri];
    var w: usize = 0;
    for (r[0..n], 0..) |x, idx| {
        if (idx == pri) continue;
        const c = rangeCells(text, x);
        const cc = modal.positionAt(text, c.cur);
        const ac = modal.positionAt(text, c.anc);
        t.sels[w] = .{
            .row = @as(i32, @intCast(cc.row)),
            .col = @intCast(cc.col),
            .arow = @as(i32, @intCast(ac.row)),
            .acol = @intCast(ac.col),
            .sticky = st[idx],
        };
        w += 1;
    }
    t.nsel = @intCast(w);
}

pub fn multiSelAction(t: *Text, text: []const u8, kind: modal.Normal.Multi, cnt: usize) void {
    var rs: [max_selections]modal.Selection = undefined;
    const got = ranges(t, text, 0, &rs);
    const n = got.n;
    const expl = (t.vsel.active and t.vsel.explicit) or t.msel.active;
    if (kind == .remove_primary) {
        if (n < 2) return; // helix: "no selections remaining"
        var out: [max_selections]modal.Selection = undefined;
        var m: usize = 0;
        for (rs[0..n], 0..) |r, i| {
            if (i == got.pri) continue;
            out[m] = r;
            m += 1;
        }
        // helix Selection::remove: the NEXT range takes over, or the
        // previous one when the primary was last
        return setRanges(t, text, out[0..m], &.{}, @min(got.pri, m - 1), expl);
    }
    if (kind == .rotate_forward or kind == .rotate_backward) {
        const turn = cnt % n;
        const pri = if (kind == .rotate_forward) (got.pri + turn) % n else (got.pri + (n - turn)) % n;
        return setRanges(t, text, rs[0..n], &.{}, pri, expl);
    }
    if (kind == .merge) {
        // helix merge_selections: first.merge(last) — the ranges are
        // sorted, so that is simply the whole span
        const lo = @min(rs[0].anchor, rs[0].head);
        const hi = @max(rs[n - 1].anchor, rs[n - 1].head);
        const rev = rs[0].anchor > rs[0].head and rs[n - 1].anchor > rs[n - 1].head;
        const one: modal.Selection = if (rev) .{ .anchor = hi, .head = lo } else .{ .anchor = lo, .head = hi };
        return setRanges(t, text, &.{one}, &.{}, 0, expl);
    }
    if (kind == .merge_consecutive) {
        var out: [max_selections]modal.Selection = undefined;
        var m: usize = 0;
        var pri: usize = 0;
        for (rs[0..n], 0..) |r, i| {
            if (m > 0 and @min(r.anchor, r.head) == @max(out[m - 1].anchor, out[m - 1].head)) {
                const lo = @min(@min(out[m - 1].anchor, out[m - 1].head), @min(r.anchor, r.head));
                const hi = @max(@max(out[m - 1].anchor, out[m - 1].head), @max(r.anchor, r.head));
                out[m - 1] = .{ .anchor = lo, .head = hi };
                if (i == got.pri) pri = m - 1;
                continue;
            }
            if (i == got.pri) pri = m;
            out[m] = r;
            m += 1;
        }
        return setRanges(t, text, out[0..m], &.{}, pri, expl);
    }
    if (kind == .ensure_forward) {
        // helix ensure_selections_forward: every range points forward
        for (rs[0..n]) |*r| {
            const from = @min(r.anchor, r.head);
            r.head = @max(r.anchor, r.head);
            r.anchor = from;
        }
        return setRanges(t, text, rs[0..n], &.{}, got.pri, expl);
    }
    if (kind == .split_newline) {
        // helix selection::split_on_newline — one range per line the
        // selection covers, the newlines themselves left out
        var out: [max_selections]modal.Selection = undefined;
        var m: usize = 0;
        for (rs[0..n]) |r| {
            const from = @min(r.anchor, r.head);
            const to = @max(r.anchor, r.head);
            if (from == to) {
                if (m < max_selections) {
                    out[m] = r;
                    m += 1;
                }
                continue;
            }
