//! TUI frontend: terminal lifecycle, rendering, and input handling.
//!
//! All communication with the core orchestrator flows through channels:
//! - [`UserAction`] sent via `action_tx` when the user submits input or quits
//! - [`UIEvent`] received via `event_rx` to display streaming assistant responses
//!
//! The terminal lifecycle follows the standard crossterm pattern:
//! 1. Enable raw mode
//! 2. Enter alternate screen
//! 3. On exit (or panic), disable raw mode and leave the alternate screen

use std::io::{self, Stdout};
use std::time::Duration;

use crossterm::event::{Event, EventStream, KeyCode, KeyEvent, KeyModifiers};
use crossterm::execute;
use crossterm::terminal::{
    EnterAlternateScreen, LeaveAlternateScreen, disable_raw_mode, enable_raw_mode,
};
use futures::StreamExt;
use ratatui::backend::CrosstermBackend;
use ratatui::layout::{Constraint, Layout, Rect};
use ratatui::style::{Color, Modifier, Style};
use ratatui::text::{Line, Span};
use ratatui::widgets::{Block, Clear, Paragraph, Wrap};
use ratatui::{Frame, Terminal};
use tokio::sync::mpsc;
use tracing::debug;

use crate::core::types::{Role, UIEvent, UserAction};

/// Errors that can occur in the TUI layer.
#[derive(Debug, thiserror::Error)]
pub enum TuiError {
    /// An underlying terminal I/O error.
    #[error("terminal IO error: {0}")]
    Io(#[from] std::io::Error),
}

/// Vim-style editing mode for the TUI.
///
/// The TUI starts in Insert mode so first-time users can type immediately.
/// Mode transitions:
///   Insert --Esc--> Normal
///   Normal --i--> Insert
///   Normal --:--> Command
///   Command --Esc/Enter--> Normal
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Mode {
    /// Navigation and command entry. Keys are interpreted as commands, not text.
    Normal,
    /// Text input. Printable keys append to the input buffer.
    Insert,
    /// Ex-style command entry. The command buffer lives in `AppState::command_buffer`.
    Command,
}

/// The UI-layer view of a conversation: rendered messages and the current input buffer.
pub struct AppState {
    /// All conversation turns rendered as (role, content) pairs.
    pub messages: Vec<(Role, String)>,
    /// The current contents of the input box.
    pub input: String,
    /// Vertical scroll offset for the output pane (lines from top).
    pub scroll: u16,
    /// Current vim-style editing mode.
    pub mode: Mode,
    /// Partial command text in Command mode (without the leading `:`).
    pub command_buffer: String,
    /// Buffered keystrokes for multi-key chord resolution in Normal mode (e.g. `gg`).
    pub pending_keys: Vec<char>,
    /// Last-known viewport height (output pane lines). Updated each frame by `update_scroll`.
    pub viewport_height: u16,
    /// Transient error message shown in the status bar, cleared on next keypress.
    pub status_error: Option<String>,
}

impl AppState {
    fn new() -> Self {
        Self {
            messages: Vec::new(),
            input: String::new(),
            scroll: 0,
            mode: Mode::Insert,
            command_buffer: String::new(),
            pending_keys: Vec::new(),
            viewport_height: 0,
            status_error: None,
        }
    }
}

/// Initialise the terminal: enable raw mode and switch to the alternate screen.
///
/// Callers must pair this with [`restore_terminal`] (and [`install_panic_hook`]) to
/// guarantee cleanup even on abnormal exit.
pub fn init_terminal() -> Result<Terminal<CrosstermBackend<Stdout>>, TuiError> {
    enable_raw_mode()?;
    let mut stdout = io::stdout();
    execute!(stdout, EnterAlternateScreen)?;
    let backend = CrosstermBackend::new(stdout);
    let terminal = Terminal::new(backend)?;
    Ok(terminal)
}

/// Restore the terminal to its pre-launch state: disable raw mode and leave the
/// alternate screen.
pub fn restore_terminal() -> io::Result<()> {
    disable_raw_mode()?;
    execute!(io::stdout(), LeaveAlternateScreen)?;
    Ok(())
}

/// Install a panic hook that restores the terminal before printing the panic message.
///
/// Without this, a panic leaves the terminal in raw mode with the alternate screen
/// active, making the shell unusable until the user runs `reset`.
pub fn install_panic_hook() {
    let original = std::panic::take_hook();
    std::panic::set_hook(Box::new(move |info| {
        // Best-effort restore; if it fails the original hook still runs.
        let _ = restore_terminal();
        original(info);
    }));
}

