use std::sync::atomic::{AtomicU32, Ordering};

use crate::collections::RawArray;
use super::unfair_se_lock::{UnfairSeLock, UnfairSeLockSharedGuard};

/// Multiple-producer single-consumer queue. Generally used in the publicly
/// accessible fields of a component. The holder of this struct should be the
/// consumer. To retrieve access to the producer-side: call `producer()`.
///
/// This is a queue that will resize (indefinitely) if it becomes full, and will
/// not shrink. So probably a temporary thing.
///
/// In debug mode we'll make sure that there are no producers when the queue is
/// dropped. We don't do this in release mode because the runtime is written
/// such that components always remain alive (hence, this queue will remain
/// accessible) while there are references to it.
// NOTE: Addendum to the above remark, not true if the thread owning the
// consumer sides crashes, unwinds, and drops the `Box` with it. Question is: do
// I want to take that into account?
pub struct QueueDynMpsc<T> {
    // Entire contents are boxed up such that we can create producers that have
    // a pointer to the contents.
    inner: Box<Shared<T>>
}

// One may move around the queue between threads, as long as there is only one
// instance of it.
unsafe impl<T> Send for QueueDynMpsc<T>{}

/// Shared data between queue consumer and the queue producers
struct Shared<T> {
    data: UnfairSeLock<Inner<T>>,
    read_head: AtomicU32,
    write_head: AtomicU32,
    limit_head: AtomicU32,
    #[cfg(debug_assertions)] dbg: AtomicU32,
}

/// Locked by an exclusive/shared lock. Exclusive lock is obtained when the
/// inner data array is resized.
struct Inner<T> {
    data: RawArray<T>,
    compare_mask: u32,
    read_mask: u32,
}

type InnerRead<'a, T> = UnfairSeLockSharedGuard<'a, Inner<T>>;

impl<T> QueueDynMpsc<T> {
    /// Constructs a new MPSC queue. Note that the initial capacity is always
    /// increased to the next power of 2 (if it isn't already).
    pub fn new(initial_capacity: usize) -> Self {
        let initial_capacity = initial_capacity.next_power_of_two();
        assert_correct_capacity(initial_capacity);

        let mut data = RawArray::new();
        data.resize(initial_capacity);

        let initial_capacity = initial_capacity as u32;

        return Self{
            inner: Box::new(Shared {
                data: UnfairSeLock::new(Inner{
                    data,
                    compare_mask: (2 * initial_capacity) - 1,
                    read_mask: initial_capacity - 1,
                }),
                read_head: AtomicU32::new(0),
                write_head: AtomicU32::new(initial_capacity),
                limit_head: AtomicU32::new(initial_capacity),
                #[cfg(debug_assertions)] dbg: AtomicU32::new(0),
            }),
        };
    }

    #[inline]
    pub fn producer(&self) -> QueueDynProducer<T> {
        return QueueDynProducer::new(self);
    }

    /// Perform an attempted read from the queue. It might be that some producer
    /// is putting something in the queue while this function is executing, and
    /// we don't get the consume it.
    pub fn pop(&mut self) -> Option<T> {
        let data_lock = self.inner.data.lock_shared();
        let cur_read = self.inner.read_head.load(Ordering::Acquire);
        let cur_limit = self.inner.limit_head.load(Ordering::Acquire);
        let buf_size = data_lock.data.cap() as u32;

        if (cur_read + buf_size) & data_lock.compare_mask != cur_limit {
            // Make a bitwise copy of the value and return it. The receiver is
            // responsible for dropping it.
            unsafe {
                let source = data_lock.data.get((cur_read & data_lock.read_mask) as usize);
                let value = std::ptr::read(source);
                // We can perform a store since we're the only ones modifying
                // the atomic.
                self.inner.read_head.store((cur_read + 1) & data_lock.compare_mask, Ordering::Release);
                return Some(value);
            }
        } else {
            return None;
        }
    }
}

impl<T> Drop for QueueDynMpsc<T> {
    fn drop(&mut self) {
        // There should be no more `QueueDynProducer` pointers to this queue
        dbg_code!(assert_eq!(self.inner.dbg.load(Ordering::Acquire), 0));
        // And so the limit head should be equal to the write head
        let data_lock = self.inner.data.lock_shared();
        let write_index = self.inner.write_head.load(Ordering::Acquire);
        assert_eq!(self.inner.limit_head.load(Ordering::Acquire), write_index);

