//! TUI entry point: terminal setup, event loop, and graceful teardown.
//!
//! The public surface is [`run_tui`], a synchronous function that:
//!
//! 1. Enables raw mode and switches to the alternate screen.
//! 2. Installs a panic hook that restores the terminal before printing the
//!    panic message, so the user's shell is never left in raw mode.
//! 3. Runs the main draw / event loop until the user quits.
//! 4. Tears down the terminal (disable raw mode, leave alternate screen) via
//!    an RAII guard, so cleanup happens even if an error is returned.

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

use ratatui::crossterm::event::{self, Event, KeyCode, KeyEventKind};
use ratatui::crossterm::execute;
use ratatui::crossterm::terminal::{
    EnterAlternateScreen, LeaveAlternateScreen, disable_raw_mode, enable_raw_mode,
};
use ratatui::Terminal;
use ratatui::backend::CrosstermBackend;

use crate::error::Result;
use crate::parser::discovery::{discover_agents_for_session, discover_sessions};
use crate::tui::modals::help_modal::{handle_help_modal_event, render_help_modal};
use crate::tui::modals::quit_dialog::{handle_quit_dialog_event, render_quit_dialog};
use crate::tui::screens::session_list::{handle_session_list_event, render_session_list};
use crate::tui::screens::transcript::{
    handle_transcript_event, load_transcript_for_agent, load_transcript_for_session,
    render_transcript,
};
use crate::tui::state::{AppState, Screen, SessionListItem};

// ---------------------------------------------------------------------------
// RAII terminal guard
// ---------------------------------------------------------------------------

/// Restores the terminal to its original state when dropped.
///
/// Wraps the `Terminal` so that `disable_raw_mode` and
/// `LeaveAlternateScreen` are always called, whether the event loop exits
/// normally or returns an error.
struct TerminalGuard {
    terminal: Terminal<CrosstermBackend<Stdout>>,
}

impl TerminalGuard {
    /// Set up raw mode + alternate screen and build the `Terminal`.
    fn new() -> Result<Self> {
        enable_raw_mode()?;
        let mut stdout = io::stdout();
        execute!(stdout, EnterAlternateScreen)?;
        let backend = CrosstermBackend::new(io::stdout());
        let terminal = Terminal::new(backend)?;
        Ok(Self { terminal })
    }
}

impl Drop for TerminalGuard {
    fn drop(&mut self) {
        // Best-effort: ignore errors during cleanup so we never panic in Drop.
        let _ = disable_raw_mode();
        let _ = execute!(self.terminal.backend_mut(), LeaveAlternateScreen);
    }
}

// ---------------------------------------------------------------------------
// Panic hook
// ---------------------------------------------------------------------------

/// Install a panic hook that restores the terminal before printing the panic.
///
/// Without this, a panic in raw mode leaves the user's shell in an unusable
/// state.  The hook disables raw mode and leaves the alternate screen, then
/// delegates to the default panic handler.
fn install_panic_hook() {
    let default_hook = std::panic::take_hook();
    std::panic::set_hook(Box::new(move |info| {
        // Ignore errors — we are already panicking.
        let _ = disable_raw_mode();
        let _ = execute!(io::stdout(), LeaveAlternateScreen);
        default_hook(info);
    }));
}

// ---------------------------------------------------------------------------
// Rendering
// ---------------------------------------------------------------------------

/// Draw a single frame, dispatching to the appropriate screen renderer.
///
/// Modals are rendered on top of the base screen when active.
fn render(frame: &mut ratatui::Frame, state: &AppState) {
    let area = frame.area();
    match &state.screen {
        Screen::SessionList => render_session_list(frame, area, state),
        Screen::Transcript { .. } | Screen::SubagentTranscript { .. } => {
            render_transcript(frame, area, state);
        }
    }

    // Overlay modals on top of the active screen.
    if state.show_quit_dialog {
        render_quit_dialog(frame, area);
    } else if state.show_help {
        render_help_modal(frame, area);
    }
}

// ---------------------------------------------------------------------------
// Event handling
// ---------------------------------------------------------------------------

/// Handle a single crossterm [`Event`], mutating `state` as needed.
///
/// Modal dialogs are checked first and intercept all input while open.
/// If no modal is active, dispatches to the appropriate screen handler,
/// then falls back to global keybindings for any event not consumed by
/// the screen.
fn handle_event(event: Event, state: &mut AppState) {
    // Modal dialogs intercept all input while visible.
    if state.show_quit_dialog {
        handle_quit_dialog_event(event, state);
        return;
    }
    if state.show_help {
        handle_help_modal_event(event, state);
        return;
    }

    let consumed = match &state.screen {
        Screen::SessionList => handle_session_list_event(event.clone(), state),
        Screen::Transcript { .. } | Screen::SubagentTranscript { .. } => {
            handle_transcript_event(event.clone(), state)
        }
    };

    if consumed {
        return;
    }

    // Global fallback for screens that don't handle quit/back themselves.
    if let Event::Key(key) = event {
        if key.kind != KeyEventKind::Press {
            return;
        }
        match key.code {
            KeyCode::Char('q') => state.should_quit = true,
            KeyCode::Char('c') => state.color_enabled = !state.color_enabled,
            KeyCode::Esc => state.go_back(),
            _ => {}
        }
    }
}

