const std = @import("std");

// Modal-editing text math, kept free of vaxis/ghostty so it can be unit-tested
// in isolation (see the `unit-test` build step). main.zig wires this onto the
// pane's cursor + (for file panes) its content.
//
// The cursor sits ON a character: col is a char index in [0, line.len]; col ==
// line.len means "on the line terminator / after the last char". Motions are
// written to land on real characters; main.zig clamps for display.

pub const Cursor = struct {
    row: usize = 0,
    col: usize = 0,

    pub fn eql(a: Cursor, b: Cursor) bool {
        return a.row == b.row and a.col == b.col;
    }
};

// word char classes (matches ad/vim/kakoune: word = alnum + _, punct = other
// non-ws, ws = space/tab/newline).
pub const Kind = enum { word, punct, ws };

pub fn kindOf(c: u8) Kind {
    if (c == ' ' or c == '\t' or c == '\n' or c == '\r') return .ws;
    if (std.ascii.isAlphanumeric(c) or c == '_') return .word;
    return .punct;
}

// "long word" (W/B/E): only whitespace separates; punct is part of a word.
fn kindOfLong(c: u8) Kind {
    if (c == ' ' or c == '\t' or c == '\n' or c == '\r') return .ws;
    return .word;
}

fn kindAt(lines: []const []const u8, c: Cursor, long: bool) Kind {
    if (c.row >= lines.len) return .ws;
    const line = lines[c.row];
    if (c.col >= line.len) return .ws; // line terminator / EOF = whitespace
    return if (long) kindOfLong(line[c.col]) else kindOf(line[c.col]);
}

fn lineLenOf(lines: []const []const u8, row: usize) usize {
    if (row >= lines.len) return 0;
    return lines[row].len;
}

// advance one position across line boundaries (line terminators are positions
// too: col == line.len is the newline). Returns false at EOF.
fn stepFwd(lines: []const []const u8, c: *Cursor) bool {
    if (c.row >= lines.len) return false;
    const llen = lineLenOf(lines, c.row);
    if (c.col < llen) {
        c.col += 1;
        return true;
    }
    // at the newline: move to next line start
    if (c.row + 1 < lines.len) {
        c.row += 1;
        c.col = 0;
        return true;
    }
    return false; // EOF
}

fn stepBwd(lines: []const []const u8, c: *Cursor) bool {
    if (c.col > 0) {
        c.col -= 1;
        return true;
    }
    if (c.row == 0) return false;
    c.row -= 1;
    c.col = lineLenOf(lines, c.row); // the previous line's newline
    return true;
}

// at EOF? (past the last line's last char)
fn atEof(lines: []const []const u8, c: Cursor) bool {
    if (c.row >= lines.len) return true;
    if (c.row + 1 < lines.len) return false;
    return c.col >= lines[c.row].len;
}

pub fn firstNonWs(line: []const u8) usize {
    var i: usize = 0;
    while (i < line.len and (line[i] == ' ' or line[i] == '\t')) i += 1;
    return i;
}

// ---- per-line motions ----

pub fn lineStart(c: Cursor) Cursor {
    return .{ .row = c.row, .col = 0 };
}

pub fn lineEnd(lines: []const []const u8, c: Cursor) Cursor {
    const llen = lineLenOf(lines, c.row);
    return .{ .row = c.row, .col = if (llen == 0) 0 else llen - 1 };
}

pub fn firstNonWsOf(lines: []const []const u8, c: Cursor) Cursor {
    return .{ .row = c.row, .col = firstNonWs(lines[c.row]) };
}

// ---- char/line motions ----

pub fn charLeft(c: Cursor) Cursor {
    return .{ .row = c.row, .col = if (c.col > 0) c.col - 1 else 0 };
}

pub fn charRight(lines: []const []const u8, c: Cursor) Cursor {
    const llen = lineLenOf(lines, c.row);
    const last = if (llen == 0) 0 else llen - 1;
    return .{ .row = c.row, .col = if (c.col < last) c.col + 1 else last };
}

pub fn lineDown(lines: []const []const u8, c: Cursor) Cursor {
    const nr = if (c.row + 1 < lines.len) c.row + 1 else c.row;
    const llen = lineLenOf(lines, nr);
    const last = if (llen == 0) 0 else llen - 1;
    return .{ .row = nr, .col = if (c.col < last) c.col else last };
}

pub fn lineUp(lines: []const []const u8, c: Cursor) Cursor {
    const nr = if (c.row > 0) c.row - 1 else c.row;
    const llen = lineLenOf(lines, nr);
    const last = if (llen == 0) 0 else llen - 1;
    return .{ .row = nr, .col = if (c.col < last) c.col else last };
}

// ---- word motions ----

// `w`/`W`: to the start of the next word.
pub fn nextWordStart(lines: []const []const u8, c: Cursor, long: bool) Cursor {
    var p = c;
    const start_kind = kindAt(lines, p, long);
    if (start_kind != .ws) {
        // skip the rest of the current word-class run
        while (!atEof(lines, p) and kindAt(lines, p, long) == start_kind) {
            if (!stepFwd(lines, &p)) break;
        }
    }
    // skip whitespace (incl. newlines) to the next word start
    while (!atEof(lines, p) and kindAt(lines, p, long) == .ws) {
        if (!stepFwd(lines, &p)) break;
    }
    // p now sits on the next word's first char (or EOF -> last valid pos)
    return clampToChar(lines, p);
}

