// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package sync

import (
	
	
)

// poolDequeue is a lock-free fixed-size single-producer,
// multi-consumer queue. The single producer can both push and pop
// from the head, and consumers can pop from the tail.
//
// It has the added feature that it nils out unused slots to avoid
// unnecessary retention of objects. This is important for sync.Pool,
// but not typically a property considered in the literature.
type poolDequeue struct {
	// headTail packs together a 32-bit head index and a 32-bit
	// tail index. Both are indexes into vals modulo len(vals)-1.
	//
	// tail = index of oldest data in queue
	// head = index of next slot to fill
	//
	// Slots in the range [tail, head) are owned by consumers.
	// A consumer continues to own a slot outside this range until
	// it nils the slot, at which point ownership passes to the
	// producer.
	//
	// The head index is stored in the most-significant bits so
	// that we can atomically add to it and the overflow is
	// harmless.
	headTail uint64

	// vals is a ring buffer of interface{} values stored in this
	// dequeue. The size of this must be a power of 2.
	//
	// vals[i].typ is nil if the slot is empty and non-nil
	// otherwise. A slot is still in use until *both* the tail
	// index has moved beyond it and typ has been set to nil. This
	// is set to nil atomically by the consumer and read
	// atomically by the producer.
	vals []eface
}

type eface struct {
	typ, val unsafe.Pointer
}

const dequeueBits = 32

// dequeueLimit is the maximum size of a poolDequeue.
//
// This must be at most (1<<dequeueBits)/2 because detecting fullness
// depends on wrapping around the ring buffer without wrapping around
// the index. We divide by 4 so this fits in an int on 32-bit.
const dequeueLimit = (1 << dequeueBits) / 4

// dequeueNil is used in poolDequeue to represent interface{}(nil).
// Since we use nil to represent empty slots, we need a sentinel value
// to represent nil.
type dequeueNil *struct{}

func ( *poolDequeue) ( uint64) (,  uint32) {
	const  = 1<<dequeueBits - 1
	 = uint32(( >> dequeueBits) & )
	 = uint32( & )
	return
}

func ( *poolDequeue) (,  uint32) uint64 {
	const  = 1<<dequeueBits - 1
	return (uint64() << dequeueBits) |
		uint64(&)
}

// pushHead adds val at the head of the queue. It returns false if the
// queue is full. It must only be called by a single producer.
func ( *poolDequeue) ( interface{}) bool {
	 := atomic.LoadUint64(&.headTail)
	,  := .unpack()
	if (+uint32(len(.vals)))&(1<<dequeueBits-1) ==  {
		// Queue is full.
		return false
	}
	 := &.vals[&uint32(len(.vals)-1)]

	// Check if the head slot has been released by popTail.
	 := atomic.LoadPointer(&.typ)
	if  != nil {
		// Another goroutine is still cleaning up the tail, so
		// the queue is actually still full.
		return false
	}

	// The head slot is free, so we own it.
	if  == nil {
		 = dequeueNil(nil)
	}
	*(*interface{})(unsafe.Pointer()) = 

	// Increment head. This passes ownership of slot to popTail
	// and acts as a store barrier for writing the slot.
	atomic.AddUint64(&.headTail, 1<<dequeueBits)
	return true
}

// popHead removes and returns the element at the head of the queue.
// It returns false if the queue is empty. It must only be called by a
// single producer.
func ( *poolDequeue) () (interface{}, bool) {
	var  *eface
	for {
		 := atomic.LoadUint64(&.headTail)
		,  := .unpack()
		if  ==  {
			// Queue is empty.
			return nil, false
		}

		// Confirm tail and decrement head. We do this before
		// reading the value to take back ownership of this
		// slot.
		--
		 := .pack(, )
		if atomic.CompareAndSwapUint64(&.headTail, , ) {
			// We successfully took back slot.
			 = &.vals[&uint32(len(.vals)-1)]
			break
		}
	}

	 := *(*interface{})(unsafe.Pointer())
	if  == dequeueNil(nil) {
		 = nil
	}
	// Zero the slot. Unlike popTail, this isn't racing with
	// pushHead, so we don't need to be careful here.
	* = eface{}
	return , true
}

// popTail removes and returns the element at the tail of the queue.
// It returns false if the queue is empty. It may be called by any
// number of consumers.
func ( *poolDequeue) () (interface{}, bool) {
	var  *eface
	for {
		 := atomic.LoadUint64(&.headTail)
		,  := .unpack()
		if  ==  {
			// Queue is empty.
			return nil, false
		}

