//! A simple, cancelable timer implementation with no external dependencies.

#![deny(missing_docs)]

use std::sync::mpsc::{self, Sender};
use std::sync::{Arc, Condvar, Mutex, TryLockError};
use std::thread;
use std::time::Duration;

/// Errors that may be thrown by ThreadTimer::start()
#[derive(Debug, Eq, PartialEq)]
pub enum TimerStartError {
  /// The timer is already waiting to execute some other thunk
  AlreadyWaiting,
}

/// Errors that may be thrown by ThreadTimer::cancel()
#[derive(Debug, Eq, PartialEq)]
pub enum TimerCancelError {
  /// The timer is not currently waiting, so there is nothing to cancel
  NotWaiting,
}

/// Message sent to tell the timer thread to start waiting
struct StartWaitMessage {
  dur: Duration,
  f: Box<dyn FnOnce() + Send + 'static>,
}

/// A simple, cancelable timer that can run a thunk after waiting for an
/// arbitrary duration.
///
/// Waiting is accomplished by using a helper thread (the "wait thread") that
/// listens for incoming wait requests and then executes the requested thunk
/// after blocking for the requested duration. Because each ThreadTimer keeps
/// only one wait thread, each ThreadTimer may only be waiting for a single
/// thunk at a time.
///
/// ```
/// use std::sync::mpsc::{self, TryRecvError};
/// use std::thread;
/// use std::time::Duration;
/// use thread_timer::ThreadTimer;
///
/// let (sender, receiver) = mpsc::channel::<bool>();
/// let timer = ThreadTimer::new();
///
/// timer.start(Duration::from_millis(50), move || { sender.send(true).unwrap() }).unwrap();
///
/// thread::sleep(Duration::from_millis(60));
/// assert_eq!(receiver.try_recv(), Ok(true));
/// ```
///
/// If a ThreadTimer is currently waiting to execute a thunk, the wait can be
/// canceled, in which case the thunk will not be run.
///
/// ```
/// use std::sync::mpsc::{self, TryRecvError};
/// use std::thread;
/// use std::time::Duration;
/// use thread_timer::ThreadTimer;
///
/// let (sender, receiver) = mpsc::channel::<bool>();
/// let timer = ThreadTimer::new();
///
/// timer.start(Duration::from_millis(50), move || { sender.send(true).unwrap() }).unwrap();
///
/// thread::sleep(Duration::from_millis(10));
/// timer.cancel().unwrap();
///
/// thread::sleep(Duration::from_millis(60));
/// assert_eq!(receiver.try_recv(), Err(TryRecvError::Disconnected));
/// ```
#[derive(Clone)]
pub struct ThreadTimer {
  // Allow only one operation at a time so that we don't need to worry about interleaving
  op_lock: Arc<Mutex<()>>,
  // Used to track whether or not the timer is currently waiting
  is_waiting: Arc<(Mutex<bool>, Condvar)>,
  // Used to wait for a cancelation signal and avoid spurious wakeups while waiting
  is_canceled: Arc<(Mutex<bool>, Condvar)>,
  // Used to tell the timer thread to start waiting
  sender: Sender<StartWaitMessage>,
}

impl ThreadTimer {
  /// Creates and returns a new ThreadTimer. Spawns a new thread to do the
  /// waiting (the "wait thread").
  ///
  /// ```
  /// use thread_timer::ThreadTimer;
  /// let timer = ThreadTimer::new();
  /// ```
  pub fn new() -> Self {
    let (sender, receiver) = mpsc::channel::<StartWaitMessage>();
    let is_waiting = Arc::new((Mutex::new(false), Condvar::new()));
    let thread_is_waiting = is_waiting.clone();
    let is_canceled = Arc::new((Mutex::new(false), Condvar::new()));
    let thread_is_canceled = is_canceled.clone();

    thread::spawn(move || {
      // Loop waiting for a new message from the client thread(s).  If all
      // senders have disconnected, we will never receive a new message so
      // we should break out of the loop.
      while let Ok(msg) = receiver.recv() {
        let (cancel_lock, cancel_condvar) = &*thread_is_canceled;
        let (mut cancel_guard, cancel_res) = cancel_condvar
          .wait_timeout_while(
            cancel_lock.lock().unwrap(),
            msg.dur,
            |&mut is_canceled| !is_canceled,
          )
          .unwrap();
        if cancel_res.timed_out() {
          // Only run the thunk if the wait completed (i.e. it was not canceled)
          (msg.f)();
        }
        // Always clear the cancel guard (even if the wait completed and we
        // executed the thunk)
        *cancel_guard = false;
        let (is_waiting_lock, is_waiting_condvar) = &*thread_is_waiting;
        *is_waiting_lock.lock().unwrap() = false;
        is_waiting_condvar.notify_one();
      }
    });

    ThreadTimer {
      op_lock: Arc::new(Mutex::new(())),
      is_waiting,
      is_canceled,
      sender,
    }
  }