// ---------------------------------------------------------------------------
// Transcript data loading
// ---------------------------------------------------------------------------

/// Load transcript entries into `state` if we are on a transcript screen and
/// the entries have not been loaded yet.
///
/// Called once per event loop iteration, after event handling.  Because
/// discovery and file I/O can be slow, this is a no-op when entries are
/// already populated.
fn maybe_load_transcript(state: &mut AppState) {
    if !state.transcript_entries.is_empty() {
        return;
    }
    match state.screen.clone() {
        Screen::Transcript { session_id } => {
            load_transcript_for_session(&session_id, state);
        }
        Screen::SubagentTranscript {
            parent_session_id,
            agent_id,
        } => {
            load_transcript_for_agent(&parent_session_id, &agent_id, state);
        }
        Screen::SessionList => {}
    }
}

// ---------------------------------------------------------------------------
// Main event loop
// ---------------------------------------------------------------------------

/// Run the TUI synchronously.
///
/// Initialises the terminal, enters the event loop, and tears down cleanly
/// whether the loop exits via `should_quit` or via an error.
pub fn run_tui() -> Result<()> {
    install_panic_hook();

    let mut guard = TerminalGuard::new()?;
    let mut state = AppState::new();

    // Populate the session list from disk before entering the event loop.
    // Silently use an empty vec if discovery fails so the TUI still starts.
    let mut session_refs = discover_sessions().unwrap_or_default();
    // Sort most-recent-first before converting to display items.
    session_refs.sort_by(|a, b| b.modified_at.cmp(&a.modified_at));
    state.sessions = session_refs
        .into_iter()
        .map(|sr| {
            let short_id = sr.session_id.chars().take(8).collect();
            let full_id = sr.session_id.clone();
            let date = sr.modified_at.format("%Y-%m-%d %H:%M:%S").to_string();
            let project = sr.project_path.clone().unwrap_or_default();
            let agent_count = discover_agents_for_session(&sr.file_path)
                .map(|v| v.len())
                .unwrap_or(0);
            SessionListItem {
                short_id,
                full_id,
                date,
                project,
                model: String::new(),
                msg_count: 0,
                agent_count,
            }
        })
        .collect();

    loop {
        // Load transcript data lazily when entering a transcript screen.
        maybe_load_transcript(&mut state);

        guard.terminal.draw(|f| render(f, &state))?;

        if event::poll(Duration::from_millis(50))? {
            handle_event(event::read()?, &mut state);
        }

        if state.should_quit {
            break;
        }
    }

    Ok(())
    // `guard` is dropped here → terminal restored automatically.
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

#[cfg(test)]
mod tests {
    use ratatui::crossterm::event::{KeyEvent, KeyEventState, KeyModifiers};

    use super::*;
    use crate::tui::state::Screen;

    fn key_press(code: KeyCode) -> Event {
        Event::Key(KeyEvent {
            code,
            modifiers: KeyModifiers::NONE,
            kind: KeyEventKind::Press,
            state: KeyEventState::NONE,
        })
    }

    /// Pressing `q` on the session-list screen shows the quit dialog.
    #[test]
    fn q_on_session_list_shows_quit_dialog() {
        let mut state = AppState::new();
        handle_event(key_press(KeyCode::Char('q')), &mut state);
        assert!(state.show_quit_dialog);
        assert!(!state.should_quit);
    }

    /// Pressing `q` on the transcript screen shows the quit dialog (same as session list).
    #[test]
    fn q_on_transcript_shows_quit_dialog() {
        let mut state = AppState::new();
        state.enter_transcript("some-session");
        handle_event(key_press(KeyCode::Char('q')), &mut state);
        assert!(state.show_quit_dialog);
        assert!(!state.should_quit);
    }

    /// Pressing `Esc` on the transcript screen calls `go_back` (returns to SessionList).
    #[test]
    fn esc_goes_back() {
        let mut state = AppState::new();
        state.enter_transcript("some-session");
        handle_event(key_press(KeyCode::Esc), &mut state);
        assert_eq!(state.screen, Screen::SessionList);
    }

    /// Key-release events are ignored.
    #[test]
    fn key_release_ignored() {
        let mut state = AppState::new();
        let release_event = Event::Key(KeyEvent {
            code: KeyCode::Char('q'),
            modifiers: KeyModifiers::NONE,
            kind: KeyEventKind::Release,
            state: KeyEventState::NONE,
        });
        handle_event(release_event, &mut state);
        assert!(!state.should_quit);
    }

    /// Pressing `c` toggles `color_enabled` off then back on.
    #[test]
    fn c_toggles_color_enabled() {
        // SAFETY: single-threaded test; no other threads reading the env.
        unsafe { std::env::remove_var("NO_COLOR") };
        let mut state = AppState::new();
        assert!(state.color_enabled, "color starts enabled");
        handle_event(key_press(KeyCode::Char('c')), &mut state);
        assert!(!state.color_enabled, "color toggled off");
        handle_event(key_press(KeyCode::Char('c')), &mut state);
        assert!(state.color_enabled, "color toggled back on");
    }
}