		// Confirm head and tail (for our speculative check
		// above) and increment tail. If this succeeds, then
		// we own the slot at tail.
		 := .pack(, +1)
		if atomic.CompareAndSwapUint64(&.headTail, , ) {
			// Success.
			 = &.vals[&uint32(len(.vals)-1)]
			break
		}
	}

	// We now own slot.
	 := *(*interface{})(unsafe.Pointer())
	if  == dequeueNil(nil) {
		 = nil
	}

	// Tell pushHead that we're done with this slot. Zeroing the
	// slot is also important so we don't leave behind references
	// that could keep this object live longer than necessary.
	//
	// We write to val first and then publish that we're done with
	// this slot by atomically writing to typ.
	.val = nil
	atomic.StorePointer(&.typ, nil)
	// At this point pushHead owns the slot.

	return , true
}

// poolChain is a dynamically-sized version of poolDequeue.
//
// This is implemented as a doubly-linked list queue of poolDequeues
// where each dequeue is double the size of the previous one. Once a
// dequeue fills up, this allocates a new one and only ever pushes to
// the latest dequeue. Pops happen from the other end of the list and
// once a dequeue is exhausted, it gets removed from the list.
type poolChain struct {
	// head is the poolDequeue to push to. This is only accessed
	// by the producer, so doesn't need to be synchronized.
	head *poolChainElt

	// tail is the poolDequeue to popTail from. This is accessed
	// by consumers, so reads and writes must be atomic.
	tail *poolChainElt
}

type poolChainElt struct {
	poolDequeue

	// next and prev link to the adjacent poolChainElts in this
	// poolChain.
	//
	// next is written atomically by the producer and read
	// atomically by the consumer. It only transitions from nil to
	// non-nil.
	//
	// prev is written atomically by the consumer and read
	// atomically by the producer. It only transitions from
	// non-nil to nil.
	next, prev *poolChainElt
}

func storePoolChainElt( **poolChainElt,  *poolChainElt) {
	atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer()), unsafe.Pointer())
}

func loadPoolChainElt( **poolChainElt) *poolChainElt {
	return (*poolChainElt)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer())))
}

func ( *poolChain) ( interface{}) {
	 := .head
	if  == nil {
		// Initialize the chain.
		const  = 8 // Must be a power of 2
		 = new(poolChainElt)
		.vals = make([]eface, )
		.head = 
		storePoolChainElt(&.tail, )
	}

	if .pushHead() {
		return
	}

	// The current dequeue is full. Allocate a new one of twice
	// the size.
	 := len(.vals) * 2
	if  >= dequeueLimit {
		// Can't make it any bigger.
		 = dequeueLimit
	}

	 := &poolChainElt{prev: }
	.vals = make([]eface, )
	.head = 
	storePoolChainElt(&.next, )
	.pushHead()
}

func ( *poolChain) () (interface{}, bool) {
	 := .head
	for  != nil {
		if ,  := .popHead();  {
			return , 
		}
		// There may still be unconsumed elements in the
		// previous dequeue, so try backing up.
		 = loadPoolChainElt(&.prev)
	}
	return nil, false
}

func ( *poolChain) () (interface{}, bool) {
	 := loadPoolChainElt(&.tail)
	if  == nil {
		return nil, false
	}

	for {
		// It's important that we load the next pointer
		// *before* popping the tail. In general, d may be
		// transiently empty, but if next is non-nil before
		// the pop and the pop fails, then d is permanently
		// empty, which is the only condition under which it's
		// safe to drop d from the chain.
		 := loadPoolChainElt(&.next)

		if ,  := .popTail();  {
			return , 
		}

		if  == nil {
			// This is the only dequeue. It's empty right
			// now, but could be pushed to in the future.
			return nil, false
		}

		// The tail of the chain has been drained, so move on
		// to the next dequeue. Try to drop it from the chain
		// so the next pop doesn't have to look at the empty
		// dequeue again.
		if atomic.CompareAndSwapPointer((*unsafe.Pointer)(unsafe.Pointer(&.tail)), unsafe.Pointer(), unsafe.Pointer()) {
			// We won the race. Clear the prev pointer so
			// the garbage collector can collect the empty
			// dequeue and so popHead doesn't back up
			// further than necessary.
			storePoolChainElt(&.prev, nil)
		}
		 = 
	}
}