            var start = from;
            while (start < to and m < max_selections) {
                const eol = modal.lineEndOffset(text, modal.lineAtOffset(text, start));
                if (eol >= to) {
                    out[m] = .{ .anchor = start, .head = to };
                    m += 1;
                    break;
                }
                out[m] = .{ .anchor = start, .head = eol };
                m += 1;
                start = eol + 1;
            }
        }
        if (m == 0) return;
        return setRanges(t, text, out[0..m], &.{}, 0, true); // helix keeps primary 0
    }
    if (kind == .trim) {
        // helix trim_selections: whitespace off both ends; ranges that are
        // empty or all whitespace are dropped entirely
        var out: [max_selections]modal.Selection = undefined;
        var m: usize = 0;
        for (rs[0..n]) |r| {
            var from = @min(r.anchor, r.head);
            var to = @max(r.anchor, r.head);
            while (from < to and std.ascii.isWhitespace(text[from])) from += 1;
            while (to > from and std.ascii.isWhitespace(text[to - 1])) to -= 1;
            if (from >= to) continue;
            out[m] = if (r.anchor > r.head) .{ .anchor = to, .head = from } else .{ .anchor = from, .head = to };
            m += 1;
        }
        if (m == 0) { // helix: collapse_selection + keep_primary_selection
            const c = modal.selectionCursor(text, rs[got.pri]);
            return setRange(t, text, 0, .{ .anchor = c, .head = c }, false);
        }
        // helix: the first survivor that OVERLAPS the old primary, else the last
        const pf = @min(rs[got.pri].anchor, rs[got.pri].head);
        const pt = @max(rs[got.pri].anchor, rs[got.pri].head);
        var pri = m - 1;
        for (out[0..m], 0..) |r, i| {
            const f = @min(r.anchor, r.head);
            const to = @max(r.anchor, r.head);
            if (f == pf or (to > pf and pt > f)) {
                pri = i;
                break;
            }
        }
        return setRanges(t, text, out[0..m], &.{}, pri, expl);
    }
    const below = kind == .copy_below;
    var out: [max_selections]modal.Selection = undefined;
    var m: usize = 0;
    var pri: usize = 0;
    const nlines = modal.cursorLineCount(text);
    for (rs[0..n], 0..) |r, ri| {
        const is_pri = ri == got.pri;
        // head-exclusive: back the leading end off onto its own cell
        const hp = modal.positionAt(text, if (r.anchor < r.head) modal.prevGrapheme(text, r.head) else r.head);
        const ap = modal.positionAt(text, if (r.anchor < r.head) r.anchor else modal.prevGrapheme(text, r.anchor));
        const height = @max(hp.row, ap.row) - @min(hp.row, ap.row) + 1;
        if (m >= max_selections) break;
        if (is_pri) pri = m;
        out[m] = r;
        m += 1;
        var made: usize = 0;
        var k: usize = 0;
        while (made < cnt and m < max_selections) : (k += 1) {
            const d = (k + 1) * height;
            const arow = if (below) ap.row + d else ap.row -| d;
            const hrow = if (below) hp.row + d else hp.row -| d;
            if (arow >= nlines or hrow >= nlines) break;
            const a2 = modal.offsetAt(text, .{ .row = arow, .col = ap.col });
            const h2 = modal.offsetAt(text, .{ .row = hrow, .col = hp.col });
            // a line too short to reach the column is skipped, not clamped
            if (modal.positionAt(text, a2).col == ap.col and modal.positionAt(text, h2).col == hp.col) {
                if (is_pri) pri = m;
                out[m] = modal.moveSelectionCursor(text, .{ .anchor = a2, .head = a2 }, h2, true);
                m += 1;
                made += 1;
            }
            if (arow == 0 and hrow == 0) break;
        }
    }
    setRanges(t, text, out[0..m], &.{}, pri, expl);
}

