//! This module provides the functionality to create thread pool to execute tasks in parallel.
//! The amount of threads to be used at maximum can be regulated by using `ThreadPool::with_limit`.
//! This implementation is aimed to be of low runtime cost with minimal sychronisation due to blocking.
//! Note that no threads will be spawned until jobs are supplied to be executed. For every supplied job
//! a new thread will be launched until the maximum number is reached. By then every launched thread will
//! be reused to process the remaining elements of the queue. If no jobs are left to be executed
//! all threads will finish and die. This means that if nothing is done, no threads will run in idle in the background.
//! # Example
//! ```rust
//! # use imsearch::multithreading::ThreadPool;
//! # use imsearch::multithreading::Task;
//! let mut pool = ThreadPool::with_limit(2);
//!
//! for i in 0..10 {
//!     pool.enqueue(Task::new(i, |i| i));
//! //                            ^^^^^^ closure or static function
//! }
//!
//! pool.join_all();
//! assert_eq!(pool.get_results().iter().sum::<i32>(), 45);
//! ```

use std::{
    any::Any,
    collections::VecDeque,
    num::NonZeroUsize,
    sync::{
        mpsc::{channel, Receiver, Sender},
        Arc, Mutex,
    },
    thread::{self, JoinHandle},
};

/// Default number if threads to be used in case [`std::thread::available_parallelism`] fails.
pub const DEFAULT_THREAD_POOL_SIZE: usize = 1;

/// Indicates the priority level of functions or closures which get supplied to the pool.
/// Use [`Priority::High`] to ensure the closue to be executed before all closures that are already supplied
/// Use [`Priority::Low`] to ensure the closue to be executed after all closures that are already supplied
#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord)]
pub enum Priority {
    /// Indicate that the closure or function supplied to the thread
    /// has higher priority than any other given to the pool until now.
    /// The item will get enqueued at the start of the waiting-queue.
    High,
    /// Indicate that the closure or function supplied to the thread pool
    /// has lower priority than the already supplied ones in this pool.
    /// The item will get enqueued at the end of the waiting-queue.
    Low,
}

/// Traits a return value has to implement when being given back by a function or closure.
pub trait Sendable: Any + Send + 'static {}

impl<T> Sendable for T where T: Any + Send + 'static {}

/// A task that will be executed at some point in the future by the thread pool
/// At the heart of this struct is the function to be executed. This may be a closure.
#[derive(Debug, Copy, Clone)]
pub struct Task<I, T>
where
    I: Sendable,
    T: Sendable,
{
    job: fn(I) -> T,
    param: I,
}

impl<I, T> Task<I, T>
where
    I: Sendable,
    T: Sendable,
{
    pub fn new(param: I, job: fn(I) -> T) -> Self {
        Self { job, param }
    }
}

/// Thread pool which can be used to execute functions or closures in parallel.
/// The amount of threads to be used at maximum can be regulated by using `ThreadPool::with_limit`.
/// This implementation is aimed to be of low runtime cost with minimal sychronisation due to blocking.
/// Note that no threads will be spawned until jobs are supplied to be executed. For every supplied job
/// a new thread will be launched until the maximum number is reached. By then every launched thread will
/// be reused to process the remaining elements of the queue. If no jobs are left to be executed
/// all threads will finish and die. This means that if nothing is done, no threads will run in idle in the background.
/// # Example
/// ```rust
/// # use imsearch::multithreading::ThreadPool;
/// # use imsearch::multithreading::Task;
/// let mut pool = ThreadPool::with_limit(2);
///
/// for i in 0..10 {
///     pool.enqueue(Task::new(i, |i| i));
/// }
///
/// pool.join_all();
/// assert_eq!(pool.get_results().iter().sum::<i32>(), 45);
/// ```
/// # Drop
/// This struct implements the `Drop` trait. Upon being dropped the pool will wait for all threads
/// to finsish. This may take up an arbitrary amount of time.
/// # Panics in the thread
/// When a function or closure panics, the executing thread will detect the unwind performed by `panic` causing the
/// thread to print a message on stderr. The thread itself captures panics and won't terminate execution but continue with
/// the next task in the queue.
/// Its not recommend to use this pool with custom panic hooks or special functions which abort the process.
/// Also panicking code from external program written in C++ or others in undefinied behavior according to [`std::panic::catch_unwind`]
#[derive(Debug)]
pub struct ThreadPool<I, T>
where
    I: Sendable,
    T: Sendable,
{
    /// queue for storing the jobs to be executed
    queue: Arc<Mutex<VecDeque<Task<I, T>>>>,
    /// handles for all threads currently running and processing jobs
    handles: Vec<JoinHandle<()>>,
    /// reciver end for channel based communication between threads
    receiver: Receiver<T>,
    /// sender end for channel based communication between threads
    sender: Sender<T>,
    /// maximum amount of threads to be used in parallel
    limit: NonZeroUsize,
}

impl<I, T> Default for ThreadPool<I, T>
where
    I: Sendable,
    T: Sendable,
{
    fn default() -> Self {
        let (sender, receiver) = channel::<T>();

        // determine default thread count to use based on the system
        let default =
            NonZeroUsize::new(DEFAULT_THREAD_POOL_SIZE).expect("Thread limit must be non-zero");
        let limit = thread::available_parallelism().unwrap_or(default);

        Self {
            queue: Arc::new(Mutex::new(VecDeque::new())),
            handles: Vec::new(),
            receiver,
            sender,
            limit,
        }
    }
}

impl<I, T> ThreadPool<I, T>
where
    I: Sendable,
    T: Sendable,
{
    /// Creates a new thread pool with default thread count determined by either
    /// [`std::thread::available_parallelism`] or [`DEFAULT_THREAD_POOL_SIZE`] in case it fails.
    /// No threads will be lauched until jobs are enqueued.
    pub fn new() -> Self {
        Default::default()
    }

