//! Built-in P1 components for the TUI (plan §6).
//!
//! Each component satisfies the `Component` vtable (`tui_component.zig`) and
//! implements the cache-derived dirty model exactly as `RenderCache` defines
//! it: any state mutation calls `markDirty`/`markDirtyFrom` (drops the cache),
//! and a successful `render` calls `cache.store(lines)` (diffs the new lines
//! against the prior cache, records the lowest differing index, re-populates
//! the cache, and marks it clean). `firstLineChanged` is therefore derived
//! purely from cache state and never a hand-managed integer that can drift.
//!
//! Data-in / lines-out: each component takes STRUCTURED DATA IN via setters or
//! delta-appenders and produces LINES OUT from `render(width, alloc)`. Every
//! returned line's visible width is <= `width` (we TRUNCATE; the engine treats
//! overflow as a hard error per plan §3.1).
//!
//! Render storage convention: a component renders into a transient list, calls
//! `cache.store(lines)` (which dupes the bytes into cache-owned storage), then
//! returns `cache.lines` re-typed as `[]const []const u8`. The returned slices
//! are owned by the cache and stay valid until the next `render`/`invalidate`
//! — satisfying the vtable's lifetime contract.

const std = @import("std");
const component = @import("tui_component.zig");
const theme = @import("tui_theme.zig");
const input = @import("tui_input.zig");
const key = @import("tui_key.zig");

const Component = component.Component;
const Focusable = component.Focusable;
const RenderCache = component.RenderCache;
const CURSOR_MARKER = component.CURSOR_MARKER;
const Style = theme.Style;
const Key = key.Key;
const KeyCode = key.KeyCode;

// ===========================================================================
// Shared helpers
// ===========================================================================

/// Number of display columns occupied by `text`, counted as one column per
/// UTF-8 codepoint. `text` here is assumed to be PLAIN (no escape sequences);
/// components wrap on plain text and only add styling escapes afterward, so
/// this is a faithful visible width. Mirrors the engine's P1 approximation
/// (1 col per codepoint; wide CJK/emoji width is a deferred refinement).
pub fn displayWidth(text: []const u8) usize {
    var cols: usize = 0;
    var i: usize = 0;
    while (i < text.len) {
        const seq_len = std.unicode.utf8ByteSequenceLength(text[i]) catch 1;
        cols += 1;
        i += @min(seq_len, text.len - i);
    }
    return cols;
}

/// Truncate `text` to at most `max_cols` display columns, returning a byte
/// slice of `text` that ends on a codepoint boundary. Never splits a multibyte
/// codepoint.
pub fn truncateToCols(text: []const u8, max_cols: usize) []const u8 {
    var cols: usize = 0;
    var i: usize = 0;
    while (i < text.len and cols < max_cols) {
        const seq_len = std.unicode.utf8ByteSequenceLength(text[i]) catch 1;
        const adv = @min(seq_len, text.len - i);
        i += adv;
        cols += 1;
    }
    return text[0..i];
}

/// Wrap `text` (a single logical paragraph, no embedded newlines) into lines of
/// at most `width` display columns, appending each produced line to `out`.
/// Greedy word-wrap on ASCII spaces; a word longer than `width` is hard-split.
/// An empty paragraph yields one empty line. Lines pushed to `out` are slices
/// borrowed from `text` (no allocation of line bytes here; `out` only stores
/// the slice headers).
fn wrapParagraph(text: []const u8, width: usize, out: *std.ArrayList([]const u8), alloc: std.mem.Allocator) !void {
    if (width == 0) {
        try out.append(alloc, "");
        return;
    }
    if (text.len == 0) {
        try out.append(alloc, "");
        return;
    }

    // Greedy word-wrap. We accumulate a line by byte range [line_start, i); on
    // overflow we break at the last space that fits, or hard-split a word that
    // is wider than `width`.
    var line_start: usize = 0;
    var line_cols: usize = 0;
    var last_break: ?usize = null; // byte index of the last space on this line
    var i: usize = 0;

    while (i < text.len) {
        const seq_len = std.unicode.utf8ByteSequenceLength(text[i]) catch 1;
        const adv = @min(seq_len, text.len - i);
        const is_space = adv == 1 and text[i] == ' ';

        if (is_space and line_cols == width) {
            // The overflowing glyph is the inter-word space itself: break here
            // and consume the space (standard word-wrap discards it) so the
            // current word group fills the line exactly.
            try out.append(alloc, text[line_start..i]);
            line_start = i + adv;
            last_break = null;
            line_cols = 0;
            i += adv;
            continue;
        }
        if (line_cols + 1 > width) {
            // Adding this glyph would overflow; break the line first.
            if (last_break) |brk| {
                // Break at the last space: emit up to (not including) it, and
                // start the next line just after it.
                try out.append(alloc, text[line_start..brk]);
                line_start = brk + 1;
                last_break = null;
                line_cols = displayWidth(text[line_start..i]);
            } else {
                // No space on this line: hard-split before the current glyph.
                try out.append(alloc, text[line_start..i]);
                line_start = i;
                line_cols = 0;
            }
        }

        if (is_space) last_break = i;
        line_cols += 1;
        i += adv;
    }
    // Flush the final line (always emit, even if empty/trailing fragment).
    try out.append(alloc, text[line_start..]);
}

