//! Executors allow easy scheduling and execution of functions or closures.
//! The `CoreExecutor` uses a single thread for scheduling and execution, while the
//! `ThreadPoolExecutor` uses multiple threads to execute the function.
//! Internally, each executor uses a `tokio_core::reactor::Core` as event loop, that will drive
//! the scheduling of the functions (and for the `CoreExecutor`, also their execution). A reference
//! to the event loop is passed to every function when executed, allowing it to register additional
//! events if needed.
use futures::future::Future;
use futures::sync::oneshot::{channel, Sender};
use futures_cpupool::{Builder, CpuPool};
use tokio_core::reactor::Timeout;
use tokio_core::reactor::{Core, Handle, Remote};

use std::io;
use std::sync::Arc;
use std::thread::{self, JoinHandle};
use std::time::{Instant, Duration};


fn fixed_interval_loop<F>(scheduled_fn: F, interval: Duration, handle: &Handle)
    where F: Fn(&Handle) + Send + 'static
{
    let start_time = Instant::now();
    scheduled_fn(&handle);
    let execution = start_time.elapsed();
    let next_iter_wait = if execution >= interval {
        Duration::from_secs(0)
    } else {
        interval - execution
    };
    let handle_clone = handle.clone();
    let t = Timeout::new(next_iter_wait, handle).unwrap()
        .then(move |_| {
            fixed_interval_loop(scheduled_fn, interval, &handle_clone);
            Ok::<(), ()>(())
        });
    handle.spawn(t);
}

fn calculate_delay(interval: Duration, execution: Duration, delay: Duration) -> (Duration, Duration) {
    if execution >= interval {
        (Duration::from_secs(0), delay + execution - interval)
    } else {
        let wait_gap = interval - execution;
        if delay == Duration::from_secs(0) {
            (wait_gap, Duration::from_secs(0))
        } else {
            if delay < wait_gap {
                (wait_gap - delay, Duration::from_secs(0))
            } else {
                (Duration::from_secs(0), delay - wait_gap)
            }
        }
    }
}

fn fixed_rate_loop<F>(scheduled_fn: F, interval: Duration, handle: &Handle, delay: Duration)
    where F: Fn(&Handle) + Send + 'static
{
    let start_time = Instant::now();
    scheduled_fn(&handle);
    let execution = start_time.elapsed();
    let (next_iter_wait, updated_delay) = calculate_delay(interval, execution, delay);
    let handle_clone = handle.clone();
    let t = Timeout::new(next_iter_wait, handle).unwrap()
        .then(move |_| {
            fixed_rate_loop(scheduled_fn, interval, &handle_clone, updated_delay);
            Ok::<(), ()>(())
        });
    handle.spawn(t);
}


struct CoreExecutorInner {
    remote: Remote,
    termination_sender: Option<Sender<()>>,
    thread_handle: Option<JoinHandle<()>>,
}

impl Drop for CoreExecutorInner {
    fn drop(&mut self) {
        let _ = self.termination_sender.take().unwrap().send(());
        let _ = self.thread_handle.take().unwrap().join();
    }
}

/// A `CoreExecutor` is the most simple executor provided. It runs a single thread, which is
/// responsible for both scheduling the function (registering the timer for the wakeup),
/// and the actual execution. The executor will stop once dropped. The `CoreExecutor`
/// can be cloned to generate a new reference to the same underlying executor.
/// Given the single threaded nature of this executor, tasks are executed sequentially, and a long
/// running task will cause delay in other subsequent executions.
pub struct CoreExecutor {
    inner: Arc<CoreExecutorInner>
}

impl Clone for CoreExecutor {
    fn clone(&self) -> Self {
        CoreExecutor { inner: Arc::clone(&self.inner) }
    }
}

impl CoreExecutor {
    /// Creates a new `CoreExecutor`.
    pub fn new() -> Result<CoreExecutor, io::Error> {
        CoreExecutor::with_name("core_executor")
    }

    /// Creates a new `CoreExecutor` with the specified thread name.
    pub fn with_name(thread_name: &str) -> Result<CoreExecutor, io::Error> {
        let (termination_tx, termination_rx) = channel();
        let (core_tx, core_rx) = channel();
        let thread_handle = thread::Builder::new()
            .name(thread_name.to_owned())
            .spawn(move || {
                debug!("Core starting");
                let mut core = Core::new().expect("Failed to start core");
                let _ = core_tx.send(core.remote());
                match core.run(termination_rx) {
                    Ok(v) => debug!("Core terminated correctly {:?}", v),
                    Err(e) => debug!("Core terminated with error: {:?}", e),
                }
            })?;
        let inner = CoreExecutorInner {
            remote: core_rx.wait().expect("Failed to receive remote"),
            termination_sender: Some(termination_tx),
            thread_handle: Some(thread_handle),
        };
        let executor = CoreExecutor {
            inner: Arc::new(inner)
        };
        debug!("Executor created");
        Ok(executor)
    }

