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    //! A concurrent, lock-free, FIFO list.

    use crate::loom::sync::atomic::{AtomicPtr, AtomicUsize};
    use crate::loom::thread;
    use crate::sync::mpsc::block::{self, Block};

    use std::fmt;
    use std::ptr::NonNull;
    use std::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release};

    /// List queue transmit handle
    pub(crate) struct Tx<T> {
        /// Tail in the `Block` mpmc list.
        block_tail: AtomicPtr<Block<T>>,

        /// Position to push the next message. This reference a block and offset
        /// into the block.
        tail_position: AtomicUsize,
    }

    /// List queue receive handle
    pub(crate) struct Rx<T> {
        /// Pointer to the block being processed
        head: NonNull<Block<T>>,

        /// Next slot index to process
        index: usize,

        /// Pointer to the next block pending release
        free_head: NonNull<Block<T>>,
    }

    pub(crate) fn channel<T>() -> (Tx<T>, Rx<T>) {
        // Create the initial block shared between the tx and rx halves.
        let initial_block = Box::new(Block::new(0));
        let initial_block_ptr = Box::into_raw(initial_block);

        let tx = Tx {
            block_tail: AtomicPtr::new(initial_block_ptr),
            tail_position: AtomicUsize::new(0),
        };

        let head = NonNull::new(initial_block_ptr).unwrap();

        let rx = Rx {
            head,
            index: 0,
            free_head: head,
        };

        (tx, rx)
    }

    impl<T> Tx<T> {
        /// Pushes a value into the list.
        pub(crate) fn push(&self, value: T) {
            // First, claim a slot for the value. `Acquire` is used here to
            // synchronize with the `fetch_add` in `reclaim_blocks`.
            let slot_index = self.tail_position.fetch_add(1, Acquire);

            // Load the current block and write the value
            let block = self.find_block(slot_index);

            unsafe {
                // Write the value to the block
                block.as_ref().write(slot_index, value);
            }
        }

        /// Closes the send half of the list
        ///
        /// Similar process as pushing a value, but instead of writing the value &
        /// setting the ready flag, the TX_CLOSED flag is set on the block.
        pub(crate) fn close(&self) {
            // First, claim a slot for the value. This is the last slot that will be
            // claimed.
            let slot_index = self.tail_position.fetch_add(1, Acquire);

            let block = self.find_block(slot_index);

            unsafe { block.as_ref().tx_close() }
        }

        fn find_block(&self, slot_index: usize) -> NonNull<Block<T>> {
            // The start index of the block that contains `index`.
            let start_index = block::start_index(slot_index);

            // The index offset into the block
            let offset = block::offset(slot_index);

            // Load the current head of the block
            let mut block_ptr = self.block_tail.load(Acquire);

            let block = unsafe { &*block_ptr };

            // Calculate the distance between the tail ptr and the target block
            let distance = block.distance(start_index);

            // Decide if this call to `find_block` should attempt to update the
            // `block_tail` pointer.
            //
            // Updating `block_tail` is not always performed in order to reduce
            // contention.
            //
            // When set, as the routine walks the linked list, it attempts to update
            // `block_tail`. If the update cannot be performed, `try_updating_tail`
            // is unset.
            let mut try_updating_tail = distance > offset;

            // Walk the linked list of blocks until the block with `start_index` is
            // found.
            loop {
                let block = unsafe { &(*block_ptr) };

                if block.is_at_index(start_index) {
                    return unsafe { NonNull::new_unchecked(block_ptr) };
                }

                let next_block = block
                    .load_next(Acquire)
                    // There is no allocated next block, grow the linked list.
                    .unwrap_or_else(|| block.grow());

                // If the block is **not** final, then the tail pointer cannot be
                // advanced any more.
                try_updating_tail &= block.is_final();

                if try_updating_tail {
                    // Advancing `block_tail` must happen when walking the linked
                    // list. `block_tail` may not advance passed any blocks that are
                    // not "final". At the point a block is finalized, it is unknown
                    // if there are any prior blocks that are unfinalized, which
                    // makes it impossible to advance `block_tail`.
                    //
                    // While walking the linked list, `block_tail` can be advanced
                    // as long as finalized blocks are traversed.
                    //
                    // Release ordering is used to ensure that any subsequent reads
                    // are able to see the memory pointed to by `block_tail`.
                    //
                    // Acquire is not needed as any "actual" value is not accessed.
                    // At this point, the linked list is walked to acquire blocks.
                    if self
                        .block_tail
                        .compare_exchange(block_ptr, next_block.as_ptr(), Release, Relaxed)
                        .is_ok()
                    {
                        // Synchronize with any senders
                        let tail_position = self.tail_position.fetch_add(0, Release);