/// Split `buffer` on newlines into paragraphs and wrap each to `width`,
/// appending all produced lines to `out`. A trailing newline produces a final
/// empty line (so a freshly-typed "\n" shows a blank row). An empty buffer
/// produces no lines.
fn wrapBuffer(buffer: []const u8, width: usize, out: *std.ArrayList([]const u8), alloc: std.mem.Allocator) !void {
    if (buffer.len == 0) return;
    var it = std.mem.splitScalar(u8, buffer, '\n');
    while (it.next()) |para| {
        try wrapParagraph(para, width, out, alloc);
    }
}

/// Build the cache-owned line set for a styled text block: each wrapped plain
/// line is wrapped in `style.open()`/`style.close()` and stored via the cache.
/// Returns the cache's owned lines re-typed for the vtable.
///
/// `width` bounds the *visible* width: the plain text is truncated to `width`
/// columns BEFORE styling escapes are added (escapes are zero visible width).
fn renderStyledLines(
    cache: *RenderCache,
    buffer: []const u8,
    style: Style,
    width: usize,
    alloc: std.mem.Allocator,
) ![]const []const u8 {
    // 1. Wrap plain text into borrowed slices.
    var plain: std.ArrayList([]const u8) = .empty;
    defer plain.deinit(alloc);
    try wrapBuffer(buffer, width, &plain, alloc);

    // 2. Style each line into a transient owned buffer.
    var styled: std.ArrayList([]const u8) = .empty;
    defer {
        for (styled.items) |s| alloc.free(s);
        styled.deinit(alloc);
    }
    for (plain.items) |line| {
        // Defensive truncate (wrap already bounds it, but a hard contract).
        const vis = truncateToCols(line, width);
        const composed = try std.fmt.allocPrint(alloc, "{s}{s}{s}", .{ style.open(), vis, style.close() });
        try styled.append(alloc, composed);
    }

    // 3. Commit to the cache (dupes), then return the cache's owned copy.
    try cache.store(styled.items);
    return cacheLines(cache);
}

/// Re-type the cache's owned `[][]u8` lines as `[]const []const u8` for the
/// vtable return. The cache guarantees these outlive the call until the next
/// render/invalidate.
fn cacheLines(cache: *RenderCache) []const []const u8 {
    const owned = cache.lines orelse return &.{};
    return @ptrCast(owned);
}

// ===========================================================================
// AssistantText — streaming assistant message (plan §6, §8)
// ===========================================================================

/// Accumulates assistant content deltas into an internal buffer and renders
/// the wrapped text with the theme's assistant style.
///
/// Streaming-tail dirty model (plan §3.3): a delta is appended to the buffer
/// and the cache is marked dirty; the engine then requests a render. Because
/// appended text only changes the LAST wrapped line(s) and leaves earlier
/// wrapped lines byte-identical, `RenderCache.store`'s diff naturally reports
/// `firstLineChanged` near the TAIL, not line 0 — the cut stays near the end
/// during streaming. (There is no per-delta render method on the interface;
/// the delta just mutates state + dirties, per plan §8.)
///
/// Markdown hosting (plan §8, DEFERRED): the buffer/render are structured so a
/// later pass can cache finished blocks and only re-render the last open block.
/// For P1 this is plain text + word wrap; the tail-near firstLineChanged
/// property already holds via the cache diff, so the markdown upgrade slots in
/// without changing the dirty model.
pub const AssistantText = struct {
    alloc: std.mem.Allocator,
    buffer: std.ArrayList(u8) = .empty,
    cache: RenderCache,

    pub fn init(alloc: std.mem.Allocator) AssistantText {
        return .{ .alloc = alloc, .cache = RenderCache.init(alloc) };
    }

    pub fn deinit(self: *AssistantText) void {
        self.buffer.deinit(self.alloc);
        self.cache.deinit();
    }

    /// Append a streaming content delta. Mutates the buffer and marks the cache
    /// dirty (the engine will requestRender). The cache diff keeps
    /// firstLineChanged near the tail.
    pub fn appendDelta(self: *AssistantText, delta: []const u8) !void {
        try self.buffer.appendSlice(self.alloc, delta);
        // markDirtyAppend RETAINS the baseline so the post-render diff recovers
        // the true tail change point; while dirty it reports a tail hint, so
        // the engine's cut stays near the end during streaming (plan §3.3/§8).
        self.cache.markDirtyAppend();
    }

    /// Replace the whole buffer (e.g. a non-streaming set). Marks dirty.
    pub fn setText(self: *AssistantText, text: []const u8) !void {
        self.buffer.clearRetainingCapacity();
        try self.buffer.appendSlice(self.alloc, text);
        self.cache.markDirty();
    }

    fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
        _ = alloc;
        const self: *AssistantText = @ptrCast(@alignCast(ptr));
        return renderStyledLines(&self.cache, self.buffer.items, theme.default.fg(.assistant), width, self.alloc);
    }

    fn firstLineChangedImpl(ptr: *anyopaque) ?usize {
        const self: *AssistantText = @ptrCast(@alignCast(ptr));
        return self.cache.firstLineChanged();
    }

    fn invalidateImpl(ptr: *anyopaque) void {
        const self: *AssistantText = @ptrCast(@alignCast(ptr));
        self.cache.invalidate();
    }

    const vtable = Component.VTable{
        .render = renderImpl,
        .firstLineChanged = firstLineChangedImpl,
        .invalidate = invalidateImpl,
    };

    pub fn comp(self: *AssistantText) Component {
        return .{ .ptr = self, .vtable = &vtable };
    }
};

// ===========================================================================
// UserText — submitted user message (plan §6)
// ===========================================================================