    /// Schedule a function for running at fixed intervals. The executor will try to run the
    /// function every `interval`, but if one execution takes longer than `interval` it will delay
    /// all the subsequent calls.
    pub fn schedule_fixed_interval<F>(&self, initial: Duration, interval: Duration, scheduled_fn: F)
        where F: Fn(&Handle) + Send + 'static
    {
        self.inner.remote.spawn(move |handle| {
            let handle_clone = handle.clone();
            let t = Timeout::new(initial, handle).unwrap()
                .then(move |_| {
                    fixed_interval_loop(scheduled_fn, interval, &handle_clone);
                    Ok::<(), ()>(())
                });
            handle.spawn(t);
            Ok::<(), ()>(())
        });
    }

    /// Schedule a function for running at fixed rate. The executor will try to run the function
    /// every `interval`, and if a task execution takes longer than `interval`, the wait time
    /// between task will be reduced to decrease the overall delay.
    pub fn schedule_fixed_rate<F>(&self, initial: Duration, interval: Duration, scheduled_fn: F)
        where F: Fn(&Handle) + Send + 'static
    {
        self.inner.remote.spawn(move |handle| {
            let handle_clone = handle.clone();
            let t = Timeout::new(initial, handle).unwrap()
                .then(move |_| {
                    fixed_rate_loop(scheduled_fn, interval, &handle_clone, Duration::from_secs(0));
                    Ok::<(), ()>(())
                });
            handle.spawn(t);
            Ok::<(), ()>(())
        });
    }
}


/// A `ThreadPoolExecutor` will use one thread for the task scheduling and a thread pool for
/// task execution, allowing multiple tasks to run in parallel.
#[derive(Clone)]
pub struct ThreadPoolExecutor {
    executor: CoreExecutor,
    pool: CpuPool
}

impl ThreadPoolExecutor {
    /// Creates a new `ThreadPoolExecutor` with the specified number of threads. Threads will
    /// be named "pool_thread_0", "pool_thread_1" and so on.
    pub fn new(threads: usize) -> Result<ThreadPoolExecutor, io::Error> {
        ThreadPoolExecutor::with_prefix(threads, "pool_thread_")
    }

    /// Creates a new `ThreadPoolExecutor` with the specified number of threads and prefix for
    /// the thread names.
    pub fn with_prefix(threads: usize, prefix: &str) -> Result<ThreadPoolExecutor, io::Error> {
        let new_executor = CoreExecutor::with_name(&format!("{}executor", prefix))?;
        Ok(ThreadPoolExecutor::with_executor(threads, prefix, new_executor))
    }

    /// Creates a new `ThreadPoolExecutor` with the specified number of threads, prefix and
    /// using the given `CoreExecutor` for scheduling.
    pub fn with_executor(threads: usize, prefix: &str, executor: CoreExecutor) -> ThreadPoolExecutor {
        let pool = Builder::new()
            .pool_size(threads)
            .name_prefix(prefix)
            .create();
        ThreadPoolExecutor { pool, executor }
    }

    /// Schedules the given function to be executed every `interval`. The function will be
    /// scheduled on one of the threads in the thread pool.
    pub fn schedule_fixed_rate<F>(&self, initial: Duration, interval: Duration, scheduled_fn: F)
        where F: Fn(&Remote) + Send + Sync + 'static
    {
        let pool_clone = self.pool.clone();
        let arc_fn = Arc::new(scheduled_fn);
        self.executor.schedule_fixed_interval(  // Fixed interval is enough
            initial,
            interval,
            move |handle| {
                let arc_fn_clone = arc_fn.clone();
                let remote = handle.remote().clone();
                let t = pool_clone.spawn_fn(move || {
                    arc_fn_clone(&remote);
                    Ok::<(),()>(())
                });
                handle.spawn(t);
            }
        );
    }

    // TODO: make pub(crate)
    /// Returns the thread pool used internally.
    pub fn pool(&self) -> &CpuPool {
        &self.pool
    }
}