/// Internal control flow signal returned by [`handle_key`].
enum LoopControl {
    /// The user pressed Enter with non-empty input; send this message to the core.
    SendMessage(String),
    /// The user pressed Ctrl+C or Ctrl+D; exit the event loop.
    Quit,
    /// The user ran `:clear`; wipe the conversation.
    ClearHistory,
}

/// Map a key event to a [`LoopControl`] signal, mutating `state` as a side-effect.
///
/// Ctrl+C / Ctrl+D quit from any mode. All other keys are dispatched to the
/// handler for the current [`Mode`].
fn handle_key(key: Option<KeyEvent>, state: &mut AppState) -> Option<LoopControl> {
    let key = key?;
    // Clear any transient status error on the next keypress.
    state.status_error = None;
    // Ctrl+C quits from any mode.
    if key.modifiers.contains(KeyModifiers::CONTROL) && key.code == KeyCode::Char('c') {
        return Some(LoopControl::Quit);
    }
    // Ctrl+D quits only from Insert/Command mode. In Normal mode it scrolls.
    if key.modifiers.contains(KeyModifiers::CONTROL)
        && key.code == KeyCode::Char('d')
        && !matches!(state.mode, Mode::Normal)
    {
        return Some(LoopControl::Quit);
    }
    match &state.mode {
        Mode::Normal => handle_normal_key(key, state),
        Mode::Insert => handle_insert_key(key, state),
        Mode::Command => handle_command_key(key, state),
    }
}

enum ChordResult {
    /// The chord was fully matched; the returned closure applies the effect.
    Executed,
    /// The keys so far are a valid prefix of at least one chord.
    Prefix,
    /// The keys don't match any chord or prefix.
    NoMatch,
}

/// Check `keys` against known multi-key chords in Normal mode.
///
/// Currently only `gg` (scroll to top) is defined. Returns the match status
/// without mutating state -- the caller applies effects for `Executed`.
fn resolve_chord(keys: &[char]) -> ChordResult {
    match keys {
        [.., 'g', 'g'] => ChordResult::Executed,
        [.., 'g'] => ChordResult::Prefix,
        _ => ChordResult::NoMatch,
    }
}

/// Handle a keypress in Normal mode.
///
/// | Key | Effect                                    |
/// |-----|-------------------------------------------|
/// | `i` | Switch to Insert mode                     |
/// | `:` | Switch to Command mode (empty buffer)     |
/// | `j` | Scroll down one line                      |
/// | `k` | Scroll up one line                        |
/// | `G` | Scroll to bottom                          |
/// | `g` | Begin chord; `gg` scrolls to top          |
/// | Ctrl+d | Scroll down half a page                |
/// | Ctrl+u | Scroll up half a page                  |
fn handle_normal_key(key: KeyEvent, state: &mut AppState) -> Option<LoopControl> {
    let is_ctrl = key.modifiers.contains(KeyModifiers::CONTROL);
    match key.code {
        KeyCode::Char('d') if is_ctrl => {
            let half = (state.viewport_height / 2).max(1);
            state.scroll = state.scroll.saturating_add(half);
            state.pending_keys.clear();
        }
        KeyCode::Char('u') if is_ctrl => {
            let half = (state.viewport_height / 2).max(1);
            state.scroll = state.scroll.saturating_sub(half);
            state.pending_keys.clear();
        }
        KeyCode::Char('i') if !is_ctrl && state.pending_keys.is_empty() => {
            state.mode = Mode::Insert;
        }
        KeyCode::Char(':') if !is_ctrl && state.pending_keys.is_empty() => {
            state.command_buffer.clear();
            state.mode = Mode::Command;
        }
        KeyCode::Char('j') if !is_ctrl && state.pending_keys.is_empty() => {
            state.scroll = state.scroll.saturating_add(1);
        }
        KeyCode::Char('k') if !is_ctrl && state.pending_keys.is_empty() => {
            state.scroll = state.scroll.saturating_sub(1);
        }
        KeyCode::Char('G') if !is_ctrl && state.pending_keys.is_empty() => {
            state.scroll = u16::MAX;
        }
        KeyCode::Char(c) => {
            state.pending_keys.push(c);
            match resolve_chord(&state.pending_keys) {
                ChordResult::Executed => {
                    // Apply the chord effect. Currently only `gg`.
                    state.scroll = 0;
                    state.pending_keys.clear();
                }
                ChordResult::NoMatch => {
                    state.pending_keys.clear();
                }
                ChordResult::Prefix => {} // wait for more keys
            }
        }
        _ => {
            state.pending_keys.clear();
        }
    }
    None
}