/// A submitted user message, rendered with the theme's user style. Static once
/// set; `setText` replaces it and marks dirty.
pub const UserText = struct {
    alloc: std.mem.Allocator,
    buffer: std.ArrayList(u8) = .empty,
    cache: RenderCache,

    pub fn init(alloc: std.mem.Allocator) UserText {
        return .{ .alloc = alloc, .cache = RenderCache.init(alloc) };
    }

    pub fn deinit(self: *UserText) void {
        self.buffer.deinit(self.alloc);
        self.cache.deinit();
    }

    /// Set the (static) message text. Marks dirty.
    pub fn setText(self: *UserText, text: []const u8) !void {
        self.buffer.clearRetainingCapacity();
        try self.buffer.appendSlice(self.alloc, text);
        self.cache.markDirty();
    }

    fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
        _ = alloc;
        const self: *UserText = @ptrCast(@alignCast(ptr));
        return renderStyledLines(&self.cache, self.buffer.items, theme.default.fg(.user), width, self.alloc);
    }

    fn firstLineChangedImpl(ptr: *anyopaque) ?usize {
        const self: *UserText = @ptrCast(@alignCast(ptr));
        return self.cache.firstLineChanged();
    }

    fn invalidateImpl(ptr: *anyopaque) void {
        const self: *UserText = @ptrCast(@alignCast(ptr));
        self.cache.invalidate();
    }

    const vtable = Component.VTable{
        .render = renderImpl,
        .firstLineChanged = firstLineChangedImpl,
        .invalidate = invalidateImpl,
    };

    pub fn comp(self: *UserText) Component {
        return .{ .ptr = self, .vtable = &vtable };
    }
};

// ===========================================================================
// InputBox — editable single-row+ input (plan §6, §3.5)
// ===========================================================================

/// A `Focusable` editor. Single row by default; ENTER submits, SHIFT+ENTER
/// inserts a newline (grows one row per line). Growth is UNBOUNDED in P1 (the
/// cap + scroll-window is deferred to P2).
///
/// Editing (raw keys via `handleInput`, decoded by `tui_input`):
///   - printable chars (UTF-8) insert at the cursor
///   - backspace deletes the codepoint before the cursor
///   - delete removes the codepoint at the cursor
///   - left/right move by one codepoint; home/end jump to start/end of the
///     current visual buffer (P1: whole buffer, not per-line)
///   - ENTER submits the whole buffer (see "Submit mechanism")
///   - SHIFT+ENTER inserts a '\n'
///
/// Cursor (plan §3.5): the box draws its OWN cursor as a reverse-video block
/// (theme `.cursor` style) over the glyph at the cursor position. When focused
/// it also emits `CURSOR_MARKER` (zero visible width) at the cursor location in
/// its render output, so the engine can later position the hardware cursor.
///
/// SHIFT+ENTER limitation: on terminals without the Kitty protocol, Enter and
/// Shift+Enter send identical bytes (`\r`); the decoder cannot distinguish them
/// and both arrive as `.enter` with no shift modifier, so only plain submit is
/// possible there. When Kitty IS active, Shift+Enter arrives as CSI-u
/// (`\x1b[13;2u`) with `mods.shift` set and this box inserts a newline. The
/// logic works wherever the distinction is available.
///
/// Submit mechanism: a POLLABLE buffer. On ENTER the current editor contents
/// are moved into `submitted` and the editor is cleared. The app calls
/// `takeSubmitted()` once per frame; it returns the submitted bytes (owned by
/// the box, valid until the next `takeSubmitted`/edit) and clears the pending
/// flag, or null if nothing was submitted. This avoids callback re-entrancy
/// into the render loop.
pub const InputBox = struct {
    alloc: std.mem.Allocator,
    focusable: Focusable = .{},
    /// Editor contents (may contain '\n' for multi-line input). UTF-8.
    text: std.ArrayList(u8) = .empty,
    /// Cursor position as a BYTE offset into `text` (always on a codepoint
    /// boundary).
    cursor: usize = 0,
    /// Pending submitted line, owned by the box. Valid until the next submit
    /// or `takeSubmitted`.
    submitted: std.ArrayList(u8) = .empty,
    has_submitted: bool = false,
    cache: RenderCache,

    pub fn init(alloc: std.mem.Allocator) InputBox {
        return .{ .alloc = alloc, .cache = RenderCache.init(alloc) };
    }

    pub fn deinit(self: *InputBox) void {
        self.text.deinit(self.alloc);
        self.submitted.deinit(self.alloc);
        self.cache.deinit();
    }

    // -- focus -------------------------------------------------------------

    /// Set focus. Re-dirties because the cursor block + marker only render when
    /// focused, so focus changes alter the output.
    pub fn setFocused(self: *InputBox, value: bool) void {
        if (self.focusable.focused != value) {
            self.focusable.setFocused(value);
            self.cache.markDirty();
        }
    }

    pub fn isFocused(self: *const InputBox) bool {
        return self.focusable.focused;
    }

    // -- submit polling ----------------------------------------------------

    /// Poll the submitted line. Returns the bytes (box-owned) and clears the
    /// pending flag, or null if nothing was submitted since the last poll.
    pub fn takeSubmitted(self: *InputBox) ?[]const u8 {
        if (!self.has_submitted) return null;
        self.has_submitted = false;
        return self.submitted.items;
    }