                        unsafe {
                            block.tx_release(tail_position);
                        }
                    } else {
                        // A concurrent sender is also working on advancing
                        // `block_tail` and this thread is falling behind.
                        //
                        // Stop trying to advance the tail pointer
                        try_updating_tail = false;
                    }
                }

                block_ptr = next_block.as_ptr();

                thread::yield_now();
            }
        }

        pub(crate) unsafe fn reclaim_block(&self, mut block: NonNull<Block<T>>) {
            // The block has been removed from the linked list and ownership
            // is reclaimed.
            //
            // Before dropping the block, see if it can be reused by
            // inserting it back at the end of the linked list.
            //
            // First, reset the data
            block.as_mut().reclaim();

            let mut reused = false;

            // Attempt to insert the block at the end
            //
            // Walk at most three times
            //
            let curr_ptr = self.block_tail.load(Acquire);

            // The pointer can never be null
            debug_assert!(!curr_ptr.is_null());

            let mut curr = NonNull::new_unchecked(curr_ptr);

            // TODO: Unify this logic with Block::grow
            for _ in 0..3 {
                match curr.as_ref().try_push(&mut block, AcqRel, Acquire) {
                    Ok(_) => {
                        reused = true;
                        break;
                    }
                    Err(next) => {
                        curr = next;
                    }
                }
            }

            if !reused {
                let _ = Box::from_raw(block.as_ptr());
            }
        }
    }

    impl<T> fmt::Debug for Tx<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Tx")
                .field("block_tail", &self.block_tail.load(Relaxed))
                .field("tail_position", &self.tail_position.load(Relaxed))
                .finish()
        }
    }

    impl<T> Rx<T> {
        /// Pops the next value off the queue
        pub(crate) fn pop(&mut self, tx: &Tx<T>) -> Option<block::Read<T>> {
            // Advance `head`, if needed
            if !self.try_advancing_head() {
                return None;
            }

            self.reclaim_blocks(tx);

            unsafe {
                let block = self.head.as_ref();

                let ret = block.read(self.index);

                if let Some(block::Read::Value(..)) = ret {
                    self.index = self.index.wrapping_add(1);
                }

                ret
            }
        }

        /// Tries advancing the block pointer to the block referenced by `self.index`.
        ///
        /// Returns `true` if successful, `false` if there is no next block to load.
        fn try_advancing_head(&mut self) -> bool {
            let block_index = block::start_index(self.index);

            loop {
                let next_block = {
                    let block = unsafe { self.head.as_ref() };

                    if block.is_at_index(block_index) {
                        return true;
                    }

                    block.load_next(Acquire)
                };

                let next_block = match next_block {
                    Some(next_block) => next_block,
                    None => {
                        return false;
                    }
                };

                self.head = next_block;

                thread::yield_now();
            }
        }

        fn reclaim_blocks(&mut self, tx: &Tx<T>) {
            while self.free_head != self.head {
                unsafe {
                    // Get a handle to the block that will be freed and update
                    // `free_head` to point to the next block.
                    let block = self.free_head;

                    let observed_tail_position = block.as_ref().observed_tail_position();

                    let required_index = match observed_tail_position {
                        Some(i) => i,
                        None => return,
                    };

                    if required_index > self.index {
                        return;
                    }

                    // We may read the next pointer with `Relaxed` ordering as it is
                    // guaranteed that the `reclaim_blocks` routine trails the `recv`
                    // routine. Any memory accessed by `reclaim_blocks` has already
                    // been acquired by `recv`.
                    let next_block = block.as_ref().load_next(Relaxed);

                    // Update the free list head
                    self.free_head = next_block.unwrap();

                    // Push the emptied block onto the back of the queue, making it
                    // available to senders.
                    tx.reclaim_block(block);
                }

                thread::yield_now();
            }
        }

        /// Effectively `Drop` all the blocks. Should only be called once, when
        /// the list is dropping.
        pub(super) unsafe fn free_blocks(&mut self) {
            debug_assert_ne!(self.free_head, NonNull::dangling());

            let mut cur = Some(self.free_head);

            #[cfg(debug_assertions)]
            {
                // to trigger the debug assert above so as to catch that we
                // don't call `free_blocks` more than once.
                self.free_head = NonNull::dangling();
                self.head = NonNull::dangling();
            }

            while let Some(block) = cur {
                cur = block.as_ref().load_next(Relaxed);
                drop(Box::from_raw(block.as_ptr()));
            }
        }
    }

    impl<T> fmt::Debug for Rx<T> {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Rx")
                .field("head", &self.head)
                .field("index", &self.index)
                .field("free_head", &self.free_head)
                .finish()
        }
    }