//! Orchestrates the main application execution, terminal events, and TUI rendering.
//!
//! The `AppRunner` owns the event loop, turns raw key events into the
//! [`Action`]s declared in the user's config, and runs the `:` command line.

use crate::app::{App, InsertVariant, Mode};
use crate::config::{Action, Config};
use crate::format::FormatHandler;
use crossterm::event::{self, Event, KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
use ratatui::Terminal;
use ratatui::backend::Backend;
use std::io;
use std::path::PathBuf;
use std::time::Duration;
use tui_input::backend::crossterm::EventHandler;

/// How often the loop wakes up with no input, so timed UI state can settle.
const TICK: Duration = Duration::from_millis(250);

/// Manages the main application execution loop, event handling, and terminal interaction.
pub struct AppRunner<'a, B: Backend> {
    /// Reference to the terminal instance.
    terminal: &'a mut Terminal<B>,
    /// Mutable reference to the application state.
    app: &'a mut App,
    /// Loaded user configuration (keybinds, theme).
    config: &'a Config,
    /// Handler for the specific file format (env, json, yaml, toml).
    handler: &'a dyn FormatHandler,
    /// Every key sequence and the action it triggers, resolved from the config once.
    bindings: Vec<(String, Action)>,
    /// Buffer for storing active command entry (e.g., ":w").
    command_buffer: String,
    /// Buffer for storing multi-key sequence of presses (e.g., "gg").
    key_sequence: String,
}

impl<'a, B: Backend> AppRunner<'a, B>
where
    io::Error: From<B::Error>,
{
    /// Creates a new runner instance with all required dependencies.
    pub fn new(
        terminal: &'a mut Terminal<B>,
        app: &'a mut App,
        config: &'a Config,
        handler: &'a dyn FormatHandler,
    ) -> Self {
        Self {
            terminal,
            app,
            config,
            handler,
            bindings: config.keybinds.resolved(),
            command_buffer: String::new(),
            key_sequence: String::new(),
        }
    }

    /// Starts the main application loop.
    /// 
    /// This loop continues until `self.app.running` is set to false. It polls
    /// rather than blocking so that a resize or an expiring status message
    /// repaints without waiting for the next keystroke.
    pub fn run(&mut self) -> io::Result<()> {
        while self.app.running {
            self.terminal
                .draw(|f| crate::ui::draw(f, self.app, self.config))?;

            if !event::poll(TICK)? {
                continue;
            }

            match event::read()? {
                // Key *releases* are delivered on some terminals; acting on both
                // edges would fire every binding twice.
                Event::Key(key) if key.kind != KeyEventKind::Release => {
                    self.handle_key_event(key)?;
                }
                _ => {}
            }
        }
        Ok(())
    }

    /// Primary dispatcher for all keyboard events.
    ///
    /// It delegates handling to specialized methods based on the 
    /// current application mode.
    fn handle_key_event(&mut self, key: KeyEvent) -> io::Result<()> {
        match self.app.mode {
            Mode::Normal => self.handle_normal_mode(key),
            Mode::Insert | Mode::InsertKey => self.handle_insert_mode(key),
            Mode::Search => self.handle_search_mode(key),
            Mode::Help => self.handle_help_mode(key),
        }
    }

    /// The help overlay scrolls with the usual movement keys; anything else closes it.
    fn handle_help_mode(&mut self, key: KeyEvent) -> io::Result<()> {
        let token = Self::key_token(key);
        let is = |action| token.as_deref() == Some(self.config.keybinds.key_for(action)).as_deref();

        if is(Action::Down) || key.code == KeyCode::Down {
            self.app.scroll_help(true);
        } else if is(Action::Up) || key.code == KeyCode::Up {
            self.app.scroll_help(false);
        } else {
            self.app.toggle_help();
        }
        Ok(())
    }

    /// Handles keys in Normal mode, separating navigation from command entry.
    fn handle_normal_mode(&mut self, key: KeyEvent) -> io::Result<()> {
        if !self.command_buffer.is_empty() {
            self.handle_command_mode(key)
        } else {
            self.handle_navigation_mode(key)
        }
    }