    // -- editing primitives (also directly unit-testable) ------------------

    fn insertText(self: *InputBox, bytes: []const u8) !void {
        try self.text.insertSlice(self.alloc, self.cursor, bytes);
        self.cursor += bytes.len;
        self.cache.markDirty();
    }

    fn backspace(self: *InputBox) void {
        if (self.cursor == 0) return;
        const start = self.prevBoundary(self.cursor);
        const removed = self.cursor - start;
        std.mem.copyForwards(u8, self.text.items[start..], self.text.items[self.cursor..]);
        self.text.items.len -= removed;
        self.cursor = start;
        self.cache.markDirty();
    }

    fn deleteForward(self: *InputBox) void {
        if (self.cursor >= self.text.items.len) return;
        const next = self.nextBoundary(self.cursor);
        const removed = next - self.cursor;
        std.mem.copyForwards(u8, self.text.items[self.cursor..], self.text.items[next..]);
        self.text.items.len -= removed;
        self.cache.markDirty();
    }

    fn moveLeft(self: *InputBox) void {
        if (self.cursor == 0) return;
        self.cursor = self.prevBoundary(self.cursor);
        self.cache.markDirty();
    }

    fn moveRight(self: *InputBox) void {
        if (self.cursor >= self.text.items.len) return;
        self.cursor = self.nextBoundary(self.cursor);
        self.cache.markDirty();
    }

    fn moveHome(self: *InputBox) void {
        if (self.cursor == 0) return;
        self.cursor = 0;
        self.cache.markDirty();
    }

    fn moveEnd(self: *InputBox) void {
        if (self.cursor == self.text.items.len) return;
        self.cursor = self.text.items.len;
        self.cache.markDirty();
    }

    fn submit(self: *InputBox) !void {
        self.submitted.clearRetainingCapacity();
        try self.submitted.appendSlice(self.alloc, self.text.items);
        self.has_submitted = true;
        self.text.clearRetainingCapacity();
        self.cursor = 0;
        self.cache.markDirty();
    }

    /// Byte index of the codepoint boundary before `i` (i > 0).
    fn prevBoundary(self: *const InputBox, i: usize) usize {
        var j = i - 1;
        while (j > 0 and isContinuation(self.text.items[j])) : (j -= 1) {}
        return j;
    }

    /// Byte index of the next codepoint boundary after `i` (i < len).
    fn nextBoundary(self: *const InputBox, i: usize) usize {
        const seq_len = std.unicode.utf8ByteSequenceLength(self.text.items[i]) catch 1;
        return @min(i + seq_len, self.text.items.len);
    }

    fn isContinuation(b: u8) bool {
        return (b & 0xc0) == 0x80;
    }

    // -- input handling ----------------------------------------------------

    /// Apply one decoded key. Split out so tests can drive editing without raw
    /// byte sequences.
    pub fn applyKey(self: *InputBox, k: Key) !void {
        if (k.event == .release) return;
        switch (k.code) {
            .char => {
                if (k.mods.ctrl or k.mods.alt or k.mods.super) return; // not a printable insert
                if (k.text) |t| {
                    try self.insertText(t);
                } else {
                    // Encode the codepoint ourselves when no text was carried.
                    var buf: [4]u8 = undefined;
                    const n = std.unicode.utf8Encode(k.code.char, &buf) catch return;
                    try self.insertText(buf[0..n]);
                }
            },
            .enter => {
                if (k.mods.shift) {
                    try self.insertText("\n"); // shift+enter newline
                } else {
                    try self.submit();
                }
            },
            .backspace => self.backspace(),
            .delete => self.deleteForward(),
            .left => self.moveLeft(),
            .right => self.moveRight(),
            .home => self.moveHome(),
            .end => self.moveEnd(),
            else => {}, // tab, arrows up/down, fkeys: ignored in P1
        }
    }

    fn handleInputImpl(ptr: *anyopaque, data: []const u8) void {
        const self: *InputBox = @ptrCast(@alignCast(ptr));
        var off: usize = 0;
        while (off < data.len) {
            const step = input.decodeOne(data[off..]) orelse break; // partial tail: drop in P1
            off += step.consumed;
            switch (step.decoded) {
                .key => |k| self.applyKey(k) catch return,
                .paste => |p| self.insertText(p) catch return,
            }
        }
    }

    // -- render ------------------------------------------------------------

    fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
        _ = alloc;
        const self: *InputBox = @ptrCast(@alignCast(ptr));
        return self.renderLines(width);
    }

    /// Render the editor: split on '\n' into visual rows, place the styled
    /// cursor block + CURSOR_MARKER at the cursor row/column when focused.
    /// Truncates each row to `width` columns.
    fn renderLines(self: *InputBox, width: usize) ![]const []const u8 {
        const a = self.alloc;
        var rows: std.ArrayList([]const u8) = .empty;
        defer {
            for (rows.items) |r| a.free(r);
            rows.deinit(a);
        }

        const cursor_style = theme.default.fg(.cursor);
        // Locate the cursor's (row, byte-col-in-row).
        const focused = self.focusable.focused;