/// Handle a keypress in Insert mode (the default text-entry mode).
///
/// | Key              | Effect                                          |
/// |------------------|-------------------------------------------------|
/// | Esc              | Switch to Normal mode                           |
/// | Printable char   | Append to input buffer                          |
/// | Backspace        | Pop last char from input buffer                  |
/// | Enter (non-empty)| Take input, push User message, return `SendMessage` |
/// | Enter (empty)    | No-op                                           |
fn handle_insert_key(key: KeyEvent, state: &mut AppState) -> Option<LoopControl> {
    match key.code {
        KeyCode::Esc => {
            state.mode = Mode::Normal;
            None
        }
        KeyCode::Char(c) if !key.modifiers.contains(KeyModifiers::CONTROL) => {
            state.input.push(c);
            None
        }
        KeyCode::Backspace => {
            state.input.pop();
            None
        }
        KeyCode::Enter => {
            let msg = std::mem::take(&mut state.input);
            if msg.is_empty() {
                None
            } else {
                state.messages.push((Role::User, msg.clone()));
                Some(LoopControl::SendMessage(msg))
            }
        }
        _ => None,
    }
}

/// Execute a parsed command string and return the appropriate control signal.
///
/// | Command       | Effect                                          |
/// |---------------|-------------------------------------------------|
/// | `quit` / `q`  | Return `LoopControl::Quit`                      |
/// | `clear`       | Clear messages and scroll, return `ClearHistory` |
/// | anything else | Set `status_error` on state                     |
fn execute_command(buf: &str, state: &mut AppState) -> Option<LoopControl> {
    match buf.trim() {
        "quit" | "q" => Some(LoopControl::Quit),
        "clear" => {
            state.messages.clear();
            state.scroll = 0;
            Some(LoopControl::ClearHistory)
        }
        other => {
            state.status_error = Some(format!("Unknown command: {other}"));
            None
        }
    }
}

/// Handle a keypress in Command mode (ex-style `:` command entry).
///
/// | Key       | Effect                                              |
/// |-----------|-----------------------------------------------------|
/// | Esc       | Abandon command, return to Normal                   |
/// | Enter     | Accept command (execution deferred to 2.3), Normal  |
/// | Backspace | Pop from buffer; if empty, return to Normal         |
/// | Char(c)   | Append to command buffer                            |
fn handle_command_key(key: KeyEvent, state: &mut AppState) -> Option<LoopControl> {
    match key.code {
        KeyCode::Esc => {
            state.mode = Mode::Normal;
        }
        KeyCode::Enter => {
            state.mode = Mode::Normal;
            return execute_command(&state.command_buffer.clone(), state);
        }
        KeyCode::Backspace => {
            state.command_buffer.pop();
        }
        KeyCode::Char(c) => {
            state.command_buffer.push(c);
        }
        _ => {}
    }
    None
}

/// Drain all pending [`UIEvent`]s from `event_rx` and apply them to `state`.
///
/// This is non-blocking: it processes all currently-available events and returns
/// immediately when the channel is empty.
///
/// | Event              | Effect                                                     |
/// |--------------------|------------------------------------------------------------|
/// | `StreamDelta(s)`   | Append `s` to last message if it's `Assistant`; else push new |
/// | `TurnComplete`     | No structural change; logged at debug level                |
/// | `Error(msg)`       | Push `(Assistant, "[error] {msg}")`                        |
fn drain_ui_events(event_rx: &mut mpsc::Receiver<UIEvent>, state: &mut AppState) {
    while let Ok(event) = event_rx.try_recv() {
        match event {
            UIEvent::StreamDelta(chunk) => {
                if let Some((Role::Assistant, content)) = state.messages.last_mut() {
                    content.push_str(&chunk);
                } else {
                    state.messages.push((Role::Assistant, chunk));
                }
            }
            UIEvent::TurnComplete => {
                debug!("turn complete");
            }
            UIEvent::Error(msg) => {
                state
                    .messages
                    .push((Role::Assistant, format!("[error] {msg}")));
            }
        }
    }
}