/// a range's start CELL (document order key) — the smaller of its two ends
pub fn selStart(s: SelRange) struct { row: i32, col: i32 } {
    if (s.arow < s.row or (s.arow == s.row and s.acol < s.col)) return .{ .row = s.arow, .col = s.acol };
    return .{ .row = s.row, .col = s.col };
}

pub fn maxLine(text: []const u8) usize {
    const nl = modal.cursorLineCount(text);
    return if (text.len == 0 or text[text.len - 1] == '\n') nl -| 2 else nl - 1;
}

/// point-target motion: collapse there (extend in select mode)
pub fn pointMove(t: *Text, text: []const u8, range: modal.Selection, target: usize) void {
    setRange(t, text, 0, modal.moveSelectionCursor(text, range, target, t.select), false);
}

/// word motions select their traversed span (extend mode: head only)
pub fn wordMove(t: *Text, text: []const u8, range: modal.Selection, cnt: usize, target: modal.WordTarget) void {
    const wr = modal.moveWord(text, range, cnt, target);
    const res = if (t.select) modal.moveSelectionCursor(text, range, modal.selectionCursor(text, wr), true) else wr;
    setRange(t, text, 0, res, false);
}

/// f/t/F/T: anchor at the old cursor cell, head on the hit (not found: no move)
pub fn findMove(t: *Text, text: []const u8, range: modal.Selection, ch: u21, fwd: bool, till: bool, cnt: usize) void {
    const cur = modal.selectionCursor(text, range);
    const target = modal.findTarget(text, cur, ch, fwd, till, cnt) orelse return;
    const res = if (t.select)
        modal.moveSelectionCursor(text, range, target, true)
    else
        modal.moveSelectionCursor(text, .{ .anchor = cur, .head = cur }, target, true);
    setRange(t, text, 0, res, false);
}

/// j/k and friends: sticky goal column, clamped onto short lines' newline
pub fn verticalMove(t: *Text, text: []const u8, range: modal.Selection, down: bool, cnt: usize) void {
    const cur = modal.selectionCursor(text, range);
    const pos = File.textPosition(t, text, cur);
    const goal: usize = if (t.sticky_col >= 0)
        @intCast(t.sticky_col)
    else
        File.rawDisplayCol(File.textLine(t, text, pos.row), pos.col);
    const last_row = File.textLineCount(t, text) - 1;
    const nline = if (down) @min(pos.row + @max(1, cnt), last_row) else pos.row -| @max(1, cnt);
    const target_col = File.rawAtDisplay(File.textLine(t, text, nline), goal);
    const target = File.textOffset(t, text, .{ .row = nline, .col = target_col });
    // extend mode never walks onto the empty trailing line (helix)
    if (t.select and target == text.len and text.len > 0 and text[text.len - 1] == '\n') return;
    setRange(t, text, 0, modal.moveSelectionCursor(text, range, target, t.select), false);
    t.sticky_col = @intCast(goal);
}

pub fn visualMove(
    t: *Text,
    text: []const u8,
    range: modal.Selection,
    down: bool,
    cnt: usize,
    width: usize,
) void {
    const cur = modal.selectionCursor(text, range);
    const pos = File.textPosition(t, text, cur);
    const last_row = File.textLineCount(t, text) - 1;
    var row = pos.row;
    var line = modal.lineSlice(text, row);
    var vrow = File.visualRow(line, pos.col, width);
    const goal: usize = if (t.sticky_col >= 0)
        @intCast(t.sticky_col)
    else
        File.rawDisplayCol(line[vrow.start..vrow.end], pos.col -| vrow.start);
    var steps = @max(1, cnt);
    while (steps > 0) : (steps -= 1) {
        if (down) {
            if (vrow.end < line.len) {
                vrow = File.visualRow(line, vrow.end, width);
                continue;
            }
            if (row == last_row) break;
            row += 1;
            line = modal.lineSlice(text, row);
            vrow = File.visualRow(line, 0, width);
        } else {
            if (vrow.start > 0) {
                vrow = File.visualRow(line, vrow.start - 1, width);
                continue;
            }
            if (row == 0) break;
            row -= 1;
            line = modal.lineSlice(text, row);
            vrow = File.visualRow(line, line.len, width);
        }
    }
    // The newline slot is a real cursor position, but the first byte of the
    // NEXT visual row is not: landing there would read as two rows moved.
    var target_col = vrow.start + File.rawAtDisplay(line[vrow.start..vrow.end], goal);
    if (vrow.end < line.len and target_col >= vrow.end)
        target_col = modal.graphemeStart(line, vrow.end - 1);
    const target = File.textOffset(t, text, .{ .row = row, .col = target_col });
    // extend mode never walks onto the empty trailing line (helix)
    if (t.select and target == text.len and text.len > 0 and text[text.len - 1] == '\n') return;
    setRange(t, text, 0, modal.moveSelectionCursor(text, range, target, t.select), false);
    t.sticky_col = @intCast(goal);
}

/// Ctrl-d/u: scroll half a page AND move the cursor by the same rows
pub fn halfPageMove(t: *Text, text: []const u8, range: modal.Selection, down: bool) void {
    // a view to move through is a body's; a tag or an answer is all shown
    if (t.what != .body) return;
    const w = t.pane() orelse return;
    const half: i32 = @max(1, @divTrunc(@as(i32, w.rows), 2));
    w.scrollBy(if (down) half else -half);
    verticalMove(t, text, range, down, @intCast(half));
}