    /// Logic for entering and executing ":" style commands.
    fn handle_command_mode(&mut self, key: KeyEvent) -> io::Result<()> {
        match key.code {
            KeyCode::Enter => {
                let cmd = self.command_buffer.clone();
                self.command_buffer.clear();
                self.app.prompt = None;
                self.execute_command(cmd.trim())
            }
            KeyCode::Esc => {
                self.command_buffer.clear();
                self.app.prompt = None;
                self.app.clear_status();
                Ok(())
            }
            KeyCode::Backspace => {
                self.command_buffer.pop();
                self.sync_command_prompt();
                Ok(())
            }
            KeyCode::Char(c) => {
                self.command_buffer.push(c);
                self.sync_command_prompt();
                Ok(())
            }
            _ => Ok(()),
        }
    }

    /// Renders a key event as the string used in the keybinding config.
    ///
    /// Named keys are spelled in angle brackets (`<Space>`) so that they can
    /// never be confused with a sequence of ordinary characters.
    fn key_token(key: KeyEvent) -> Option<String> {
        let base = match key.code {
            KeyCode::Char(' ') => "<Space>".to_string(),
            KeyCode::Char(c) => c.to_string(),
            KeyCode::Tab => "<Tab>".to_string(),
            KeyCode::BackTab => "<S-Tab>".to_string(),
            _ => return None,
        };

        if key.modifiers.contains(KeyModifiers::ALT) {
            Some(format!("alt+{}", base))
        } else {
            Some(base)
        }
    }

    /// Handles primary navigation and transitions to insert or command modes.
    ///
    /// This method manages multi-key sequences (like `gg`) and immediate 
    /// actions (like `i`). It correctly re-evaluates sequences to prevent 
    /// "one-key-behind" responsiveness bugs.
    fn handle_navigation_mode(&mut self, key: KeyEvent) -> io::Result<()> {
        let Some(token) = Self::key_token(key) else {
            // Reset the sequence buffer if a non-character key is pressed.
            self.key_sequence.clear();
            match key.code {
                KeyCode::Down => self.app.next(),
                KeyCode::Up => self.app.previous(),
                KeyCode::Enter => {
                    self.save_file(None);
                }
                KeyCode::Esc => self.app.clear_status(),
                _ => {}
            }
            return Ok(());
        };

        self.key_sequence.push_str(&token);

        let mut action = self.find_binding();
        if action.is_none() {
            if self.is_prefix_binding() {
                // A partial multi-key bind (like the first 'g' of 'gg'): wait.
                return Ok(());
            }
            // Not a match and not a prefix: restart from the current key.
            self.key_sequence = token;
            action = self.find_binding();
            if action.is_none() && !self.is_prefix_binding() {
                self.key_sequence.clear();
            }
        }

        let Some(action) = action else { return Ok(()) };
        self.key_sequence.clear();
        self.dispatch(action);
        Ok(())
    }

    /// Applies a resolved action to the application state.
    fn dispatch(&mut self, action: Action) {
        match action {
            Action::Down => self.app.next(),
            Action::Up => self.app.previous(),
            Action::JumpTop => self.app.jump_top(),
            Action::JumpBottom => self.app.jump_bottom(),
            Action::Edit => self.app.enter_insert(InsertVariant::Start),
            Action::EditAppend => self.app.enter_insert(InsertVariant::End),
            Action::EditSubstitute => self.app.enter_insert(InsertVariant::Substitute),
            Action::Rename => self.app.enter_insert_key(),
            Action::Search => {
                self.app.begin_search();
                self.sync_search_prompt();
            }
            Action::NextMatch => self.app.jump_next_match(),
            Action::PreviousMatch => self.app.jump_previous_match(),
            Action::AppendItem => self.app.add_item(true, false, false),
            Action::PrependItem => self.app.add_item(false, false, false),
            Action::AppendGroup => self.app.add_item(true, true, true),
            Action::PrependGroup => self.app.add_item(false, true, true),
            Action::DeleteItem => self.app.delete_selected(),
            Action::ToggleGroup => {
                self.app.toggle_group_selected();
                self.app.save_undo_state();
            }
            Action::AddMissing => self.app.accept_selected(),
            Action::AcceptAll => self.app.accept_all_missing(),
            Action::ToggleFold => self.app.toggle_fold(),
            Action::FoldAll => self.app.set_all_folds(true),
            Action::UnfoldAll => self.app.set_all_folds(false),
            Action::Undo => self.app.undo(),
            Action::Redo => self.app.redo(),
            Action::UndoBranchPrev => self.app.cycle_undo_branch(false),
            Action::UndoBranchNext => self.app.cycle_undo_branch(true),
            Action::Help => self.app.toggle_help(),
            Action::Command => {
                self.command_buffer.push(':');
                self.sync_command_prompt();
            }
            Action::Quit => self.request_quit(false),
        }
    }