/// Estimate the total rendered line count for all messages and update `state.scroll`
/// so the bottom of the content is visible.
///
/// When content fits within the viewport, `state.scroll` is set to 0.
///
/// Line estimation per message:
/// - 1 line for the role header
/// - `ceil(chars / width).max(1)` lines per newline-separated content line
/// - 1 blank separator line
fn update_scroll(state: &mut AppState, area: Rect) {
    // 4 = input pane (3: border top + content + border bottom) + status bar (1)
    let viewport_height = area.height.saturating_sub(4);
    state.viewport_height = viewport_height;
    let width = area.width.max(1) as usize;

    let mut total_lines: u16 = 0;
    for (_, content) in &state.messages {
        total_lines = total_lines.saturating_add(1); // role header
        for line in content.lines() {
            let chars = line.chars().count();
            let wrapped = chars.div_ceil(width).max(1) as u16;
            total_lines = total_lines.saturating_add(wrapped);
        }
        total_lines = total_lines.saturating_add(1); // blank separator
    }

    let max_scroll = total_lines.saturating_sub(viewport_height);

    match state.mode {
        // In Insert mode, auto-follow the bottom of the conversation.
        Mode::Insert => {
            state.scroll = max_scroll;
        }
        // In Normal/Command mode, the user controls scroll -- just clamp to bounds.
        _ => {
            state.scroll = state.scroll.min(max_scroll);
        }
    }
}

/// Compute the overlay rectangle for the command palette, centered on `output_area`.
fn command_overlay_rect(output_area: Rect) -> Rect {
    let overlay_w = (output_area.width / 2).max(80).min(output_area.width);
    let overlay_h: u16 = 3; // border + content + border
    let overlay_x = output_area.x + (output_area.width.saturating_sub(overlay_w)) / 2;
    let overlay_y = output_area.y + 2;
    Rect {
        x: overlay_x,
        y: overlay_y,
        width: overlay_w,
        height: overlay_h.min(output_area.height),
    }
}

/// Render the full TUI into `frame`.
///
/// Layout (top to bottom):
/// ```text
/// +--------------------------------+
/// |  conversation history          |  Fill(1)
/// |                                |
/// +--------------------------------+
/// |  Input                         |  Length(3)
/// |  > _                           |
/// +--------------------------------+
/// | NORMAL          tokens: --     |  Length(1)
/// +--------------------------------+
/// ```
///
/// Role headers are coloured: `"You:"` in cyan, `"Assistant:"` in green.
/// In Command mode, a one-line overlay appears at row 1 of the output pane.
fn render(frame: &mut Frame, state: &AppState) {
    let chunks = Layout::vertical([
        Constraint::Fill(1),
        Constraint::Length(3),
        Constraint::Length(1),
    ])
    .split(frame.area());

    // --- Output pane ---
    let mut lines: Vec<Line> = Vec::new();
    for (role, content) in &state.messages {
        let (label, color) = match role {
            Role::User => ("You:", Color::Cyan),
            Role::Assistant => ("Assistant:", Color::Green),
        };
        lines.push(Line::from(Span::styled(label, Style::default().fg(color))));
        for body_line in content.lines() {
            lines.push(Line::from(body_line.to_string()));
        }
        lines.push(Line::from("")); // blank separator
    }

    let output = Paragraph::new(lines)
        .wrap(Wrap { trim: false })
        .scroll((state.scroll, 0));
    frame.render_widget(output, chunks[0]);

    // --- Command overlay (floating box centered on output pane) ---
    if state.mode == Mode::Command {
        let overlay_area = command_overlay_rect(chunks[0]);
        // Clear the area behind the overlay so it appears floating.
        frame.render_widget(Clear, overlay_area);
        let overlay = Paragraph::new(format!(":{}", state.command_buffer)).block(
            Block::bordered()
                .border_style(Style::default().fg(Color::Yellow))
                .title("Command"),
        );
        frame.render_widget(overlay, overlay_area);
    }

    // --- Input pane ---
    let (input_title, input_style) = match state.mode {
        Mode::Normal | Mode::Command => (
            "Input (normal)",
            Style::default()
                .fg(Color::DarkGray)
                .add_modifier(Modifier::ITALIC),
        ),
        Mode::Insert => ("Input", Style::default()),
    };
    let input_text = format!("> {}", state.input);
    let input_widget = Paragraph::new(input_text)
        .style(input_style)
        .block(Block::bordered().title(input_title));
    frame.render_widget(input_widget, chunks[1]);