/// helix `scroll` without cursor sync (Ctrl-f/b, PgUp/PgDn, zj/zk): shift
/// the view, then snap a fallen-out cursor to the near scrolloff edge, col 0
pub fn scrollViewMove(t: *Text, text: []const u8, range: modal.Selection, delta: i32) void {
    // a view to move through is a body's; a tag or an answer is all shown
    if (t.what != .body) return;
    const w = t.pane() orelse return;
    const margin: i32 = @min(config.scroll_off, @divTrunc(@as(i32, w.rows) - 1, 2));
    w.scrollBy(delta);
    const top = w.scroll();
    const last_row: i32 = @intCast(File.textLineCount(t, text) - 1);
    const cur = modal.selectionCursor(text, range);
    if (delta > 0) {
        const snap: i32 = @max(0, @min(top + margin, last_row));
        const head = File.textLineStart(t, text, @intCast(snap));
        if (head <= cur) return;
        const anchor = if (t.select) range.anchor else head;
        setRange(t, text, 0, .{ .anchor = anchor, .head = head }, false);
    } else {
        const snap: i32 = @max(0, @min(top + @as(i32, w.rows) - margin - 1, last_row));
        const head = File.textLineStart(t, text, @intCast(snap));
        if (head >= cur) return;
        const anchor = if (t.select) range.anchor else head;
        setRange(t, text, 0, .{ .anchor = anchor, .head = head }, false);
    }
}

/// gt/gc/gb: view-relative rows, col 0, scrolloff clamped (helix goto_window)
pub fn gotoWindow(t: *Text, text: []const u8, range: modal.Selection, which: enum { top, center, bottom }, cnt: usize) void {
    // a view to move through is a body's; a tag or an answer is all shown
    if (t.what != .body) return;
    const w = t.pane() orelse return;
    const margin: i32 = @min(config.scroll_off, @divTrunc(@as(i32, w.rows) - 1, 2));
    const top = w.scroll();
    const last_row: i32 = @intCast(File.textLineCount(t, text) - 1);
    const last_vis: i32 = @min(@as(i32, w.rows) - 1, last_row - top);
    const n: i32 = @intCast(cnt - 1);
    var vline: i32 = switch (which) {
        .top => top + margin + n,
        .center => top + @divTrunc(last_vis, 2),
        .bottom => top + last_vis - (margin + n),
    };
    vline = @max(vline, top + margin);
    vline = @min(vline, top + last_vis - margin);
    const row: i32 = std.math.clamp(vline, 0, last_row);
    pointMove(t, text, range, File.textLineStart(t, text, @intCast(row)));
}

/// helix Range::line_range — the inclusive line span a range covers
pub fn rangeLineSpan(text: []const u8, r: modal.Selection) struct { start: usize, end: usize } {
    const from = @min(r.anchor, r.head);
    const to = @max(r.anchor, r.head);
    const to_adj = if (from == to) to else @max(modal.prevGrapheme(text, to), from);
    return .{ .start = modal.lineAtOffset(text, from), .end = modal.lineAtOffset(text, to_adj) };
}

fn lineStartOrEof(text: []const u8, line: usize) usize {
    if (line >= modal.cursorLineCount(text)) return text.len;
    return modal.lineStartOffset(text, line);
}

/// helix `x` extend_line_below: full lines incl. the newline, cursor ON
/// the last one's '\n'; an already-line-bounded selection grows downward
pub fn lineSelect(t: *Text, text: []const u8, range: modal.Selection, cnt: usize) void {
    const span = rangeLineSpan(text, range);
    const start = modal.lineStartOffset(text, span.start);
    const end = lineStartOrEof(text, span.end + 1);
    const full = @min(range.anchor, range.head) == start and @max(range.anchor, range.head) == end;
    const head = lineStartOrEof(text, span.end + cnt + @intFromBool(full));
    setRange(t, text, 0, .{ .anchor = start, .head = head }, true);
}

/// helix `X` extend_to_line_bounds (direction kept)
pub fn lineBoundsSelect(t: *Text, text: []const u8, range: modal.Selection) void {
    const span = rangeLineSpan(text, range);
    const start = modal.lineStartOffset(text, span.start);
    const end = lineStartOrEof(text, span.end + 1);
    const r: modal.Selection = if (range.head < range.anchor)
        .{ .anchor = end, .head = start }
    else
        .{ .anchor = start, .head = end };
    setRange(t, text, 0, r, true);
}