    /// Looks up the current `key_sequence` in the configured keybindings.
    fn find_binding(&self) -> Option<Action> {
        self.bindings
            .iter()
            .find(|(bind, _)| *bind == self.key_sequence)
            .map(|(_, action)| *action)
    }

    /// Returns true if the current `key_sequence` is a partial prefix of any configured bind.
    fn is_prefix_binding(&self) -> bool {
        self.bindings
            .iter()
            .any(|(bind, _)| bind.starts_with(&self.key_sequence) && bind != &self.key_sequence)
    }

    /// Delegates key events to the `tui_input` handler during active editing.
    fn handle_insert_mode(&mut self, key: KeyEvent) -> io::Result<()> {
        match key.code {
            KeyCode::Esc => self.app.cancel_insert(),
            KeyCode::Enter => self.app.enter_normal(),
            _ => {
                self.app.input.handle_event(&Event::Key(key));
            }
        }
        Ok(())
    }

    /// Handles search mode key events and live-updates the match position.
    fn handle_search_mode(&mut self, key: KeyEvent) -> io::Result<()> {
        match key.code {
            // Enter keeps the query so `n`/`N` continue to work; Esc abandons it
            // and puts the cursor back where the search started.
            KeyCode::Enter => self.app.end_search(true),
            KeyCode::Esc => self.app.end_search(false),
            KeyCode::Backspace => {
                self.app.search_query.pop();
                self.sync_search_prompt();
                self.app.jump_next_match();
            }
            KeyCode::Char(c) => {
                self.app.search_query.push(c);
                self.sync_search_prompt();
                self.app.jump_next_match();
            }
            _ => {}
        }
        Ok(())
    }

    /// Maps command strings (like `:w`) to internal application actions.
    fn execute_command(&mut self, cmd: &str) -> io::Result<()> {
        let kb = &self.config.keybinds;
        let save = kb.save.clone();
        let quit = kb.quit.clone();

        // `:w some/other/path` retargets the write without changing the session.
        if let Some(rest) = cmd.strip_prefix(&format!("{} ", save)) {
            let target = PathBuf::from(rest.trim());
            self.save_file(Some(target));
            return Ok(());
        }

        match cmd {
            c if c == save => {
                self.save_file(None);
            }
            c if c == quit => self.request_quit(false),
            ":q!" => self.request_quit(true),
            ":wq" | ":x" => {
                if self.save_file(None) {
                    self.app.running = false;
                }
            }
            ":help" | ":h" => self.app.toggle_help(),
            other => self.app.set_status(format!("Unknown command: {}", other)),
        }
        Ok(())
    }

    /// Quits, refusing to discard unsaved work unless `force` is set.
    fn request_quit(&mut self, force: bool) {
        if self.app.is_dirty && !force {
            self.app.set_status(format!(
                "Unsaved changes — {} to save, :q! to discard",
                self.config.keybinds.save
            ));
            return;
        }
        self.app.running = false;
    }

    /// Writes the current state to disk. Returns true on success.
    fn save_file(&mut self, target: Option<PathBuf>) -> bool {
        let path = target.unwrap_or_else(|| self.app.output_path.clone());
        match self.handler.write(&path, &self.app.vars) {
            Ok(()) => {
                self.app.output_path = path.clone();
                self.app.is_dirty = false;
                for var in self.app.vars.iter_mut() {
                    var.dirty = false;
                }
                self.app.set_status(format!("Saved to {}", path.display()));
                true
            }
            Err(e) => {
                // The reason matters: a read-only path and an unrepresentable
                // document fail very differently.
                self.app.set_status(format!("Could not save {}: {}", path.display(), e));
                false
            }
        }
    }

    /// Synchronizes the prompt line with the active command buffer.
    fn sync_command_prompt(&mut self) {
        self.app.prompt = if self.command_buffer.is_empty() {
            None
        } else {
            Some(self.command_buffer.clone())
        };
    }

    /// Synchronizes the prompt line with the active search query.
    fn sync_search_prompt(&mut self) {
        self.app.prompt = Some(format!(
            "{}{}",
            self.config.keybinds.key_for(Action::Search),
            self.app.search_query
        ));
    }
}