// `b`/`B`: to the start of the previous word.
pub fn prevWordStart(lines: []const []const u8, c: Cursor, long: bool) Cursor {
    var p = c;
    if (!stepBwd(lines, &p)) return c; // at buffer start
    // skip whitespace backward
    while (kindAt(lines, p, long) == .ws) {
        if (!stepBwd(lines, &p)) return .{ .row = 0, .col = 0 };
    }
    // now on the end of the previous word; walk back to its start
    const k = kindAt(lines, p, long);
    while (true) {
        var q = p;
        if (!stepBwd(lines, &q)) {
            p.col = 0;
            break;
        }
        if (kindAt(lines, q, long) != k) break; // crossed into prior class
        p = q;
    }
    return clampToChar(lines, p);
}

// `e`/`E`: to the end of the current/next word.
pub fn nextWordEnd(lines: []const []const u8, c: Cursor, long: bool) Cursor {
    var p = c;
    if (!stepFwd(lines, &p)) return clampToChar(lines, c);
    // skip whitespace forward
    while (!atEof(lines, p) and kindAt(lines, p, long) == .ws) {
        if (!stepFwd(lines, &p)) break;
    }
    if (atEof(lines, p)) return clampToChar(lines, p);
    // now on a word's first char; advance to the last char of this run
    const k = kindAt(lines, p, long);
    while (!atEof(lines, p)) {
        var q = p;
        if (!stepFwd(lines, &q)) break;
        if (kindAt(lines, q, long) != k) break;
        p = q;
    }
    return clampToChar(lines, p);
}

// ---- goto ----

pub fn gotoFirst() Cursor {
    return .{ .row = 0, .col = 0 };
}

pub fn gotoLast(lines: []const []const u8) Cursor {
    const r = if (lines.len == 0) 0 else lines.len - 1;
    return .{ .row = r, .col = 0 };
}

// half/full page: target row, caller clamps + scrolls.
pub fn pageDown(c: Cursor, page: usize) Cursor {
    return .{ .row = c.row + page, .col = c.col };
}

pub fn pageUp(c: Cursor, page: usize) Cursor {
    return .{ .row = if (c.row > page) c.row - page else 0, .col = c.col };
}

// ---- helpers used by motions + main.zig ----

// clamp a (possibly terminator/EOF) position onto a real character.
pub fn clampToChar(lines: []const []const u8, c: Cursor) Cursor {
    if (c.row >= lines.len) {
        return .{ .row = if (lines.len == 0) 0 else lines.len - 1, .col = 0 };
    }
    const llen = lineLenOf(lines, c.row);
    if (llen == 0) return .{ .row = c.row, .col = 0 };
    return .{ .row = c.row, .col = @min(c.col, llen - 1) };
}

pub fn lineCount(content: []const u8) usize {
    if (content.len == 0) return 0;
    return std.mem.count(u8, content, "\n") + 1;
}

// byte offset of the start of line `row` (0-based). row may == lineCount()
// (== content.len, the end).
pub fn lineStartOffset(content: []const u8, row: usize) usize {
    var off: usize = 0;
    var r: usize = 0;
    while (r < row) : (r += 1) {
        const nl = std.mem.indexOfPos(u8, content, off, "\n") orelse return content.len;
        off = nl + 1;
    }
    return off;
}

// the text of line `row` (no terminator), a slice into `content`.
pub fn lineSlice(content: []const u8, row: usize) []const u8 {
    const start = lineStartOffset(content, row);
    if (start >= content.len) return "";
    const nl = std.mem.indexOfPos(u8, content, start, "\n") orelse content.len;
    return content[start..nl];
}

// ---- file content mutations. caller frees the returned slice + the old one. ----

// insert `text` at (row, col). col is clamped to the line length.
pub fn insertAt(alloc: std.mem.Allocator, content: []const u8, c: Cursor, text: []const u8) ![]u8 {
    const row = if (c.row >= lineCount(content)) lineCount(content) -| 1 else c.row;
    const line = lineSlice(content, row);
    const col = @min(c.col, line.len);
    const off = lineStartOffset(content, row) + col;
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(alloc, content[0..off]);
    try out.appendSlice(alloc, text);
    try out.appendSlice(alloc, content[off..]);
    return out.toOwnedSlice(alloc);
}

// delete the character at (row, col). no-op if col is past the line end.
pub fn deleteChar(alloc: std.mem.Allocator, content: []const u8, c: Cursor) ![]u8 {
    const line = lineSlice(content, c.row);
    if (c.col >= line.len) return alloc.dupe(u8, content);
    const off = lineStartOffset(content, c.row) + c.col;
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(alloc, content[0..off]);
    try out.appendSlice(alloc, content[off + 1 ..]);
    return out.toOwnedSlice(alloc);
}

