use color_eyre::eyre::WrapErr;
use ratatui::crossterm::event::{self, Event as CrosstermEvent};
use std::{
    sync::mpsc,
    thread,
    time::{Duration, Instant},
};

/// The frequency at which tick events are emitted.
const TICK_FPS: f64 = 30.0;

/// Representation of all possible events.
#[derive(Clone, Debug)]
pub enum Event {
    /// An event that is emitted on a regular schedule.
    ///
    /// Use this event to run any code which has to run outside of being a direct response to a user
    /// event. e.g. polling exernal systems, updating animations, or rendering the UI based on a
    /// fixed frame rate.
    Tick,
    /// Crossterm events.
    ///
    /// These events are emitted by the terminal.
    Crossterm(CrosstermEvent),
    /// Application events.
    ///
    /// Use this event to emit custom events that are specific to your application.
    App(AppEvent),
}

/// Application events.
///
/// You can extend this enum with your own custom events.
#[derive(Clone, Debug)]
pub enum AppEvent {
    /// Increment the counter.
    Increment,
    /// Decrement the counter.
    Decrement,
    /// Quit the application.
    Quit,
}

/// Terminal event handler.
#[derive(Debug)]
pub struct EventHandler {
    /// Event sender channel.
    sender: mpsc::Sender<Event>,
    /// Event receiver channel.
    receiver: mpsc::Receiver<Event>,
}

impl EventHandler {
    /// Constructs a new instance of [`EventHandler`] and spawns a new thread to handle events.
    pub fn new() -> Self {
        let (sender, receiver) = mpsc::channel();
        let actor = EventThread::new(sender.clone());
        thread::spawn(|| actor.run());
        Self { sender, receiver }
    }

    /// Receives an event from the sender.
    ///
    /// This function blocks until an event is received.
    ///
    /// # Errors
    ///
    /// This function returns an error if the sender channel is disconnected. This can happen if an
    /// error occurs in the event thread. In practice, this should not happen unless there is a
    /// problem with the underlying terminal.
    pub fn next(&self) -> color_eyre::Result<Event> {
        Ok(self.receiver.recv()?)
    }

    /// Queue an app event to be sent to the event receiver.
    ///
    /// This is useful for sending events to the event handler which will be processed by the next
    /// iteration of the application's event loop.
    pub fn send(&self, app_event: AppEvent) {
        // Ignore the result as the reciever cannot be dropped while this struct still has a
        // reference to it
        let _ = self.sender.send(Event::App(app_event));
    }
}

/// A thread that handles reading crossterm events and emitting tick events on a regular schedule.
struct EventThread {
    /// Event sender channel.
    sender: mpsc::Sender<Event>,
}

impl EventThread {
    /// Constructs a new instance of [`EventThread`].
    const fn new(sender: mpsc::Sender<Event>) -> Self {
        Self { sender }
    }

    /// Runs the event thread.
    ///
    /// This function emits tick events at a fixed rate and polls for crossterm events in between.
    fn run(self) -> color_eyre::Result<()> {
        let tick_interval = Duration::from_secs_f64(1.0 / TICK_FPS);
        let mut last_tick = Instant::now();
        loop {
            // emit tick events at a fixed rate
            let timeout = tick_interval.saturating_sub(last_tick.elapsed());
            if timeout == Duration::ZERO {
                last_tick = Instant::now();
                self.send(Event::Tick);
            }
            // poll for crossterm events, ensuring that we don't block the tick interval
            if event::poll(timeout).wrap_err("failed to poll for crossterm events")? {
                let event = event::read().wrap_err("failed to read crossterm event")?;
                self.send(Event::Crossterm(event));
            }
        }
    }

    /// Sends an event to the receiver.
    fn send(&self, event: Event) {
        // Ignores the result because shutting down the app drops the receiver, which causes the send
        // operation to fail. This is expected behavior and should not panic.
        let _ = self.sender.send(event);
    }
}

impl Default for EventHandler {
    fn default() -> Self {
        Self::new()
    }
}