    // --- Cursor positioning ---
    match state.mode {
        Mode::Insert => {
            // Cursor at end of input text: border(1) + "> " (2) + input len
            let cursor_x = chunks[1].x + 1 + 2 + state.input.len() as u16;
            let cursor_y = chunks[1].y + 1; // inside the border
            frame.set_cursor_position((cursor_x, cursor_y));
        }
        Mode::Command => {
            // Cursor in the floating overlay
            let overlay = command_overlay_rect(chunks[0]);
            // border(1) + ":" (1) + buf len
            let cursor_x = overlay.x + 1 + 1 + state.command_buffer.len() as u16;
            let cursor_y = overlay.y + 1; // inside the border
            frame.set_cursor_position((cursor_x, cursor_y));
        }
        Mode::Normal => {} // no cursor
    }

    // --- Status bar ---
    let (mode_label, mode_style) = match state.mode {
        Mode::Normal => (
            " NORMAL ",
            Style::default().bg(Color::Blue).fg(Color::White),
        ),
        Mode::Insert => (
            " INSERT ",
            Style::default().bg(Color::Green).fg(Color::White),
        ),
        Mode::Command => (
            " COMMAND ",
            Style::default().bg(Color::Yellow).fg(Color::Black),
        ),
    };

    let right_text = if let Some(ref err) = state.status_error {
        err.clone()
    } else {
        "tokens: --".to_string()
    };

    let right_style = if state.status_error.is_some() {
        Style::default().fg(Color::Red)
    } else {
        Style::default()
    };

    let bar_width = chunks[2].width as usize;
    let left_len = mode_label.len();
    let right_len = right_text.len();
    let pad = bar_width.saturating_sub(left_len + right_len);

    let status_line = Line::from(vec![
        Span::styled(mode_label, mode_style),
        Span::raw(" ".repeat(pad)),
        Span::styled(right_text, right_style),
    ]);
    let status_widget = Paragraph::new(status_line);
    frame.render_widget(status_widget, chunks[2]);
}

/// Run the TUI event loop.
///
/// This function owns the terminal for its entire lifetime. It initialises the
/// terminal, installs the panic hook, then spins in a ~60 fps loop:
///
/// ```text
/// loop:
///   1. drain UIEvents (non-blocking try_recv)
///   2. poll keyboard for up to 16 ms via EventStream (async, no blocking thread)
///   3. handle key event -> Option<LoopControl>
///   4. render frame (scroll updated inside draw closure)
///   5. act on LoopControl: send message or break
/// ```
///
/// On `Ctrl+C` / `Ctrl+D`: sends [`UserAction::Quit`], restores the terminal, and
/// returns `Ok(())`.
pub async fn run(
    action_tx: mpsc::Sender<UserAction>,
    mut event_rx: mpsc::Receiver<UIEvent>,
) -> Result<(), TuiError> {
    install_panic_hook();
    let mut terminal = init_terminal()?;
    let mut state = AppState::new();
    let mut event_stream = EventStream::new();

    loop {
        // 1. Drain pending UI events.
        drain_ui_events(&mut event_rx, &mut state);

        // 2. Poll keyboard for up to 16 ms.  EventStream integrates with the
        //    Tokio runtime via futures::Stream, so no blocking thread is needed.
        //    Timeout expiry, stream end, non-key events, and I/O errors all map
        //    to None -- the frame is rendered regardless.
        let key_event: Option<KeyEvent> =
            match tokio::time::timeout(Duration::from_millis(16), event_stream.next()).await {
                Ok(Some(Ok(Event::Key(k)))) => Some(k),
                _ => None,
            };

        // 3. Handle key.
        let control = handle_key(key_event, &mut state);

        // 4. Render (scroll updated inside draw closure to use current frame area).
        terminal.draw(|frame| {
            update_scroll(&mut state, frame.area());
            render(frame, &state);
        })?;

        // 5. Act on control signal after render so the user sees the submitted message.
        match control {
            Some(LoopControl::SendMessage(msg)) => {
                if action_tx.send(UserAction::SendMessage(msg)).await.is_err() {
                    break;
                }
            }
            Some(LoopControl::Quit) => {
                let _ = action_tx.send(UserAction::Quit).await;
                break;
            }
            Some(LoopControl::ClearHistory) => {
                let _ = action_tx.send(UserAction::ClearHistory).await;
            }
            None => {}
        }
    }

    restore_terminal()?;
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::*;
    use ratatui::backend::TestBackend;

    fn make_key(code: KeyCode) -> Option<KeyEvent> {
        Some(KeyEvent::new(code, KeyModifiers::empty()))
    }

    fn ctrl_key(c: char) -> Option<KeyEvent> {
        Some(KeyEvent::new(KeyCode::Char(c), KeyModifiers::CONTROL))
    }