        // Every item that has not yet been taken out of the queue needs to
        // have its destructor called. We immediately apply the
        // increment-by-size trick and wait until we've hit the write head.
        let mut read_index = self.inner.read_head.load(Ordering::Acquire);
        read_index += data_lock.data.cap() as u32;
        while read_index & data_lock.compare_mask != write_index {
            unsafe {
                let target = data_lock.data.get((read_index & data_lock.read_mask) as usize);
                std::ptr::drop_in_place(target);
            }
            read_index += 1;
        }
    }
}

pub struct QueueDynProducer<T> {
    queue: *const Shared<T>,
}

impl<T> QueueDynProducer<T> {
    fn new(consumer: &QueueDynMpsc<T>) -> Self {
        dbg_code!(consumer.inner.dbg.fetch_add(1, Ordering::AcqRel));
        unsafe {
            // If you only knew the power of the dark side! Obi-Wan never told
            // you what happened to your father!
            let queue: *const _ = std::mem::transmute(consumer.inner.as_ref());
            return Self{ queue };
        }
    }

    pub fn push(&self, value: T) {
        let queue = unsafe{ &*self.queue };

        let mut data_lock = queue.data.lock_shared();
        let mut write_index = queue.write_head.load(Ordering::Acquire);

        'attempt_write: loop {
            let read_index = queue.read_head.load(Ordering::Acquire);

            if write_index == read_index { // both stored as [0, 2*capacity), so we can check equality without bitwise ANDing
                // Need to resize, try loading read/write index afterwards
                let expected_capacity = data_lock.data.cap();
                data_lock = self.resize(data_lock, expected_capacity);
                write_index = queue.write_head.load(Ordering::Acquire);
                continue 'attempt_write;
            }

            // If here try to advance write index
            let new_write_index = (write_index + 1) & data_lock.compare_mask;
            if let Err(actual_write_index) = queue.write_head.compare_exchange(
                write_index, new_write_index, Ordering::AcqRel, Ordering::Acquire
            ) {
                write_index = actual_write_index;
                continue 'attempt_write;
            }

            // We're now allowed to write at `write_index`
            unsafe {
                std::ptr::write(data_lock.data.get((write_index & data_lock.read_mask) as usize), value);
            }

            // Update limit head to let reader obtain the written value in a
            // CAS-loop
            while let Err(_) = queue.limit_head.compare_exchange_weak(
                write_index, new_write_index,
                Ordering::AcqRel, Ordering::Relaxed
            ) {}

            return;
        }
    }

    fn resize(&self, shared_lock: InnerRead<T>, expected_capacity: usize) -> InnerRead<T> {
        drop(shared_lock);
        let queue = unsafe{ &*self.queue };

        {
            let mut exclusive_lock = queue.data.lock_exclusive();

            // We hold the exclusive lock, but someone else might have done the resizing, and so:
            if exclusive_lock.data.cap() == expected_capacity {
                let old_capacity = expected_capacity;
                let new_capacity = 2 * old_capacity;
                assert_correct_capacity(new_capacity);

                // Resize by a factor of two, and make the two halves identical.
                exclusive_lock.data.resize(new_capacity);
                for idx in old_capacity..new_capacity {
                    unsafe {
                        let target = exclusive_lock.data.get(idx);
                        let source = exclusive_lock.data.get(idx - old_capacity);
                        std::ptr::write(target, std::ptr::read(source));
                    }
                }