    /// Creates a new thread pool with the given thread count. The pool will continue to launch new threads even if
    /// the system does not allow for that count of parallelism.
    /// No threads will be lauched until jobs are enqueued.
    /// # Panic
    /// This function will fails if `max_threads` is zero.
    pub fn with_limit(max_threads: usize) -> Self {
        let (sender, receiver) = channel::<T>();
        Self {
            limit: NonZeroUsize::new(max_threads).expect("Thread limit must be non-zero"),
            queue: Arc::new(Mutex::new(VecDeque::new())),
            handles: Vec::new(),
            sender,
            receiver,
        }
    }

    /// Put a new job into the queue to be executed by a thread in the future.
    /// The priority of the job will determine if the job will be put at the start or end of the queue.
    /// See [`crate::multithreading::Priority`].
    /// This function will create a new thread if the maximum number of threads in not reached.
    /// In case the maximum number of threads is already used, the job is stalled and will get executed
    /// when a thread is ready and its at the start of the queue.
    pub fn enqueue_priorize(&mut self, func: Task<I, T>, priority: Priority) {
        // put job into queue
        let mut queue = self.queue.lock().unwrap();

        // insert new job into queue depending on its priority
        match priority {
            Priority::High => queue.push_front(func),
            Priority::Low => queue.push_back(func),
        }

        if self.handles.len() < self.limit.get() {
            // we can still launch threads to run in parallel

            // clone the sender
            let tx = self.sender.clone();
            let queue = self.queue.clone();

            self.handles.push(thread::spawn(move || {
                while let Some(task) = queue.lock().unwrap().pop_front() {
                    tx.send((task.job)(task.param))
                        .expect("unable to send result over channel");
                }
            }));
        }

        self.handles.retain(|h| !h.is_finished());
    }

    /// Put a new job into the queue to be executed by a thread in the future.
    /// The priority of the job is automatically set to [`crate::multithreading::Priority::Low`].
    /// This function will create a new thread if the maximum number of threads in not reached.
    /// In case the maximum number of threads is already used, the job is stalled and will get executed
    /// when a thread is ready and its at the start of the queue.
    pub fn enqueue(&mut self, func: Task<I, T>) {
        self.enqueue_priorize(func, Priority::Low);
    }

    /// Wait for all threads to finish executing. This means that by the time all threads have finished
    /// every task will have been executed too. In other words the threads finsish when the queue of jobs is empty.
    /// This function will block the caller thread.
    pub fn join_all(&mut self) {
        while let Some(handle) = self.handles.pop() {
            handle.join().unwrap();
        }
    }

    /// Sendables all results that have been Sendableed by the threads until now
    /// and haven't been consumed yet.
    /// All results retrieved from this call won't be Sendableed on a second call.
    /// This function is non blocking.
    pub fn try_get_results(&mut self) -> Vec<T> {
        self.receiver.try_iter().collect()
    }

    /// Sendables all results that have been returned by the threads until now
    /// and haven't been consumed yet. The function will also wait for all threads to finish executing (empty the queue).
    /// All results retrieved from this call won't be returned on a second call.
    /// This function will block the caller thread.
    pub fn get_results(&mut self) -> Vec<T> {
        self.join_all();
        self.try_get_results()
    }
}

impl<I, T> Drop for ThreadPool<I, T>
where
    I: Sendable,
    T: Sendable,
{
    fn drop(&mut self) {
        self.join_all();
    }
}

#[cfg(test)]
mod test {
    use std::panic::UnwindSafe;

    use super::*;

    #[test]
    fn test_default() {
        let mut pool = ThreadPool::default();

        for i in 0..10 {
            pool.enqueue_priorize(Task::new(i, |i| i), Priority::High);
        }

        pool.join_all();

        assert_eq!(pool.try_get_results().iter().sum::<i32>(), 45);
    }

    #[test]
    fn test_limit() {
        let mut pool = ThreadPool::with_limit(2);

        for i in 0..10 {
            pool.enqueue(Task::new(i, |i| i));
        }

        assert_eq!(pool.handles.len(), 2);
        assert_eq!(pool.limit.get(), 2);

        pool.join_all();

        assert_eq!(pool.get_results().iter().sum::<i32>(), 45);
    }

    trait Object: Send + UnwindSafe {
        fn get(&mut self) -> i32;
    }

    #[derive(Default)]
    struct Test1 {
        _int: i32,
    }

    impl Object for Test1 {
        fn get(&mut self) -> i32 {
            0
        }
    }

    #[derive(Default)]
    struct Test2 {
        _c: char,
    }

    impl Object for Test2 {
        fn get(&mut self) -> i32 {
            0
        }
    }

    #[derive(Default)]
    struct Test3 {
        _s: String,
    }

    impl Object for Test3 {
        fn get(&mut self) -> i32 {
            0
        }
    }

    #[test]
    fn test_1() {
        let mut pool = ThreadPool::with_limit(2);

        let feats: Vec<Box<dyn Object>> = vec![
            Box::new(Test1::default()),
            Box::new(Test2::default()),
            Box::new(Test3::default()),
        ];

        for feat in feats {
            pool.enqueue(Task::new(feat, |mut i| {
                let _ = i.get();
                i
            }));
        }

        pool.join_all();

        let _feats: Vec<Box<dyn Object>> = pool.get_results();
    }
}