/// helix `Alt-x` shrink_to_line_bounds (single-line selections untouched)
pub fn shrinkSelToLineBounds(t: *Text, text: []const u8, range: modal.Selection) void {
    const span = rangeLineSpan(text, range);
    if (span.start == span.end) return;
    const from = @min(range.anchor, range.head);
    const to = @max(range.anchor, range.head);
    var start = modal.lineStartOffset(text, span.start);
    var end = lineStartOrEof(text, span.end + 1);
    if (start != from) start = lineStartOrEof(text, span.start + 1);
    if (end != to) end = modal.lineStartOffset(text, span.end);
    const expl = (t.vsel.active and t.vsel.explicit) or t.msel.active;
    const r: modal.Selection = if (range.head < range.anchor)
        .{ .anchor = end, .head = start }
    else
        .{ .anchor = start, .head = end };
    setRange(t, text, 0, r, expl);
}

/// pull the cursor back inside `text` after a rewrite; `row0` is the
/// absolute surface row of its first line (0 for a file)
pub fn clampCursor(t: *Text, text: []const u8, row0: i32) void {
    const n = modal.lineCount(text);
    const row: usize = @min(@as(usize, @intCast(@max(0, t.cur_row - row0))), if (n == 0) 0 else n - 1);
    const llen = modal.lineSlice(text, row).len;
    t.cur_row = @as(i32, @intCast(row)) + row0;
    t.cur_col = @intCast(@min(@as(usize, @intCast(@max(0, t.cur_col))), llen));
    t.cur_pinned = true;
    t.vsel.active = false;
    t.msel.active = false;
    t.show();
}

fn pushHistory(gpa: std.mem.Allocator, slots: []Terminal.Snapshot, len: *usize, value: Terminal.Snapshot) void {
    if (len.* == slots.len) {
        if (slots[0].ovl) |overlay| gpa.free(overlay.text);
        std.mem.copyForwards(Terminal.Snapshot, slots[0 .. slots.len - 1], slots[1..]);
        len.* -= 1;
    }
    slots[len.*] = value;
    len.* += 1;
}

/// Record an edit buffer (a terminal's overlay, a tag's own text) as it
/// stands before an edit, once per change of it, and forget what could have
/// been redone. Null is a buffer not made yet.
pub fn remember(t: *Text, gpa: std.mem.Allocator, current: ?Terminal.EditBuffer) void {
    if (t.ed_undo_len > 0) {
        const top = t.ed_undo[t.ed_undo_len - 1];
        const same = if (top.ovl) |overlay| if (current) |now| overlay.row == now.row and
            overlay.rows == now.rows and std.mem.eql(u8, overlay.text, now.text) else false else current == null;
        if (same) return;
    }
    const copy: ?Terminal.EditBuffer = if (current) |now| .{ .row = now.row, .rows = now.rows, .text = gpa.dupe(u8, now.text) catch return } else null;
    pushHistory(gpa, &t.ed_undo, &t.ed_undo_len, .{ .ovl = copy, .cur_row = t.cur_row, .cur_col = t.cur_col, .vsel = t.vsel });
    for (t.ed_redo[0..t.ed_redo_len]) |item| if (item.ovl) |overlay| gpa.free(overlay.text);
    t.ed_redo_len = 0;
}

/// Undo (back) or redo one edit of an edit buffer: `current` goes on the
/// other history and the state to return to comes back, its cursor and
/// selection already restored here. The caller installs its buffer, which
/// it then owns. Null when there is nothing to step to.
pub fn step(t: *Text, gpa: std.mem.Allocator, current: ?Terminal.EditBuffer, back: bool) ?Terminal.Snapshot {
    const from, const from_len, const to, const to_len = if (back)
        .{ &t.ed_undo, &t.ed_undo_len, &t.ed_redo, &t.ed_redo_len }
    else
        .{ &t.ed_redo, &t.ed_redo_len, &t.ed_undo, &t.ed_undo_len };
    if (from_len.* == 0) return null;
    const copy: ?Terminal.EditBuffer = if (current) |now| .{ .row = now.row, .rows = now.rows, .text = gpa.dupe(u8, now.text) catch return null } else null;
    pushHistory(gpa, to, to_len, .{ .ovl = copy, .cur_row = t.cur_row, .cur_col = t.cur_col, .vsel = t.vsel });
    from_len.* -= 1;
    const back_to = from[from_len.*];
    t.cur_row = back_to.cur_row;
    t.cur_col = back_to.cur_col;
    t.cur_pinned = true;
    t.vsel = back_to.vsel;
    t.msel.active = false;
    return back_to;
}