        // Walk lines, tracking byte offset so we know which row holds cursor.
        var line_byte_start: usize = 0;
        var produced_any = false;
        var it = std.mem.splitScalar(u8, self.text.items, '\n');
        while (it.next()) |line| {
            const line_start = line_byte_start;
            const line_end = line_start + line.len;
            const cursor_in_line = focused and self.cursor >= line_start and self.cursor <= line_end and
                // The cursor belongs to the FIRST line whose range contains it
                // (at a '\n' boundary it stays on the line before the break,
                // i.e. == line_end). Disambiguate the boundary: if cursor ==
                // line_end and there are more lines, it belongs to the NEXT
                // line's start unless this is the last line.
                (self.cursor < line_end or it.peek() == null);

            const row = try self.renderRow(line, if (cursor_in_line) self.cursor - line_start else null, cursor_style, width, focused);
            try rows.append(a, row);
            produced_any = true;
            line_byte_start = line_end + 1; // skip the '\n'
        }
        if (!produced_any) {
            // Empty buffer: a single (possibly cursor-bearing) row.
            const row = try self.renderRow("", if (focused) @as(?usize, 0) else null, cursor_style, width, focused);
            try rows.append(a, row);
        }

        try self.cache.store(rows.items);
        return cacheLines(&self.cache);
    }

    /// Render one visual row. `cursor_col` is the byte offset within `line`
    /// where the cursor sits (null if the cursor isn't on this row). When
    /// present and focused, draws a reverse-video block over the glyph at the
    /// cursor (or a space at end-of-line) and emits CURSOR_MARKER there.
    fn renderRow(self: *InputBox, line: []const u8, cursor_col: ?usize, cursor_style: Style, width: usize, focused: bool) ![]u8 {
        const a = self.alloc;
        // The cursor block consumes one visible column, so usable text width
        // is width-1 when the cursor sits at/after the truncated end and we
        // must show the block. To keep it simple and always-safe: truncate the
        // plain line to `width` columns; if a cursor block would push us to
        // width+1, the block replaces the last column instead.
        const vis = truncateToCols(line, width);

        var buf: std.ArrayList(u8) = .empty;
        errdefer buf.deinit(a);

        if (cursor_col == null or !focused) {
            try buf.appendSlice(a, vis);
            return buf.toOwnedSlice(a);
        }

        // Cursor is on this row. Find the byte position within `vis`.
        const cc = cursor_col.?;
        const before_cols = displayWidth(line[0..@min(cc, line.len)]);

        if (cc >= line.len) {
            // Cursor at end-of-line: block over a trailing space. Ensure room:
            // if the visible text already fills `width`, drop its last column.
            var head = vis;
            if (before_cols >= width) {
                head = truncateToCols(line, width - 1);
            }
            try buf.appendSlice(a, head);
            try buf.appendSlice(a, CURSOR_MARKER);
            try buf.appendSlice(a, cursor_style.open());
            try buf.appendSlice(a, " ");
            try buf.appendSlice(a, cursor_style.close());
            return buf.toOwnedSlice(a);
        }

        // Cursor over an interior glyph. Split: head | glyph | tail.
        const glyph_len = blk: {
            const sl = std.unicode.utf8ByteSequenceLength(line[cc]) catch 1;
            break :blk @min(sl, line.len - cc);
        };
        const head = line[0..cc];
        const glyph = line[cc .. cc + glyph_len];
        const tail = line[cc + glyph_len ..];

        // Compose head + marker + [reverse]glyph[/] + tail, then truncate the
        // whole visible width to `width` columns (escapes + marker are
        // zero-width, so truncation acts on glyphs).
        try buf.appendSlice(a, head);
        try buf.appendSlice(a, CURSOR_MARKER);
        try buf.appendSlice(a, cursor_style.open());
        try buf.appendSlice(a, glyph);
        try buf.appendSlice(a, cursor_style.close());
        try buf.appendSlice(a, tail);

        // The composed row's visible width == displayWidth(line). If that
        // exceeds `width`, rebuild with a width-bounded tail. (Rare: cursor
        // near a long line's start.) Simpler safe path: if over, truncate the
        // tail.
        if (displayWidth(line) > width) {
            buf.clearRetainingCapacity();
            // Keep head+glyph; truncate tail to remaining columns.
            const used = before_cols + 1; // head cols + the glyph
            try buf.appendSlice(a, head);
            try buf.appendSlice(a, CURSOR_MARKER);
            try buf.appendSlice(a, cursor_style.open());
            try buf.appendSlice(a, glyph);
            try buf.appendSlice(a, cursor_style.close());
            if (used < width) {
                const remaining = width - used;
                try buf.appendSlice(a, truncateToCols(tail, remaining));
            }
        }
        return buf.toOwnedSlice(a);
    }

    fn firstLineChangedImpl(ptr: *anyopaque) ?usize {
        const self: *InputBox = @ptrCast(@alignCast(ptr));
        return self.cache.firstLineChanged();
    }

    fn invalidateImpl(ptr: *anyopaque) void {
        const self: *InputBox = @ptrCast(@alignCast(ptr));
        self.cache.invalidate();
    }

    const vtable = Component.VTable{
        .render = renderImpl,
        .firstLineChanged = firstLineChangedImpl,
        .invalidate = invalidateImpl,
        .handleInput = handleInputImpl,
    };

    pub fn comp(self: *InputBox) Component {
        return .{ .ptr = self, .vtable = &vtable };
    }
};

// ===========================================================================
// Footer — persistent bottom line with frame-timing element (plan §6)
// ===========================================================================

/// The persistent bottom line. For P1 it renders a FRAME-TIMING element: the
/// last frame's render time as a theoretical-max fps (1000/ms), shown inverted
/// (reverse-video). It optionally shows model info passed in by the app. The
/// fps element is TEMPORARY (removed after perf validation) but REQUIRED for
/// P1.
///
/// Frame-time input: the app calls `setFrameTime(ms)` after each rendered frame
/// with the measured render duration in milliseconds; this updates the fps and
/// marks dirty so the footer repaints. `setModel(name)` sets the model info.
pub const Footer = struct {
    alloc: std.mem.Allocator,
    cache: RenderCache,
    /// Last frame's render time in milliseconds (null = not measured yet).
    frame_ms: ?f64 = null,
    /// Model info string (borrowed; copied into a small owned buffer on set).
    model: std.ArrayList(u8) = .empty,

    pub fn init(alloc: std.mem.Allocator) Footer {
        return .{ .alloc = alloc, .cache = RenderCache.init(alloc) };
    }

    pub fn deinit(self: *Footer) void {
        self.model.deinit(self.alloc);
        self.cache.deinit();
    }