                // Modify all atomics to reflect that we just resized the
                // underlying buffer. We have that everything between the read
                // index and the write index is readable. And the following
                // preserves that property, while increasing the size from
                // `old_capacity` to `new_capacity`.
                // Note that the addition of `new_capacity` to `write_head` is
                // to ensure the ringbuffer can distinguish the cases where the
                // ringbuffer is full, and when it is empty.
                let mut read_index = queue.read_head.load(Ordering::Acquire);
                let mut write_index = queue.write_head.load(Ordering::Acquire);
                debug_assert_eq!(write_index, queue.limit_head.load(Ordering::Acquire)); // since we have exclusive access

                let is_full = read_index == write_index; // before bitwise AND-mask
                read_index &= exclusive_lock.read_mask;
                write_index &= exclusive_lock.read_mask;

                let new_capacity = new_capacity as u32;
                if read_index <= write_index && !is_full { // which means: (read index < write_index) || buffer_is_empty
                    // The readable elements do not wrap around the ringbuffer
                    write_index += new_capacity;
                } else {
                    // The readable elements do wrap around the ringbuffer
                    write_index += old_capacity as u32;
                    write_index += new_capacity;
                }

                queue.read_head.store(read_index, Ordering::Release);
                queue.limit_head.store(write_index, Ordering::Release);
                queue.write_head.store(write_index, Ordering::Release);

                // Update the masks
                exclusive_lock.read_mask = new_capacity - 1;
                exclusive_lock.compare_mask = (2 * new_capacity) - 1;
            }
        }

        // Reacquire shared lock
        return queue.data.lock_shared();
    }
}

impl<T> Drop for QueueDynProducer<T> {
    fn drop(&mut self) {
        dbg_code!(unsafe{ (*self.queue).dbg.fetch_sub(1, Ordering::AcqRel) });
    }
}

// producer end is `Send`, because in debug mode we make sure that there are no
// more producers when the queue is destroyed. But is not sync, because that
// would circumvent our atomic counter shenanigans.
unsafe impl<T> Send for QueueDynProducer<T>{}

#[inline]
fn assert_correct_capacity(capacity: usize) {
    assert!(capacity.is_power_of_two() && capacity < (u32::MAX as usize) / 2);
}

#[cfg(test)]
mod tests {
    use super::*;

    fn queue_size<T>(queue: &QueueDynMpsc<T>) -> usize {
        let lock = queue.inner.data.lock_exclusive();
        return lock.data.cap();
    }

    #[test]
    fn single_threaded_fixed_size_push_pop() {
        const INIT_SIZE: usize = 16;
        let mut cons = QueueDynMpsc::new(INIT_SIZE);
        let prod = cons.producer();

        for _round in 0..3 {
            // Fill up with indices
            for idx in 0..INIT_SIZE {
                prod.push(idx);
            }

            // Take out indices and check
            for idx in 0..INIT_SIZE {
                let gotten = cons.pop().unwrap();
                assert_eq!(idx, gotten);
            }

            assert!(cons.pop().is_none()); // nothing left in queue
            assert_eq!(queue_size(&cons), INIT_SIZE); // queue still of same size
        }
    }

    #[test]
    fn single_threaded_resizing_push_pop() {
        const INIT_SIZE: usize = 8;
        const NUM_RESIZE: usize = 3; // note: each resize increases capacity by factor of two

        let mut cons = QueueDynMpsc::new(INIT_SIZE);
        let prod = cons.producer();

        for resize_idx in 0..NUM_RESIZE {
            // Fill up with indices, one more than the size
            let cur_size = INIT_SIZE << resize_idx;
            let new_size = cur_size << 1;
            for idx in 0..new_size {
                prod.push(idx);
            }

            for idx in 0..new_size {
                let gotten = cons.pop().unwrap();
                assert_eq!(idx, gotten);
            }

            assert!(cons.pop().is_none());
            assert_eq!(queue_size(&cons), new_size);
        }