// delete whole lines [r0, r1] inclusive (the line content + their terminators).
// returns the new content; `deleted` is the joined removed text (no terminators).
pub const Deleted = struct { content: []u8, deleted: []u8 };

pub fn deleteLines(alloc: std.mem.Allocator, content: []const u8, r0: usize, r1: usize) !Deleted {
    const n = lineCount(content);
    const lo = @min(r0, r1);
    const hi = @min(@max(r0, r1), if (n == 0) 0 else n - 1);
    if (n == 0 or hi < lo) return .{ .content = try alloc.dupe(u8, content), .deleted = try alloc.dupe(u8, "") };

    const start = lineStartOffset(content, lo);
    // end = start of line (hi+1), or content.len if hi is the last line.
    const end = if (hi + 1 < n) lineStartOffset(content, hi + 1) else content.len;
    // if we're removing the last line and there's a preceding newline, also
    // drop that newline so we don't leave a trailing blank line.
    var cut_lo = start;
    const cut_hi = end;
    if (hi + 1 == n and start > 0) cut_lo -= 1; // remove the '\n' before the last line

    var deleted: std.ArrayList(u8) = .empty;
    {
        var r = lo;
        while (r <= hi) : (r += 1) {
            if (r > lo) try deleted.append(alloc, '\n');
            try deleted.appendSlice(alloc, lineSlice(content, r));
        }
    }
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(alloc, content[0..cut_lo]);
    try out.appendSlice(alloc, content[cut_hi..]);
    return .{ .content = try out.toOwnedSlice(alloc), .deleted = try deleted.toOwnedSlice(alloc) };
}

// normalize two cursors into document order (lo <= hi), then byte offsets of the
// INCLUSIVE range [lo .. hi] (the char under hi is included). e == s means empty.
fn rangeBytes(content: []const u8, a: Cursor, b: Cursor) struct { s: usize, e: usize } {
    var lo = a;
    var hi = b;
    if (hi.row < lo.row or (hi.row == lo.row and hi.col < lo.col)) {
        lo = b;
        hi = a;
    }
    const s = lineStartOffset(content, lo.row) + @min(lo.col, lineSlice(content, lo.row).len);
    var e = lineStartOffset(content, hi.row) + @min(hi.col, lineSlice(content, hi.row).len);
    if (e < content.len) e += 1; // include the char under the head
    return .{ .s = s, .e = @max(s, e) };
}

// the text of the inclusive char range [a, b] (cursors in either order). Caller frees.
pub fn rangeText(alloc: std.mem.Allocator, content: []const u8, a: Cursor, b: Cursor) ![]u8 {
    const r = rangeBytes(content, a, b);
    return alloc.dupe(u8, content[r.s..r.e]);
}

// delete the inclusive char range [a, b]. returns new content + the removed text.
pub fn deleteRange(alloc: std.mem.Allocator, content: []const u8, a: Cursor, b: Cursor) !Deleted {
    const r = rangeBytes(content, a, b);
    const deleted = try alloc.dupe(u8, content[r.s..r.e]);
    errdefer alloc.free(deleted);
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(alloc, content[0..r.s]);
    try out.appendSlice(alloc, content[r.e..]);
    return .{ .content = try out.toOwnedSlice(alloc), .deleted = deleted };
}

// replace line `row`'s text with "" (keep the line, empty it). For `c`hange line.
pub fn clearLine(alloc: std.mem.Allocator, content: []const u8, row: usize) ![]u8 {
    const line = lineSlice(content, row);
    const start = lineStartOffset(content, row);
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(alloc, content[0..start]);
    try out.appendSlice(alloc, content[start + line.len ..]);
    return out.toOwnedSlice(alloc);
}

// paste `text` as a new line BELOW `row`. Multiline `text` becomes several lines.
pub fn pasteLineBelow(alloc: std.mem.Allocator, content: []const u8, row: usize, text: []const u8) ![]u8 {
    const n = lineCount(content);
    const off = if (row + 1 < n) lineStartOffset(content, row + 1) else content.len;
    var out: std.ArrayList(u8) = .empty;
    try out.appendSlice(alloc, content[0..off]);
    if (off < content.len) {
        try out.appendSlice(alloc, text);
        try out.append(alloc, '\n');
    } else {
        if (off > 0) try out.append(alloc, '\n');
        try out.appendSlice(alloc, text);
    }
    try out.appendSlice(alloc, content[off..]);
    return out.toOwnedSlice(alloc);
}

// ---- file-pane modal step ----
// One keypress on an editable FILE pane. This is the single source of truth for
// file-modal behavior: main.zig delegates file panes here, and the tutor-driven
// unit tests (below) exercise it directly. Pure (no vaxis/ghostty/term.zig) —
// only std + the modal primitives above. Returns the new content (caller frees
// the old), or null if the key only moved the cursor / was a no-op (caller keeps
// the old content, no realloc).