  /// Start waiting. Wait for `dur` to elapse then execute `f`. Will not
  /// execute `f` if the timer is canceled before `dur` elapses.
  /// Returns [TimerStartError](enum.TimerStartError.html)::AlreadyWaiting if
  /// the timer is already waiting to execute a thunk.
  /// ```
  /// use std::sync::mpsc::{self, TryRecvError};
  /// use std::thread;
  /// use std::time::Duration;
  /// use thread_timer::ThreadTimer;
  ///
  /// let (sender, receiver) = mpsc::channel::<bool>();
  /// let timer = ThreadTimer::new();
  ///
  /// timer.start(Duration::from_millis(50), move || { sender.send(true).unwrap() }).unwrap();
  /// assert_eq!(
  ///   receiver.try_recv(),
  ///   Err(TryRecvError::Empty),
  ///   "Received response before wait elapsed!",
  /// );
  ///
  /// thread::sleep(Duration::from_millis(60));
  /// assert_eq!(
  ///   receiver.try_recv(),
  ///   Ok(true),
  ///   "Did not receive response after wait elapsed!",
  /// );
  /// ```
  pub fn start<F>(&self, dur: Duration, f: F) -> Result<(), TimerStartError>
  where
    F: FnOnce() + Send + 'static,
  {
    let _guard = self.op_lock.lock().unwrap();
    let (is_waiting_lock, _) = &*self.is_waiting;
    let mut is_waiting = is_waiting_lock.lock().unwrap();
    if *is_waiting {
      return Err(TimerStartError::AlreadyWaiting);
    }
    *is_waiting = true;
    let msg = StartWaitMessage {
      dur,
      f: Box::new(f),
    };
    self.sender.send(msg).unwrap();
    Ok(())
  }

  /// Cancel the current timer (the thunk will not be executed and the timer
  /// will be able to start waiting to execute another thunk). This function
  /// waits until the wait thread has confirmed that it is ready to start
  /// waiting again, so it is safe to call start immediately after calling this
  /// function.
  /// Returns [TimerCancelError](enum.TimerCancelError.html)::NotWaiting if
  /// the timer is not currently waiting.
  /// ```
  /// use std::sync::mpsc::{self, TryRecvError};
  /// use std::thread;
  /// use std::time::Duration;
  /// use thread_timer::ThreadTimer;
  ///
  /// let (sender, receiver) = mpsc::channel::<bool>();
  /// let timer = ThreadTimer::new();
  ///
  /// timer.start(Duration::from_millis(50), move || { sender.send(true).unwrap() }).unwrap();
  ///
  /// // Make sure the wait has actually started before we cancel
  /// thread::sleep(Duration::from_millis(10));
  /// timer.cancel().unwrap();
  ///
  /// thread::sleep(Duration::from_millis(60));
  /// assert_eq!(
  ///   receiver.try_recv(),
  ///   // When the wait is canceled, the thunk and its Sender will be dropped
  ///   Err(TryRecvError::Disconnected),
  ///   "Received response from canceled wait!",
  /// );
  /// ```
  pub fn cancel(&self) -> Result<(), TimerCancelError> {
    let _guard = self.op_lock.lock().unwrap();
    let (is_waiting_lock, is_waiting_condvar) = &*self.is_waiting;
    if !*is_waiting_lock.lock().unwrap() {
      return Err(TimerCancelError::NotWaiting);
    }

    let (cancel_lock, cancel_condvar) = &*self.is_canceled;

    // This must be try_lock() not lock() in order to avoid a deadlock with
    // the wait thread. At this point the client thread holds the wait
    // lock. If the wait thread holds the cancel lock (if it has finished
    // waiting and is running the task), then we will not be able to get the
    // cancel lock here and the wait thread will not be able to get the wait
    // lock to indicate that it has finished waiting.
    match cancel_lock.try_lock() {
      // We were able to acquire the cancel lock, so cancel the wait
      Ok(mut cancel_guard) => {
        *cancel_guard = true;
        cancel_condvar.notify_one();
        // Let go of the cancel lock so that the wait thread can acquire it
        // (this is necessary for the wait thread's call to
        // cancel_condvar.wait_timeout_while() to terminate). If this thread is
        // still holding the cancel lock when it starts to wait on the
        // is_waiting_condvar, we'll hit a deadlock.
        drop(cancel_guard);
        // Wait until the wait thread acknowledges the cancellation (this allows
        // a client to call start() immediately after cancel() without worrying
        // about a race condition)
        let _ = is_waiting_condvar
          .wait_while(is_waiting_lock.lock().unwrap(), |&mut is_waiting| {
            is_waiting
          })
          .unwrap();
        Ok(())
      }
      // The wait thread holds the cancel lock, so return an error
      // indicating that we were unable to cancel
      Err(TryLockError::WouldBlock) => Err(TimerCancelError::NotWaiting),
      Err(TryLockError::Poisoned(_)) => panic!("Cancel lock was poisoned"),
    }
  }
}

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