writer.go

package disruptor

import "runtime"

type Writer struct {
	written  *Cursor
	upstream Barrier
	capacity int64
	previous int64
	gate     int64
}

func NewWriter(written *Cursor, upstream Barrier, capacity int64) *Writer {
	assertPowerOfTwo(capacity)

	return &Writer{
		upstream: upstream,
		written:  written,
		capacity: capacity,
		previous: InitialSequenceValue,
		gate:     InitialSequenceValue,
	}
}

func assertPowerOfTwo(value int64) {
	if value > 0 && (value&(value-1)) != 0 {
		// Wikipedia entry: http://bit.ly/1krhaSB
		panic("The ring capacity must be a power of two, e.g. 2, 4, 8, 16, 32, 64, etc.")
	}
}

func (this *Writer) Reserve(count int64) int64 {
	this.previous += count

	for spin := int64(0); this.previous-this.capacity > this.gate; spin++ {
		if spin&SpinMask == 0 {
			runtime.Gosched() // LockSupport.parkNanos(1L); http://bit.ly/1xiDINZ
		}

		this.gate = this.upstream.Read(0)
	}

	return this.previous
}

func (this *Writer) Await(next int64) {
	for next-this.capacity > this.gate {
		this.gate = this.upstream.Read(0)
	}
}

const SpinMask = 1024*16 - 1 // arbitrary; we'll want to experiment with different values