pub const Key = union(enum) {
    ch: u21, // a printable codepoint (insert inserts its UTF-8 bytes)
    esc,
    enter,
    backspace,
    left,
    right,
    up,
    down,
    home,
    end,
    page_down,
    page_up,
    half_down, // Ctrl-d
    half_up, // Ctrl-u
};

pub const FileMode = enum { normal, insert };

pub const FileState = struct {
    cur: Cursor = .{},
    mode: FileMode = .normal,
    pending: u21 = 0, // prefix codepoint in flight: 'g' or 'z'
    msel_active: bool = false,
    msel_r0: i32 = 0,
    msel_r1: i32 = 0,
};

// The view the cursor must stay inside. `scroll` is read/written so viewport
// alignment (zt/zz/zb) and page motions can adjust it; the app syncs it back to
// the file pane's scroll. The unit-test Sim passes a large rows + a dummy scroll
// so visibility is a no-op (content is what's asserted).
pub const View = struct {
    rows: usize,
    scroll: *usize,
};

fn chEq(key: Key, cp: u21) bool {
    return switch (key) {
        .ch => |c| c == cp,
        else => false,
    };
}

fn tagOf(key: Key) std.meta.Tag(Key) {
    return std.meta.activeTag(key);
}

fn splitLines(alloc: std.mem.Allocator, content: []const u8) ![][]const u8 {
    var ls: std.ArrayList([]const u8) = .empty;
    var it = std.mem.splitScalar(u8, content, '\n');
    while (it.next()) |ln| try ls.append(alloc, ln);
    return ls.toOwnedSlice(alloc);
}

fn ensureVisible(view: View, row: usize) void {
    const s = view.scroll.*;
    if (row < s) {
        view.scroll.* = row;
        return;
    }
    const last = s +| (view.rows -| 1);
    if (row > last) view.scroll.* = row -| (view.rows -| 1);
}

// helix `x`: start a line selection at the cursor, or extend it down by one.
fn lineSelectFile(st: *FileState, lines: []const []const u8) void {
    if (!st.msel_active) {
        st.msel_active = true;
        st.msel_r0 = @intCast(st.cur.row);
        st.msel_r1 = @intCast(st.cur.row);
    } else {
        const last: i32 = @intCast(if (lines.len == 0) 0 else lines.len - 1);
        st.msel_r1 = @min(st.msel_r1 + 1, last);
        st.cur.row = @intCast(st.msel_r1);
    }
    st.cur.col = 0;
    st.pending = 0;
}

// `d`: delete the selected lines (yanking them), or the char at the cursor.
fn deleteFile(alloc: std.mem.Allocator, content: []const u8, st: *FileState, lines: []const []const u8, yank: *?[]u8) !?[]u8 {
    if (st.msel_active) {
        const r0: i32 = @min(st.msel_r0, st.msel_r1);
        const r1: i32 = @max(st.msel_r0, st.msel_r1);
        const d = try deleteLines(alloc, content, @intCast(@max(0, r0)), @intCast(@max(0, r1)));
        if (yank.*) |old| alloc.free(old);
        yank.* = try alloc.dupe(u8, d.deleted);
        alloc.free(d.deleted);
        const n = lineCount(d.content);
        st.cur = .{ .row = @intCast(@min(@as(usize, @intCast(@max(0, r0))), if (n == 0) 0 else n - 1)), .col = 0 };
        st.msel_active = false;
        st.pending = 0;
        return d.content;
    }
    // no selection: delete the char at the cursor (no-op if past end)
    const llen = lineLenOf(lines, st.cur.row);
    if (st.cur.col >= llen) {
        st.pending = 0;
        return null;
    }
    const new = try deleteChar(alloc, content, st.cur);
    st.pending = 0;
    return new;
}

// `y`: yank the selected lines, or the current line. Cursor moves to the
// selection start; the selection is collapsed.
fn yankFile(alloc: std.mem.Allocator, content: []const u8, st: *FileState, yank: *?[]u8) !void {
    if (st.msel_active) {
        const r0: usize = @intCast(@max(0, @min(st.msel_r0, st.msel_r1)));
        const r1: usize = @intCast(@max(0, @max(st.msel_r0, st.msel_r1)));
        var buf: std.ArrayList(u8) = .empty;
        var r: usize = r0;
        while (r <= r1) : (r += 1) {
            if (r > r0) try buf.append(alloc, '\n');
            try buf.appendSlice(alloc, lineSlice(content, r));
        }
        if (yank.*) |old| alloc.free(old);
        yank.* = try buf.toOwnedSlice(alloc);
        st.cur = .{ .row = r0, .col = 0 };
        st.msel_active = false;
    } else {
        if (yank.*) |old| alloc.free(old);
        yank.* = try alloc.dupe(u8, lineSlice(content, st.cur.row));
    }
    st.pending = 0;
}

// `p`: paste the yanked text as a new line below the cursor.
fn pasteFile(alloc: std.mem.Allocator, content: []const u8, st: *FileState, yank: *?[]u8) !?[]u8 {
    const y = yank.* orelse {
        st.pending = 0;
        return null;
    };
    const new = try pasteLineBelow(alloc, content, st.cur.row, y);
    st.cur = .{ .row = st.cur.row + 1, .col = 0 };
    st.pending = 0;
    return new;
}