        assert_eq!(queue_size(&cons), INIT_SIZE << NUM_RESIZE);
    }

    #[test]
    fn single_threaded_alternating_push_pop() {
        const INIT_SIZE: usize = 32;
        const NUM_PROD: usize = 4;
        assert!(INIT_SIZE % NUM_PROD == 0);

        let mut cons = QueueDynMpsc::new(INIT_SIZE);
        let mut prods = Vec::with_capacity(NUM_PROD);
        for _ in 0..NUM_PROD {
            prods.push(cons.producer());
        }

        for _round_idx in 0..4 {
            // Fill up, alternating per producer
            let mut prod_idx = 0;
            for idx in 0..INIT_SIZE {
                let prod = &prods[prod_idx];
                prod_idx += 1;
                prod_idx %= NUM_PROD;
                prod.push(idx);
            }

            // Retrieve and check again
            for idx in 0..INIT_SIZE {
                let gotten = cons.pop().unwrap();
                assert_eq!(idx, gotten);
            }

            assert!(cons.pop().is_none());
            assert_eq!(queue_size(&cons), INIT_SIZE);
        }
    }

    #[test]
    fn multithreaded_production_and_consumption() {
        use std::sync::{Arc, Mutex};

        // Rather randomized test. Kind of a stress test. We let the producers
        // produce `u64` values with the high bits containing their identifier.
        // The consumer will try receive as fast as possible until each thread
        // has produced the expected number of values.
        const NUM_STRESS_TESTS: usize = 2;
        const NUM_PER_THREAD: usize = 4096;
        const NUM_PROD_THREADS: usize = 4;

        fn take_num_thread_idx(number: u64) -> u64 { return (number >> 32) & 0xFFFFFFFF; }
        fn take_num(number: u64) -> u64 { return number & 0xFFFFFFFF; }

        // Span queue and producers
        for _stress_idx in 0..NUM_STRESS_TESTS {
            let mut queue = QueueDynMpsc::new(4);
            let mut producers = Vec::with_capacity(NUM_PROD_THREADS);
            for _idx in 0..NUM_PROD_THREADS {
                producers.push(queue.producer());
            }

            // Start up consume thread and let it spin immediately. Note that it
            // must die last.
            let can_exit_lock = Arc::new(Mutex::new(false));
            let mut held_exit_lock = can_exit_lock.lock().unwrap();

            let consume_handle = {
                let can_exit_lock = can_exit_lock.clone();
                std::thread::spawn(move || {
                    let mut counters = [0u64; NUM_PROD_THREADS];
                    let mut num_done = 0;
                    while num_done != NUM_PROD_THREADS {
                        // Spin until we get something
                        let new_value = loop {
                            if let Some(value) = queue.pop() {
                                break value;
                            }
                        };

                        let thread_idx = take_num_thread_idx(new_value);
                        let counter = &mut counters[thread_idx as usize];
                        assert_eq!(*counter, take_num(new_value)); // values per thread arrive in order

                        *counter += 1;
                        if *counter == NUM_PER_THREAD as u64 {
                            // Finished this one
                            num_done += 1;
                        }
                    }

                    let _exit_guard = can_exit_lock.lock().unwrap();
                })
            };

            // Set up producer threads
            let mut handles = Vec::with_capacity(NUM_PROD_THREADS);
            for prod_idx in 0..NUM_PROD_THREADS {
                let prod_handle = producers.pop().unwrap();

                let handle = std::thread::spawn(move || {
                    let base_value = (prod_idx as u64) << 32;
                    for number in 0..NUM_PER_THREAD as u64 {
                        prod_handle.push(base_value + number);
                    }
                });

                handles.push(handle);
            }

            // Wait until all producers finished, then we unlock our held lock and
            // we wait until the consumer finishes
            for handle in handles {
                handle.join().expect("clean producer exit");
            }

            drop(held_exit_lock);
            consume_handle.join().expect("clean consumer exit");
        }
    }
}