#[cfg(test)]
mod tests {
    use std::sync::{Arc, RwLock};
    use std::thread;
    use std::time::{Duration, Instant};

    use super::{CoreExecutor, ThreadPoolExecutor, calculate_delay};

    #[test]
    fn fixed_interval_test() {
        let timings = Arc::new(RwLock::new(Vec::new()));
        {
            let executor = CoreExecutor::new().unwrap();
            let timings_clone = Arc::clone(&timings);
            executor.schedule_fixed_rate(
                Duration::from_secs(0),
                Duration::from_secs(1),
                move |_handle| {
                    timings_clone.write().unwrap().push(Instant::now());
                }
            );
            thread::sleep(Duration::from_millis(5500));
        }

        let timings = timings.read().unwrap();
        assert!(timings.len() == 6);
        for i in 1..6 {
            let execution_interval = timings[i] - timings[i-1];
            assert!(execution_interval < Duration::from_millis(1020));
            assert!(execution_interval > Duration::from_millis(980));
        }
    }

    #[test]
    fn fixed_interval_slow_task_test() {
        let counter = Arc::new(RwLock::new(0));
        let counter_clone = Arc::clone(&counter);
        {
            let executor = CoreExecutor::new().unwrap();
            executor.schedule_fixed_interval(
                Duration::from_secs(0),
                Duration::from_secs(1),
                move |_handle| {
                    // TODO: use atomic int when available
                    let counter = {
                        let mut counter = counter_clone.write().unwrap();
                        (*counter) += 1;
                        *counter
                    };
                    if counter == 1 {
                        thread::sleep(Duration::from_secs(3));
                    }
                }
            );
            thread::sleep(Duration::from_millis(5500));
        }
        assert_eq!(*counter.read().unwrap(), 4);
    }

    #[test]
    fn calculate_delay_test() {
        fn s(n: u64) -> Duration { Duration::from_secs(n) };
        assert_eq!(calculate_delay(s(10), s(3), s(0)), (s(7), s(0)));
        assert_eq!(calculate_delay(s(10), s(11), s(0)), (s(0), s(1)));
        assert_eq!(calculate_delay(s(10), s(3), s(3)), (s(4), s(0)));
        assert_eq!(calculate_delay(s(10), s(3), s(9)), (s(0), s(2)));
        assert_eq!(calculate_delay(s(10), s(12), s(15)), (s(0), s(17)));
    }

    #[test]
    fn fixed_rate_test() {
        let counter = Arc::new(RwLock::new(0));
        let counter_clone = Arc::clone(&counter);
        {
            let executor = CoreExecutor::new().unwrap();
            executor.schedule_fixed_rate(
                Duration::from_secs(0),
                Duration::from_secs(1),
                move |_handle| {
                    let mut counter = counter_clone.write().unwrap();
                    (*counter) += 1;
                }
            );
            thread::sleep(Duration::from_millis(5500));
        }
        assert_eq!(*counter.read().unwrap(), 6);
    }

    #[test]
    fn fixed_rate_slow_task_test() {
        let counter = Arc::new(RwLock::new(0));
        let counter_clone = Arc::clone(&counter);
        {
            let executor = CoreExecutor::new().unwrap();
            executor.schedule_fixed_rate(
                Duration::from_secs(0),
                Duration::from_secs(1),
                move |_handle| {
                    // TODO: use atomic int when available
                    let counter = {
                        let mut counter = counter_clone.write().unwrap();
                        (*counter) += 1;
                        *counter
                    };
                    if counter == 1 {
                        thread::sleep(Duration::from_secs(3));
                    }
                }
            );
            thread::sleep(Duration::from_millis(5500));
        }
        assert_eq!(*counter.read().unwrap(), 6);
    }

    #[test]
    fn fixed_rate_slow_task_test_pool() {
        let counter = Arc::new(RwLock::new(0));
        let counter_clone = Arc::clone(&counter);
        {
            let executor = ThreadPoolExecutor::new(20).unwrap();
            executor.schedule_fixed_rate(
                Duration::from_secs(0),
                Duration::from_secs(1),
                move |_remote| {
                    // TODO: use atomic int when available
                    let counter = {
                        let mut counter = counter_clone.write().unwrap();
                        (*counter) += 1;
                        *counter
                    };
                    if counter == 1 {
                        thread::sleep(Duration::from_secs(3));
                    }
                }
            );
            thread::sleep(Duration::from_millis(5500));
        }
        assert_eq!(*counter.read().unwrap(), 6);
    }
}