    // --- handle_key tests ---

    #[test]
    fn handle_key_printable_appends() {
        let mut state = AppState::new();
        handle_key(make_key(KeyCode::Char('h')), &mut state);
        assert_eq!(state.input, "h");
    }

    #[test]
    fn handle_key_backspace_pops() {
        let mut state = AppState::new();
        state.input = "ab".to_string();
        handle_key(make_key(KeyCode::Backspace), &mut state);
        assert_eq!(state.input, "a");
    }

    #[test]
    fn handle_key_backspace_empty_noop() {
        let mut state = AppState::new();
        handle_key(make_key(KeyCode::Backspace), &mut state);
        assert_eq!(state.input, "");
    }

    #[test]
    fn handle_key_enter_empty_noop() {
        let mut state = AppState::new();
        let result = handle_key(make_key(KeyCode::Enter), &mut state);
        assert!(result.is_none());
        assert!(state.messages.is_empty());
    }

    #[test]
    fn handle_key_enter_sends_and_clears() {
        let mut state = AppState::new();
        state.input = "hello".to_string();
        let result = handle_key(make_key(KeyCode::Enter), &mut state);
        assert!(state.input.is_empty());
        assert_eq!(state.messages.len(), 1);
        assert!(matches!(result, Some(LoopControl::SendMessage(ref m)) if m == "hello"));
    }

    #[test]
    fn handle_key_ctrl_c_quits() {
        let mut state = AppState::new();
        let result = handle_key(ctrl_key('c'), &mut state);
        assert!(matches!(result, Some(LoopControl::Quit)));
    }

    #[test]
    fn insert_ctrl_char_is_noop() {
        let mut state = AppState::new();
        handle_key(ctrl_key('a'), &mut state);
        assert_eq!(state.input, "", "Ctrl+A should not insert 'a'");
    }

    // --- drain_ui_events tests ---

    #[tokio::test]
    async fn drain_appends_to_existing_assistant() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        state.messages.push((Role::Assistant, "hello".to_string()));
        tx.send(UIEvent::StreamDelta(" world".to_string()))
            .await
            .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        assert_eq!(state.messages.last().unwrap().1, "hello world");
    }

    #[tokio::test]
    async fn drain_creates_assistant_on_user_last() {
        let (tx, mut rx) = tokio::sync::mpsc::channel(8);
        let mut state = AppState::new();
        state.messages.push((Role::User, "hi".to_string()));
        tx.send(UIEvent::StreamDelta("hello".to_string()))
            .await
            .unwrap();
        drop(tx);
        drain_ui_events(&mut rx, &mut state);
        assert_eq!(state.messages.len(), 2);
        assert_eq!(state.messages[1].0, Role::Assistant);
        assert_eq!(state.messages[1].1, "hello");
    }

    // --- render tests ---

    #[test]
    fn render_smoke_test() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let state = AppState::new();
        terminal.draw(|frame| render(frame, &state)).unwrap();
        // no panic is the assertion
    }

    #[test]
    fn render_shows_role_prefixes() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.messages.push((Role::User, "hi".to_string()));
        state.messages.push((Role::Assistant, "hello".to_string()));
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(
            all_text.contains("You:"),
            "expected 'You:' in buffer: {all_text:.100}"
        );
        assert!(
            all_text.contains("Assistant:"),
            "expected 'Assistant:' in buffer"
        );
    }

    // --- update_scroll tests ---

    #[test]
    fn update_scroll_zero_when_fits() {
        let mut state = AppState::new();
        state.messages.push((Role::User, "hello".to_string()));
        let area = Rect::new(0, 0, 80, 24);
        update_scroll(&mut state, area);
        assert_eq!(state.scroll, 0);
    }

    #[test]
    fn update_scroll_positive_when_overflow() {
        let mut state = AppState::new();
        for i in 0..50 {
            state.messages.push((Role::User, format!("message {i}")));
        }
        let area = Rect::new(0, 0, 80, 24);
        update_scroll(&mut state, area);
        assert!(state.scroll > 0, "expected scroll > 0 with 50 messages");
    }

    // --- mode tests ---

    #[test]
    fn mode_starts_insert() {
        assert_eq!(AppState::new().mode, Mode::Insert);
    }

