}

// headTailIndex represents a combined 32-bit head and 32-bit tail
// of a queue into a single 64-bit value.
type headTailIndex uint64

// makeHeadTailIndex creates a headTailIndex value from a separate
// head and tail.
func makeHeadTailIndex(head, tail uint32) headTailIndex {
	return headTailIndex(uint64(head)<<32 | uint64(tail))
}

// head returns the head of a headTailIndex value.