    /// Feed the last frame's render time (milliseconds). Marks dirty so the
    /// footer's fps element repaints next frame.
    pub fn setFrameTime(self: *Footer, ms: f64) void {
        self.frame_ms = ms;
        self.cache.markDirty();
    }

    /// Set the model info shown in the footer.
    pub fn setModel(self: *Footer, name: []const u8) !void {
        self.model.clearRetainingCapacity();
        try self.model.appendSlice(self.alloc, name);
        self.cache.markDirty();
    }

    /// Format the theoretical-max fps element from the last frame time.
    /// `fps = 1000 / ms`; a zero/sub-millisecond frame is reported as a capped
    /// ">9999" sentinel rather than infinity. "--" when unmeasured.
    fn fpsText(self: *const Footer, buf: []u8) []const u8 {
        const ms = self.frame_ms orelse return std.fmt.bufPrint(buf, "fps: --", .{}) catch "fps: --";
        if (ms <= 0.0) return std.fmt.bufPrint(buf, "fps: >9999", .{}) catch "fps: >9999";
        const fps = 1000.0 / ms;
        if (fps > 9999.0) return std.fmt.bufPrint(buf, "fps: >9999", .{}) catch "fps: >9999";
        return std.fmt.bufPrint(buf, "fps: {d:.0} ({d:.2}ms)", .{ fps, ms }) catch "fps: ?";
    }

    fn renderImpl(ptr: *anyopaque, width: usize, alloc: std.mem.Allocator) anyerror![]const []const u8 {
        _ = alloc;
        const self: *Footer = @ptrCast(@alignCast(ptr));
        const a = self.alloc;

        var fps_buf: [48]u8 = undefined;
        const fps = self.fpsText(&fps_buf);

        // Build the PLAIN content: "<fps>   <model>" (model only if set).
        var plain: std.ArrayList(u8) = .empty;
        defer plain.deinit(a);
        try plain.appendSlice(a, fps);
        if (self.model.items.len != 0) {
            try plain.appendSlice(a, "   ");
            try plain.appendSlice(a, self.model.items);
        }

        const vis = truncateToCols(plain.items, width);

        // The fps element is shown INVERTED (reverse video). The whole footer
        // line uses reverse video so the timing element stands out; the model
        // rides along in the same inverted run. (Temporary perf chrome.)
        const cursor_style = theme.default.fg(.cursor); // reverse video
        const composed = try std.fmt.allocPrint(a, "{s}{s}{s}", .{ cursor_style.open(), vis, cursor_style.close() });
        defer a.free(composed);

        const lines = [_][]const u8{composed};
        try self.cache.store(&lines);
        return cacheLines(&self.cache);
    }

    fn firstLineChangedImpl(ptr: *anyopaque) ?usize {
        const self: *Footer = @ptrCast(@alignCast(ptr));
        return self.cache.firstLineChanged();
    }

    fn invalidateImpl(ptr: *anyopaque) void {
        const self: *Footer = @ptrCast(@alignCast(ptr));
        self.cache.invalidate();
    }

    const vtable = Component.VTable{
        .render = renderImpl,
        .firstLineChanged = firstLineChangedImpl,
        .invalidate = invalidateImpl,
    };

    pub fn comp(self: *Footer) Component {
        return .{ .ptr = self, .vtable = &vtable };
    }
};

// ===========================================================================
// Tests
// ===========================================================================

const testing = std.testing;
const engine = @import("tui_engine.zig");

/// Visible width of a rendered (possibly styled, possibly marker-bearing) line,
/// reusing the engine's authoritative measure.
fn vw(line: []const u8) usize {
    return engine.visibleWidth(line);
}

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

test "displayWidth counts codepoints; truncateToCols respects boundaries" {
    try testing.expectEqual(@as(usize, 3), displayWidth("abc"));
    try testing.expectEqual(@as(usize, 3), displayWidth("aé✓"));
    try testing.expectEqualStrings("aé", truncateToCols("aé✓", 2));
    try testing.expectEqualStrings("abc", truncateToCols("abcdef", 3));
    // Never splits a multibyte codepoint.
    const t = truncateToCols("é", 1);
    try testing.expectEqualStrings("é", t);
}

test "wrapParagraph word-wraps and hard-splits long words" {
    var out: std.ArrayList([]const u8) = .empty;
    defer out.deinit(testing.allocator);
    try wrapParagraph("hello world foo", 7, &out, testing.allocator);
    try testing.expectEqual(@as(usize, 3), out.items.len);
    try testing.expectEqualStrings("hello", out.items[0]);
    try testing.expectEqualStrings("world", out.items[1]);
    try testing.expectEqualStrings("foo", out.items[2]);

    out.clearRetainingCapacity();
    try wrapParagraph("abcdefghij", 4, &out, testing.allocator);
    try testing.expectEqual(@as(usize, 3), out.items.len);
    try testing.expectEqualStrings("abcd", out.items[0]);
    try testing.expectEqualStrings("efgh", out.items[1]);
    try testing.expectEqualStrings("ij", out.items[2]);
}