    #[test]
    fn insert_esc_enters_normal() {
        let mut state = AppState::new();
        handle_key(make_key(KeyCode::Esc), &mut state);
        assert_eq!(state.mode, Mode::Normal);
    }

    #[test]
    fn normal_i_enters_insert() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        handle_key(make_key(KeyCode::Char('i')), &mut state);
        assert_eq!(state.mode, Mode::Insert);
    }

    #[test]
    fn normal_colon_enters_command() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        handle_key(make_key(KeyCode::Char(':')), &mut state);
        assert_eq!(state.mode, Mode::Command);
    }

    #[test]
    fn command_esc_enters_normal() {
        let mut state = AppState::new();
        state.mode = Mode::Command;
        state.command_buffer = "q".to_string();
        handle_key(make_key(KeyCode::Esc), &mut state);
        assert_eq!(state.mode, Mode::Normal);
    }

    #[test]
    fn command_enter_enters_normal() {
        let mut state = AppState::new();
        state.mode = Mode::Command;
        handle_key(make_key(KeyCode::Enter), &mut state);
        assert_eq!(state.mode, Mode::Normal);
    }

    #[test]
    fn command_chars_append_to_buffer() {
        let mut state = AppState::new();
        state.mode = Mode::Command;
        handle_key(make_key(KeyCode::Char('q')), &mut state);
        assert_eq!(state.mode, Mode::Command);
        assert_eq!(state.command_buffer, "q");
    }

    #[test]
    fn command_backspace_empty_stays_in_command() {
        let mut state = AppState::new();
        state.mode = Mode::Command;
        state.command_buffer = "ab".to_string();
        handle_key(make_key(KeyCode::Backspace), &mut state);
        assert_eq!(state.command_buffer, "a");
        handle_key(make_key(KeyCode::Backspace), &mut state);
        assert_eq!(state.command_buffer, "");
        handle_key(make_key(KeyCode::Backspace), &mut state);
        assert_eq!(state.command_buffer, "");
        handle_key(make_key(KeyCode::Backspace), &mut state);
        assert_eq!(state.command_buffer, "");
    }

    #[test]
    fn normal_j_increments_scroll() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        handle_key(make_key(KeyCode::Char('j')), &mut state);
        assert_eq!(state.scroll, 1);
    }

    #[test]
    fn normal_k_decrements_scroll() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        state.scroll = 5;
        handle_key(make_key(KeyCode::Char('k')), &mut state);
        assert_eq!(state.scroll, 4);
    }

    #[test]
    fn normal_k_clamps_at_zero() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        handle_key(make_key(KeyCode::Char('k')), &mut state);
        assert_eq!(state.scroll, 0);
    }

    #[test]
    fn normal_big_g_scrolls_bottom() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        handle_key(make_key(KeyCode::Char('G')), &mut state);
        assert_eq!(state.scroll, u16::MAX);
    }

    #[test]
    fn normal_gg_scrolls_top() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        state.scroll = 50;
        handle_key(make_key(KeyCode::Char('g')), &mut state);
        assert_eq!(state.pending_keys, vec!['g']); // prefix, waiting
        handle_key(make_key(KeyCode::Char('g')), &mut state);
        assert_eq!(state.scroll, 0);
        assert!(state.pending_keys.is_empty());
    }

    #[test]
    fn normal_g_then_other_clears_pending() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        state.scroll = 50;
        handle_key(make_key(KeyCode::Char('g')), &mut state);
        handle_key(make_key(KeyCode::Char('x')), &mut state);
        assert!(state.pending_keys.is_empty());
        assert_eq!(state.scroll, 50); // unchanged
    }

    #[test]
    fn ctrl_c_quits_from_any_mode() {
        for mode in [Mode::Normal, Mode::Insert, Mode::Command] {
            let mut state = AppState::new();
            state.mode = mode;
            let result = handle_key(ctrl_key('c'), &mut state);
            assert!(matches!(result, Some(LoopControl::Quit)));
        }
    }

    #[test]
    fn ctrl_d_quits_from_insert() {
        let mut state = AppState::new();
        let result = handle_key(ctrl_key('d'), &mut state);
        assert!(matches!(result, Some(LoopControl::Quit)));
    }

    #[test]
    fn ctrl_d_scrolls_in_normal() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        state.viewport_height = 20;
        let result = handle_key(ctrl_key('d'), &mut state);
        assert!(result.is_none());
        assert_eq!(state.scroll, 10);
    }

