ring.go

package storage

import (
	"errors"
	"runtime"
	"sync/atomic"
)

var (
	// ErrDisposed is returned when an operation is performed on a disposed
	// queue.
	ErrDisposed = errors.New(`queue: disposed`)

	// ErrTimeout is returned when an applicable queue operation times out.
	ErrTimeout = errors.New(`queue: poll timed out`)

	// ErrEmptyQueue is returned when an non-applicable queue operation was called
	// due to the queue's empty item state
	ErrEmptyQueue = errors.New(`queue: empty queue`)
)

// roundUp takes a uint64 greater than 0 and rounds it up to the next
// power of 2.
func roundUp(v uint64) uint64 {
	v--
	v |= v >> 1
	v |= v >> 2
	v |= v >> 4
	v |= v >> 8
	v |= v >> 16
	v |= v >> 32
	v++
	return v
}

type node struct {
	position uint64
	data     interface{}
}

// RingBuffer is a MPMC buffer that achieves threadsafety with CAS operations
// only.  A put on full or get on empty call will block until an item
// is put or retrieved.  Calling Dispose on the RingBuffer will unblock
// any blocked threads with an error.  This buffer is similar to the buffer
// described here: http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
// with some minor additions.
type RingBuffer struct {
	_padding0      [8]uint64
	queue          uint64
	_padding1      [8]uint64
	dequeue        uint64
	_padding2      [8]uint64
	mask, disposed uint64
	_padding3      [8]uint64
	nodes          []*node
}

func (rb *RingBuffer) init(size uint64) {
	size = roundUp(size)
	rb.nodes = make([]*node, size)
	for i := uint64(0); i < size; i++ {
		rb.nodes[i] = &node{position: i}
	}
	rb.mask = size - 1 // so we don't have to do this with every put/get operation
}

// Put adds the provided item to the queue.  If the queue is full, this
// call will block until an item is added to the queue or Dispose is called
// on the queue.  An error will be returned if the queue is disposed.
func (rb *RingBuffer) Put(item interface{}) error {
	_, err := rb.put(item, false)
	return err
}

// Offer adds the provided item to the queue if there is space.  If the queue
// is full, this call will return false.  An error will be returned if the
// queue is disposed.
func (rb *RingBuffer) Offer(item interface{}) (bool, error) {
	return rb.put(item, true)
}

func (rb *RingBuffer) put(item interface{}, offer bool) (bool, error) {
	var n *node
	pos := atomic.LoadUint64(&rb.queue)
L:
	for {
		if atomic.LoadUint64(&rb.disposed) == 1 {
			return false, ErrDisposed
		}

		n = rb.nodes[pos&rb.mask]
		seq := atomic.LoadUint64(&n.position)
		switch dif := seq - pos; {
		case dif == 0:
			if atomic.CompareAndSwapUint64(&rb.queue, pos, pos+1) {
				break L
			}
		case dif < 0:
			panic(`Ring buffer in a compromised state during a put operation.`)
		default:
			pos = atomic.LoadUint64(&rb.queue)
		}

		if offer {
			return false, nil
		}
		runtime.Gosched() // free up the cpu before the next iteration
	}

	n.data = item
	atomic.StoreUint64(&n.position, pos+1)
	return true, nil
}

// Poll will return the next item in the queue.  This call will block
// if the queue is empty.  This call will unblock when an item is added
// to the queue, Dispose is called on the queue, or the timeout is reached. An
// error will be returned if the queue is disposed or a timeout occurs. A
// non-positive timeout will block indefinitely.
func (rb *RingBuffer) Poll(block bool) (interface{}, error) {
	var (
		n   *node
		pos = atomic.LoadUint64(&rb.dequeue)
	)
L:
	for {
		if atomic.LoadUint64(&rb.disposed) == 1 {
			return nil, ErrDisposed
		}

		n = rb.nodes[pos&rb.mask]
		seq := atomic.LoadUint64(&n.position)
		switch dif := seq - (pos + 1); {
		case dif == 0:
			if atomic.CompareAndSwapUint64(&rb.dequeue, pos, pos+1) {
				break L
			}
		case dif < 0:
			panic(`Ring buffer in compromised state during a get operation.`)
		default:
			pos = atomic.LoadUint64(&rb.dequeue)
		}

		if !block {
			return nil, nil
		}
		runtime.Gosched() // free up the cpu before the next iteration
	}
	data := n.data
	n.data = nil
	atomic.StoreUint64(&n.position, pos+rb.mask+1)
	return data, nil
}

// Len returns the number of items in the queue.
func (rb *RingBuffer) Len() uint64 {
	return atomic.LoadUint64(&rb.queue) - atomic.LoadUint64(&rb.dequeue)
}

// Cap returns the capacity of this ring buffer.
func (rb *RingBuffer) Cap() uint64 {
	return uint64(len(rb.nodes))
}

// Dispose will dispose of this queue and free any blocked threads
// in the Put and/or Get methods.  Calling those methods on a disposed
// queue will return an error.
func (rb *RingBuffer) Dispose() {
	atomic.CompareAndSwapUint64(&rb.disposed, 0, 1)
}

// IsDisposed will return a bool indicating if this queue has been
// disposed.
func (rb *RingBuffer) IsDisposed() bool {
	return atomic.LoadUint64(&rb.disposed) == 1
}

// NewRingBuffer will allocate, initialize, and return a ring buffer
// with the specified size.
func NewRingBuffer(size uint64) *RingBuffer {
	rb := &RingBuffer{}
	rb.init(size)
	return rb
}