// -- AssistantText ---------------------------------------------------------

test "AssistantText: renders wrapped text within width" {
    var at = AssistantText.init(testing.allocator);
    defer at.deinit();
    try at.setText("hello world foo");
    const lines = try at.comp().render(7, testing.allocator);
    try testing.expectEqual(@as(usize, 3), lines.len);
    for (lines) |l| try testing.expect(vw(l) <= 7);
    // First render after empty cache => changed from 0.
    try testing.expectEqual(@as(?usize, 0), at.comp().firstLineChanged());
}

test "AssistantText: streaming keeps firstLineChanged near the tail, not 0" {
    var at = AssistantText.init(testing.allocator);
    defer at.deinit();
    // Seed several wrapped lines.
    try at.setText("alpha beta gamma delta epsilon");
    _ = try at.comp().render(11, testing.allocator);
    const lines1 = at.cache.lines.?.len;
    try testing.expect(lines1 >= 3);

    // Append a delta to the tail; earlier wrapped lines stay byte-identical.
    try at.appendDelta(" zeta");
    // While dirty, firstLineChanged reports 0 (full markDirty). The KEY
    // property is what the cache reports AFTER the render: the lowest line that
    // actually changed must be near the tail, not 0.
    _ = try at.comp().render(11, testing.allocator);
    const fc = at.comp().firstLineChanged();
    // The change landed on the last line(s); the cut must be > 0.
    try testing.expect(fc == null or fc.? > 0);
    // Stronger: the first changed line should be at the tail region.
    if (fc) |v| try testing.expect(v >= lines1 - 1);
}

test "AssistantText: width truncation on a long unbroken word" {
    var at = AssistantText.init(testing.allocator);
    defer at.deinit();
    try at.setText("supercalifragilistic");
    const lines = try at.comp().render(5, testing.allocator);
    for (lines) |l| try testing.expect(vw(l) <= 5);
}

// -- UserText ---------------------------------------------------------------

test "UserText: renders user-styled lines within width" {
    var ut = UserText.init(testing.allocator);
    defer ut.deinit();
    try ut.setText("a user message that wraps");
    const lines = try ut.comp().render(10, testing.allocator);
    for (lines) |l| try testing.expect(vw(l) <= 10);
    // Static: re-render with no change => clean.
    _ = try ut.comp().render(10, testing.allocator);
    try testing.expectEqual(@as(?usize, null), ut.comp().firstLineChanged());
}

// -- InputBox ---------------------------------------------------------------

fn charKey(c: u21, text: []const u8) Key {
    return .{ .code = .{ .char = c }, .text = text };
}

test "InputBox: insert printable chars and render with cursor block when focused" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    ib.setFocused(true);
    try ib.applyKey(charKey('h', "h"));
    try ib.applyKey(charKey('i', "i"));
    const lines = try ib.comp().render(20, testing.allocator);
    try testing.expectEqual(@as(usize, 1), lines.len);
    try testing.expect(vw(lines[0]) <= 20);
    // Focused => emits CURSOR_MARKER and reverse-video style.
    try testing.expect(std.mem.indexOf(u8, lines[0], CURSOR_MARKER) != null);
    try testing.expect(std.mem.indexOf(u8, lines[0], "\x1b[7m") != null);
    try testing.expect(std.mem.indexOf(u8, lines[0], "hi") != null);
}

test "InputBox: not focused emits no cursor marker" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    try ib.applyKey(charKey('x', "x"));
    const lines = try ib.comp().render(20, testing.allocator);
    try testing.expect(std.mem.indexOf(u8, lines[0], CURSOR_MARKER) == null);
}

test "InputBox: backspace, delete, and cursor movement" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    for ("abc") |c| try ib.applyKey(charKey(c, &[_]u8{c}));
    try testing.expectEqual(@as(usize, 3), ib.cursor);
    ib.backspace(); // "ab"
    try testing.expectEqualStrings("ab", ib.text.items);
    try testing.expectEqual(@as(usize, 2), ib.cursor);
    ib.moveLeft(); // cursor at 1
    try testing.expectEqual(@as(usize, 1), ib.cursor);
    ib.deleteForward(); // delete 'b' -> "a"
    try testing.expectEqualStrings("a", ib.text.items);
    ib.moveHome();
    try testing.expectEqual(@as(usize, 0), ib.cursor);
    ib.moveEnd();
    try testing.expectEqual(@as(usize, 1), ib.cursor);
}

test "InputBox: multibyte backspace removes a whole codepoint" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    try ib.applyKey(charKey('é', "é")); // 2 bytes
    try testing.expectEqual(@as(usize, 2), ib.cursor);
    ib.backspace();
    try testing.expectEqual(@as(usize, 0), ib.text.items.len);
}

test "InputBox: shift+enter inserts newline, enter submits" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    for ("ab") |c| try ib.applyKey(charKey(c, &[_]u8{c}));
    // Shift+Enter => newline (grows a row).
    try ib.applyKey(.{ .code = .enter, .mods = .{ .shift = true } });
    for ("cd") |c| try ib.applyKey(charKey(c, &[_]u8{c}));
    try testing.expectEqualStrings("ab\ncd", ib.text.items);
    const lines = try ib.comp().render(20, testing.allocator);
    try testing.expectEqual(@as(usize, 2), lines.len);