// `c`: change the selection (delete + enter INSERT, keeping one empty line), or
// change the char at the cursor. Yanks the removed text.
fn changeFile(alloc: std.mem.Allocator, content: []const u8, st: *FileState, lines: []const []const u8, yank: *?[]u8) !?[]u8 {
    if (st.msel_active) {
        const r0: usize = @intCast(@max(0, @min(st.msel_r0, st.msel_r1)));
        const r1: usize = @intCast(@max(0, @max(st.msel_r0, st.msel_r1)));
        // yank the original lines first (from the un-mutated content)
        var buf: std.ArrayList(u8) = .empty;
        var r: usize = r0;
        while (r <= r1) : (r += 1) {
            if (r > r0) try buf.append(alloc, '\n');
            try buf.appendSlice(alloc, lineSlice(content, r));
        }
        if (yank.*) |old| alloc.free(old);
        yank.* = try buf.toOwnedSlice(alloc);
        // drop the extra lines (keep one), then empty the kept line
        const d = if (r1 > r0) try deleteLines(alloc, content, r0 + 1, r1) else null;
        defer if (d) |dd| alloc.free(dd.deleted);
        const after_del: []const u8 = if (d) |dd| dd.content else content;
        const cl = try clearLine(alloc, after_del, r0);
        if (d) |dd| alloc.free(dd.content); // free the intermediate (after_del copy)
        st.cur = .{ .row = r0, .col = 0 };
        st.mode = .insert;
        st.msel_active = false;
        st.pending = 0;
        return cl;
    }
    // no selection: delete the char at the cursor (if any), then enter INSERT
    const llen = lineLenOf(lines, st.cur.row);
    if (st.cur.col >= llen) {
        st.mode = .insert;
        st.msel_active = false;
        st.pending = 0;
        return null;
    }
    const new = try deleteChar(alloc, content, st.cur);
    st.mode = .insert;
    st.msel_active = false;
    st.pending = 0;
    return new;
}

// INSERT-mode key on a file pane. Returns new content or null (no change).
fn stepInsertFile(alloc: std.mem.Allocator, content: []const u8, st: *FileState, lines: []const []const u8, key: Key) !?[]u8 {
    switch (key) {
        .esc => {
            st.mode = .normal;
            st.msel_active = false;
            st.pending = 0;
            return null;
        },
        .ch => |cp| {
            if (cp == '\n') return try stepInsertFile(alloc, content, st, lines, .enter);
            var buf: [4]u8 = undefined;
            const n = std.unicode.utf8Encode(cp, &buf) catch return null;
            const new = try insertAt(alloc, content, st.cur, buf[0..n]);
            st.cur.col += n; // col is a byte offset; advance by the char's byte length
            st.pending = 0;
            return new;
        },
        .enter => {
            const new = try insertAt(alloc, content, st.cur, "\n");
            st.cur = .{ .row = st.cur.row + 1, .col = 0 };
            st.pending = 0;
            return new;
        },
        .backspace => {
            if (st.cur.col > 0) {
                const new = try deleteChar(alloc, content, .{ .row = st.cur.row, .col = st.cur.col - 1 });
                st.cur.col -= 1;
                return new;
            } else if (st.cur.row > 0) {
                const prevlen = lineSlice(content, st.cur.row - 1).len;
                const new = try deleteChar(alloc, content, .{ .row = st.cur.row - 1, .col = prevlen });
                st.cur = .{ .row = st.cur.row - 1, .col = prevlen };
                return new;
            }
            return null;
        },
        .left => {
            st.cur = charLeft(st.cur);
            return null;
        },
        .right => {
            st.cur = charRight(lines, st.cur);
            return null;
        },
        .up => {
            st.cur = lineUp(lines, st.cur);
            return null;
        },
        .down => {
            st.cur = lineDown(lines, st.cur);
            return null;
        },
        .home => {
            st.cur = lineStart(st.cur);
            return null;
        },
        .end => {
            st.cur = lineEnd(lines, st.cur);
            return null;
        },
        else => return null,
    }
}

// The full NORMAL-mode file step (motions + insert entry + select/edit). See
// handleNormal in main.zig for the terminal counterpart (which differs in its
// edit ops: terminals can't delete shell output, only pasted runs).
pub fn stepFile(alloc: std.mem.Allocator, content: []const u8, st: *FileState, view: View, key: Key, yank: *?[]u8) !?[]u8 {
    const lines = try splitLines(alloc, content);
    defer alloc.free(lines);
    if (st.mode == .insert) return try stepInsertFile(alloc, content, st, lines, key);

    var cur = clampToChar(lines, st.cur);