    #[test]
    fn ctrl_u_scrolls_up_in_normal() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        state.viewport_height = 20;
        state.scroll = 15;
        let result = handle_key(ctrl_key('u'), &mut state);
        assert!(result.is_none());
        assert_eq!(state.scroll, 5);
    }

    #[test]
    fn ctrl_u_clamps_at_zero() {
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        state.viewport_height = 20;
        state.scroll = 3;
        handle_key(ctrl_key('u'), &mut state);
        assert_eq!(state.scroll, 0);
    }

    // --- execute_command tests ---

    #[test]
    fn command_quit_returns_quit() {
        let mut state = AppState::new();
        let result = execute_command("quit", &mut state);
        assert!(matches!(result, Some(LoopControl::Quit)));
    }

    #[test]
    fn command_q_returns_quit() {
        let mut state = AppState::new();
        let result = execute_command("q", &mut state);
        assert!(matches!(result, Some(LoopControl::Quit)));
    }

    #[test]
    fn command_clear_empties_messages() {
        let mut state = AppState::new();
        state.messages.push((Role::User, "hi".to_string()));
        state.messages.push((Role::Assistant, "hello".to_string()));
        state.scroll = 10;
        let result = execute_command("clear", &mut state);
        assert!(matches!(result, Some(LoopControl::ClearHistory)));
        assert!(state.messages.is_empty());
        assert_eq!(state.scroll, 0);
    }

    #[test]
    fn command_unknown_sets_status_error() {
        let mut state = AppState::new();
        let result = execute_command("foo", &mut state);
        assert!(result.is_none());
        assert_eq!(state.status_error.as_deref(), Some("Unknown command: foo"));
    }

    #[test]
    fn status_error_cleared_on_next_keypress() {
        let mut state = AppState::new();
        state.status_error = Some("some error".to_string());
        handle_key(make_key(KeyCode::Char('h')), &mut state);
        assert!(state.status_error.is_none());
    }

    #[test]
    fn command_enter_executes_quit() {
        let mut state = AppState::new();
        state.mode = Mode::Command;
        state.command_buffer = "q".to_string();
        let result = handle_key(make_key(KeyCode::Enter), &mut state);
        assert!(matches!(result, Some(LoopControl::Quit)));
        assert_eq!(state.mode, Mode::Normal);
    }

    // --- render snapshot tests ---

    #[test]
    fn render_status_bar_normal_mode() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(all_text.contains("NORMAL"), "expected 'NORMAL' in buffer");
        assert!(
            all_text.contains("tokens: --"),
            "expected 'tokens: --' in buffer"
        );
    }

    #[test]
    fn render_status_bar_insert_mode() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let state = AppState::new(); // starts in Insert
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(all_text.contains("INSERT"), "expected 'INSERT' in buffer");
    }

    #[test]
    fn render_status_bar_command_mode() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.mode = Mode::Command;
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(all_text.contains("COMMAND"), "expected 'COMMAND' in buffer");
    }

    #[test]
    fn render_command_overlay_visible_in_command_mode() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.mode = Mode::Command;
        state.command_buffer = "quit".to_string();
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(
            all_text.contains(":quit"),
            "expected ':quit' overlay in buffer"
        );
    }

    #[test]
    fn render_no_overlay_in_normal_mode() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        // Row 1 should not have a ":" prefix from the overlay
        let row1: String = (0..80)
            .map(|x| buf.cell((x, 1)).unwrap().symbol().to_string())
            .collect();
        assert!(
            !row1.starts_with(':'),
            "overlay should not appear in Normal mode"
        );
    }

    #[test]
    fn render_input_pane_dimmed_in_normal() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.mode = Mode::Normal;
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(
            all_text.contains("Input (normal)"),
            "expected 'Input (normal)' title in Normal mode"
        );
    }

    #[test]
    fn render_status_bar_shows_error() {
        let backend = TestBackend::new(80, 24);
        let mut terminal = Terminal::new(backend).unwrap();
        let mut state = AppState::new();
        state.status_error = Some("Unknown command: foo".to_string());
        terminal.draw(|frame| render(frame, &state)).unwrap();
        let buf = terminal.backend().buffer().clone();
        let all_text: String = buf
            .content()
            .iter()
            .map(|c| c.symbol().to_string())
            .collect();
        assert!(
            all_text.contains("Unknown command: foo"),
            "expected error in status bar"
        );
    }
}