    // Plain Enter => submit, editor cleared, pollable buffer set.
    try ib.applyKey(.{ .code = .enter });
    const got = ib.takeSubmitted();
    try testing.expect(got != null);
    try testing.expectEqualStrings("ab\ncd", got.?);
    try testing.expectEqual(@as(usize, 0), ib.text.items.len);
    // Second poll returns null.
    try testing.expect(ib.takeSubmitted() == null);
}

test "InputBox: handleInput decodes raw bytes (typing + enter)" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    ib.comp().handleInput("hi\r"); // 'h' 'i' Enter
    const got = ib.takeSubmitted();
    try testing.expect(got != null);
    try testing.expectEqualStrings("hi", got.?);
}

test "InputBox: handleInput kitty shift+enter inserts newline" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    ib.comp().handleInput("a\x1b[13;2ub"); // 'a', shift+enter, 'b'
    try testing.expectEqualStrings("a\nb", ib.text.items);
    try testing.expect(ib.takeSubmitted() == null); // no plain enter yet
}

test "InputBox: firstLineChanged is cache-derived (clean after stable render)" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    ib.setFocused(true);
    try ib.applyKey(charKey('x', "x"));
    _ = try ib.comp().render(20, testing.allocator);
    // Re-render with no state change => clean.
    _ = try ib.comp().render(20, testing.allocator);
    try testing.expectEqual(@as(?usize, null), ib.comp().firstLineChanged());
    // Edit => dirty again.
    try ib.applyKey(charKey('y', "y"));
    try testing.expectEqual(@as(?usize, 0), ib.comp().firstLineChanged());
}

test "InputBox: cursor block fits within width at end of a full line" {
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    ib.setFocused(true);
    for ("abcde") |c| try ib.applyKey(charKey(c, &[_]u8{c}));
    // width 5, cursor at end: the block must not overflow.
    const lines = try ib.comp().render(5, testing.allocator);
    try testing.expect(vw(lines[0]) <= 5);
}

// -- Footer -----------------------------------------------------------------

test "Footer: renders fps from frame time, inverted, within width" {
    var ft = Footer.init(testing.allocator);
    defer ft.deinit();
    ft.setFrameTime(8.0); // 1000/8 = 125 fps
    const lines = try ft.comp().render(80, testing.allocator);
    try testing.expectEqual(@as(usize, 1), lines.len);
    try testing.expect(vw(lines[0]) <= 80);
    // Inverted (reverse video) styling present.
    try testing.expect(std.mem.indexOf(u8, lines[0], "\x1b[7m") != null);
    // fps value 125 present.
    try testing.expect(std.mem.indexOf(u8, lines[0], "125") != null);
}

test "Footer: unmeasured frame shows placeholder; submillisecond capped" {
    var ft = Footer.init(testing.allocator);
    defer ft.deinit();
    var buf: [48]u8 = undefined;
    try testing.expectEqualStrings("fps: --", ft.fpsText(&buf));
    ft.setFrameTime(0.0);
    try testing.expectEqualStrings("fps: >9999", ft.fpsText(&buf));
    ft.setFrameTime(0.05); // 20000 fps -> capped
    try testing.expectEqualStrings("fps: >9999", ft.fpsText(&buf));
}

test "Footer: shows model info and truncates to width" {
    var ft = Footer.init(testing.allocator);
    defer ft.deinit();
    ft.setFrameTime(10.0);
    try ft.setModel("gpt-test-model");
    const lines = try ft.comp().render(12, testing.allocator);
    try testing.expect(vw(lines[0]) <= 12);
}

test "Footer: setFrameTime dirties; stable re-render is clean" {
    var ft = Footer.init(testing.allocator);
    defer ft.deinit();
    ft.setFrameTime(8.0);
    _ = try ft.comp().render(80, testing.allocator);
    _ = try ft.comp().render(80, testing.allocator);
    try testing.expectEqual(@as(?usize, null), ft.comp().firstLineChanged());
    ft.setFrameTime(16.0);
    try testing.expectEqual(@as(?usize, 0), ft.comp().firstLineChanged());
}

// -- Integration with the real Engine (no TTY) ------------------------------

test "components drive the real engine without a TTY" {
    var buf = std.Io.Writer.Allocating.init(testing.allocator);
    defer buf.deinit();
    var eng = engine.Engine.init(testing.allocator, &buf.writer, 40, 24, false);
    defer eng.deinit();

    var user = UserText.init(testing.allocator);
    defer user.deinit();
    var assistant = AssistantText.init(testing.allocator);
    defer assistant.deinit();
    var ib = InputBox.init(testing.allocator);
    defer ib.deinit();
    var footer = Footer.init(testing.allocator);
    defer footer.deinit();

    try user.setText("hi there");
    try assistant.appendDelta("hello");
    ib.setFocused(true);
    try ib.applyKey(charKey('q', "q"));
    footer.setFrameTime(8.0);

    try eng.addComponent(user.comp());
    try eng.addComponent(assistant.comp());
    try eng.addComponent(ib.comp());
    try eng.addComponent(footer.comp());

    try eng.render(); // first paint: must not error (width contract holds)
    const out = buf.written();
    try testing.expect(std.mem.indexOf(u8, out, "hi there") != null);
    try testing.expect(std.mem.indexOf(u8, out, "hello") != null);
    // Cursor marker is consumed by the engine and recorded as a hint.
    try testing.expect(eng.cursor_hint != null);

    // Stream another delta -> only the assistant should re-render; the engine
    // stays on the differential path (no full clear after first paint).
    try assistant.appendDelta(" world");
    footer.setFrameTime(9.0);
    buf.clearRetainingCapacity();
    try eng.render();
    const out2 = buf.written();
    try testing.expect(std.mem.indexOf(u8, out2, "world") != null);
}