    // prefix continuations
    if (st.pending != 0) {
        const p = st.pending;
        st.pending = 0;
        if (p == 'g') {
            if (chEq(key, 'g')) cur = gotoFirst()
            else if (chEq(key, 'e')) cur = gotoLast(lines)
            else if (chEq(key, 'h')) cur = lineStart(cur)
            else if (chEq(key, 'l')) cur = lineEnd(lines, cur)
            else return null; // unknown continuation: consume, no-op
            st.cur = cur;
            st.msel_active = false;
            return null;
        }
        if (p == 'z') {
            if (chEq(key, 't')) {
                view.scroll.* = cur.row;
            } else if (chEq(key, 'z')) {
                view.scroll.* = @intCast(@max(0, @as(i32, @intCast(cur.row)) - @as(i32, @intCast(view.rows / 2))));
            } else if (chEq(key, 'b')) {
                view.scroll.* = @intCast(@max(0, @as(i32, @intCast(cur.row)) - @as(i32, @intCast(view.rows)) + 1));
            } else return null;
            st.msel_active = false;
            return null;
        }
    }
    if (chEq(key, 'g')) {
        st.pending = 'g';
        return null;
    }
    if (chEq(key, 'z')) {
        st.pending = 'z';
        return null;
    }

    // motions -> move cursor, collapse selection, keep visible
    var moved: ?Cursor = null;
    if (chEq(key, 'h') or tagOf(key) == .left) moved = charLeft(cur)
    else if (chEq(key, 'l') or tagOf(key) == .right) moved = charRight(lines, cur)
    else if (chEq(key, 'j') or tagOf(key) == .down) moved = lineDown(lines, cur)
    else if (chEq(key, 'k') or tagOf(key) == .up) moved = lineUp(lines, cur)
    else if (chEq(key, 'w')) moved = nextWordStart(lines, cur, false)
    else if (chEq(key, 'W')) moved = nextWordStart(lines, cur, true)
    else if (chEq(key, 'b')) moved = prevWordStart(lines, cur, false)
    else if (chEq(key, 'B')) moved = prevWordStart(lines, cur, true)
    else if (chEq(key, 'e')) moved = nextWordEnd(lines, cur, false)
    else if (chEq(key, 'E')) moved = nextWordEnd(lines, cur, true)
    else if (chEq(key, '0') or tagOf(key) == .home) moved = lineStart(cur)
    else if (chEq(key, '$') or tagOf(key) == .end) moved = lineEnd(lines, cur)
    else if (chEq(key, '^')) moved = firstNonWsOf(lines, cur)
    else if (chEq(key, 'G')) moved = gotoLast(lines)
    else if (tagOf(key) == .half_down) moved = clampToChar(lines, pageDown(cur, @max(1, view.rows / 2)))
    else if (tagOf(key) == .half_up) moved = clampToChar(lines, pageUp(cur, @max(1, view.rows / 2)))
    else if (tagOf(key) == .page_down) moved = clampToChar(lines, pageDown(cur, view.rows))
    else if (tagOf(key) == .page_up) moved = clampToChar(lines, pageUp(cur, view.rows));
    if (moved) |m| {
        st.cur = m;
        st.msel_active = false;
        st.pending = 0;
        ensureVisible(view, m.row);
        return null;
    }

    // insert entry
    if (chEq(key, 'i')) {
        st.mode = .insert;
        st.cur = cur;
        st.msel_active = false;
        st.pending = 0;
        return null;
    }
    if (chEq(key, 'a')) {
        st.mode = .insert;
        st.cur = charRight(lines, cur);
        st.msel_active = false;
        st.pending = 0;
        return null;
    }
    if (chEq(key, 'I')) {
        st.mode = .insert;
        st.cur = firstNonWsOf(lines, cur);
        st.msel_active = false;
        st.pending = 0;
        return null;
    }
    if (chEq(key, 'A')) {
        st.mode = .insert;
        st.cur = .{ .row = cur.row, .col = lineLenOf(lines, cur.row) };
        st.msel_active = false;
        st.pending = 0;
        return null;
    }
    if (chEq(key, 'o')) {
        const new = try insertAt(alloc, content, .{ .row = cur.row, .col = lineLenOf(lines, cur.row) }, "\n");
        st.cur = .{ .row = cur.row + 1, .col = 0 };
        st.mode = .insert;
        st.msel_active = false;
        st.pending = 0;
        ensureVisible(view, st.cur.row);
        return new;
    }
    if (chEq(key, 'O')) {
        const new = try insertAt(alloc, content, .{ .row = cur.row, .col = 0 }, "\n");
        st.cur = .{ .row = cur.row, .col = 0 };
        st.mode = .insert;
        st.msel_active = false;
        st.pending = 0;
        ensureVisible(view, st.cur.row);
        return new;
    }

    // select / edit
    if (chEq(key, 'x')) {
        lineSelectFile(st, lines);
        return null;
    }
    if (chEq(key, 'd')) return try deleteFile(alloc, content, st, lines, yank);
    if (chEq(key, 'c')) return try changeFile(alloc, content, st, lines, yank);
    if (chEq(key, 'y')) {
        try yankFile(alloc, content, st, yank);
        return null;
    }
    if (chEq(key, 'p')) return try pasteFile(alloc, content, st, yank);

    // Enter / other keys: no-op on a file (no tty mode)
    return null;
}

// ---- tests ----

const testEq = struct {
    fn cur(lines: []const []const u8, s: []const u8) Cursor {
        var row: usize = 0;
        for (lines) |ln| {
            if (std.mem.eql(u8, ln, s)) return .{ .row = row, .col = 0 };
            row += 1;
        }
        unreachable;
    }
};

test "kindOf" {
    try std.testing.expectEqual(Kind.word, kindOf('a'));
    try std.testing.expectEqual(Kind.word, kindOf('_'));
    try std.testing.expectEqual(Kind.word, kindOf('9'));
    try std.testing.expectEqual(Kind.punct, kindOf('.'));
    try std.testing.expectEqual(Kind.punct, kindOf('('));
    try std.testing.expectEqual(Kind.ws, kindOf(' '));
    try std.testing.expectEqual(Kind.ws, kindOf('\n'));
}

test "char/line motions" {
    const lines = [_][]const u8{ "alpha beta", "  two words", "x" };
    const c = Cursor{ .row = 0, .col = 5 };
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 4 }, charLeft(c));
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 6 }, charRight(&lines, c));
    try std.testing.expectEqual(Cursor{ .row = 1, .col = 5 }, lineDown(&lines, c));
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 5 }, lineUp(&lines, Cursor{ .row = 1, .col = 5 }));
    // line ends
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 9 }, lineEnd(&lines, c));
    try std.testing.expectEqual(Cursor{ .row = 2, .col = 0 }, lineEnd(&lines, Cursor{ .row = 2, .col = 0 }));
    // first non-ws
    try std.testing.expectEqual(Cursor{ .row = 1, .col = 2 }, firstNonWsOf(&lines, Cursor{ .row = 1, .col = 0 }));
}

test "word motions w/b/e" {
    const lines = [_][]const u8{"this is a test"};
    const w = &lines;
    // "this is a test", indices 0..13
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 5 }, nextWordStart(w, Cursor{ .row = 0, .col = 0 }, false)); // t->next word "is"
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 8 }, nextWordStart(w, Cursor{ .row = 0, .col = 5 }, false)); // -> "a"
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 10 }, nextWordStart(w, Cursor{ .row = 0, .col = 8 }, false)); // -> "test"
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 10 }, nextWordStart(w, Cursor{ .row = 0, .col = 9 }, false)); // from ws
    // b
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 8 }, prevWordStart(w, Cursor{ .row = 0, .col = 10 }, false)); // test -> "a"
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 5 }, prevWordStart(w, Cursor{ .row = 0, .col = 8 }, false)); // -> "is"
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 0 }, prevWordStart(w, Cursor{ .row = 0, .col = 5 }, false)); // -> "this"
    // e
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 3 }, nextWordEnd(w, Cursor{ .row = 0, .col = 0 }, false)); // this[3]
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 6 }, nextWordEnd(w, Cursor{ .row = 0, .col = 3 }, false)); // -> "is"[6]
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 13 }, nextWordEnd(w, Cursor{ .row = 0, .col = 10 }, false)); // -> "test"[13]
}

test "word motions cross line" {
    const lines = [_][]const u8{ "foo bar", "", "baz" };
    const w = &lines;
    // from end of "foo bar" (row0 col6) w crosses the blank line to "baz"
    try std.testing.expectEqual(Cursor{ .row = 2, .col = 0 }, nextWordStart(w, Cursor{ .row = 0, .col = 6 }, false));
    // b from "baz" crosses back to "bar"
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 4 }, prevWordStart(w, Cursor{ .row = 2, .col = 0 }, false));
    // e from row0 col0 -> "foo" end (col2)
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 2 }, nextWordEnd(w, Cursor{ .row = 0, .col = 0 }, false));
}

test "long word W treats punct as word" {
    // "foo.bar baz" : W from 0 -> "baz" at 8 (foo.bar is one long word)
    const lines = [_][]const u8{"foo.bar baz"};
    const w = &lines;
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 8 }, nextWordStart(w, Cursor{ .row = 0, .col = 0 }, true));
    // w (non-long) from 0 -> '.' at 3 (punct is its own word, like vim/helix)
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 3 }, nextWordStart(w, Cursor{ .row = 0, .col = 0 }, false));
}

test "goto" {
    const lines = [_][]const u8{ "a", "b", "c" };
    try std.testing.expectEqual(Cursor{ .row = 0, .col = 0 }, gotoFirst());
    try std.testing.expectEqual(Cursor{ .row = 2, .col = 0 }, gotoLast(&lines));
}

test "lineStartOffset + lineSlice" {
    const content = "alpha\nbeta\n\ngamma";
    try std.testing.expectEqual(@as(usize, 0), lineStartOffset(content, 0));
    try std.testing.expectEqual(@as(usize, 6), lineStartOffset(content, 1));
    try std.testing.expectEqual(@as(usize, 11), lineStartOffset(content, 2));
    try std.testing.expectEqual(@as(usize, 12), lineStartOffset(content, 3));
    try std.testing.expectEqual(@as(usize, 17), lineStartOffset(content, 4)); // past end
    try std.testing.expectEqualStrings("alpha", lineSlice(content, 0));
    try std.testing.expectEqualStrings("beta", lineSlice(content, 1));
    try std.testing.expectEqualStrings("", lineSlice(content, 2));
    try std.testing.expectEqualStrings("gamma", lineSlice(content, 3));
    try std.testing.expectEqual(@as(usize, 4), lineCount(content));
}

test "insertAt mid-line and at end" {
    const content = "hello world";
    const a = std.testing.allocator;
    const r1 = try insertAt(a, content, .{ .row = 0, .col = 5 }, "!");
    defer a.free(r1);
    try std.testing.expectEqualStrings("hello! world", r1);
    const r2 = try insertAt(a, content, .{ .row = 0, .col = 99 }, "!");
    defer a.free(r2);
    try std.testing.expectEqualStrings("hello world!", r2);
}

test "insertAt multiline creates lines" {
    const content = "a\nb";
    const a = std.testing.allocator;
    const r = try insertAt(a, content, .{ .row = 0, .col = 1 }, "X\nY");
    defer a.free(r);
    try std.testing.expectEqualStrings("aX\nY\nb", r);
    try std.testing.expectEqual(@as(usize, 3), lineCount(r));
}

test "deleteChar" {
    const content = "abc";
    const a = std.testing.allocator;
    const r = try deleteChar(a, content, .{ .row = 0, .col = 1 });
    defer a.free(r);
    try std.testing.expectEqualStrings("ac", r);
    // past end: no-op
    const r2 = try deleteChar(a, content, .{ .row = 0, .col = 5 });
    defer a.free(r2);
    try std.testing.expectEqualStrings("abc", r2);
}

test "deleteLines middle" {
    const content = "one\ntwo\nthree\nfour";
    const a = std.testing.allocator;
    const d = try deleteLines(a, content, 1, 2);
    defer a.free(d.content);
    defer a.free(d.deleted);
    try std.testing.expectEqualStrings("one\nfour", d.content);
    try std.testing.expectEqualStrings("two\nthree", d.deleted);
}

test "deleteLines last line drops preceding newline" {
    const content = "one\ntwo\nthree";
    const a = std.testing.allocator;
    const d = try deleteLines(a, content, 2, 2);
    defer a.free(d.content);
    defer a.free(d.deleted);
    try std.testing.expectEqualStrings("one\ntwo", d.content);
    try std.testing.expectEqualStrings("three", d.deleted);
}

test "deleteLines only line" {
    const content = "only";
    const a = std.testing.allocator;
    const d = try deleteLines(a, content, 0, 0);
    defer a.free(d.content);
    defer a.free(d.deleted);
    try std.testing.expectEqualStrings("", d.content);
    try std.testing.expectEqualStrings("only", d.deleted);
}

test "rangeText + deleteRange (char-wise select)" {
    const a = std.testing.allocator;
    const content = "hello\nworld\nfoo";
    // same-line inclusive range: "hello"[1..3] -> "ell"
    const t1 = try rangeText(a, content, .{ .row = 0, .col = 1 }, .{ .row = 0, .col = 3 });
    defer a.free(t1);
    try std.testing.expectEqualStrings("ell", t1);
    // reversed cursors give the same range
    const t2 = try rangeText(a, content, .{ .row = 0, .col = 3 }, .{ .row = 0, .col = 1 });
    defer a.free(t2);
    try std.testing.expectEqualStrings("ell", t2);
    // cross-line range includes the newline: row0 col3 .. row1 col1 -> "lo\nwo"
    const t3 = try rangeText(a, content, .{ .row = 0, .col = 3 }, .{ .row = 1, .col = 1 });
    defer a.free(t3);
    try std.testing.expectEqualStrings("lo\nwo", t3);
    // delete the same cross-line range
    const d = try deleteRange(a, content, .{ .row = 0, .col = 3 }, .{ .row = 1, .col = 1 });
    defer a.free(d.content);
    defer a.free(d.deleted);
    try std.testing.expectEqualStrings("helrld\nfoo", d.content);
    try std.testing.expectEqualStrings("lo\nwo", d.deleted);
}

test "clearLine" {
    const content = "keep\nzap me\nkeep2";
    const a = std.testing.allocator;
    const r = try clearLine(a, content, 1);
    defer a.free(r);
    try std.testing.expectEqualStrings("keep\n\nkeep2", r);
}

test "pasteLineBelow" {
    const content = "one\ntwo";
    const a = std.testing.allocator;
    const r = try pasteLineBelow(a, content, 0, "INSERTED");
    defer a.free(r);
    try std.testing.expectEqualStrings("one\nINSERTED\ntwo", r);
    // paste below last line
    const r2 = try pasteLineBelow(a, content, 1, "END");
    defer a.free(r2);
    try std.testing.expectEqualStrings("one\ntwo\nEND", r2);
    // multiline yanked text
    const r3 = try pasteLineBelow(a, content, 0, "a\nb");
    defer a.free(r3);
    try std.testing.expectEqualStrings("one\na\nb\ntwo", r3);
}