KDOC 663: リングキューは満杯になるといちばん古い要素を上書きする

https://github.com/kijimad/opentelemetry-go/blob/f013f0ec436a436795beaa57a181ebecd34aad3d/sdk/log/batch.go#L331-L335

// queue holds a queue of logging records.
//
// When the queue becomes full, the oldest records in the queue are
// overwritten.
type queue struct {

// 容量 n の輪の上で、read(いちばん古い要素)と write(次に書く場所)の2つの位置を進める
type queue struct {
	head        *ring.Ring // 表示のための起点。動かさない
	read, write *ring.Ring
	cap, len    int
	dropped     int
}

func newQueue(n int) *queue {
	r := ring.New(n)
	return &queue{head: r, read: r, write: r, cap: n}
}

func (q *queue) Enqueue(v int) {
	q.write.Value = v
	q.write = q.write.Next()
	q.len++
	// 満杯なら、いちばん古い要素を上書きし、read も1つ進める
	if q.len > q.cap {
		q.len = q.cap
		q.read = q.read.Next()
		q.dropped++
	}
}

func (q *queue) Dequeue() (int, bool) {
	if q.len == 0 {
		return 0, false
	}
	v := q.read.Value.(int)
	q.read.Value = nil
	q.read = q.read.Next()
	q.len--
	return v, true
}

func (q *queue) String() string {
	slots := []string{}
	var read, write int
	p := q.head
	for i := 0; i < q.cap; i++ {
		if p == q.read {
			read = i
		}
		if p == q.write {
			write = i
		}
		if p.Value == nil {
			slots = append(slots, "_")
		} else {
			slots = append(slots, fmt.Sprint(p.Value))
		}
		p = p.Next()
	}
	return fmt.Sprintf("%v read=%d write=%d len=%d dropped=%d", slots, read, write, q.len, q.dropped)
}

func main() {
	q := newQueue(3)
	fmt.Println("初期         ", q)
	for i := 1; i <= 5; i++ {
		q.Enqueue(i)
		fmt.Printf("Enqueue(%d)    %v\n", i, q)
	}
	for range 2 {
		v, _ := q.Dequeue()
		fmt.Printf("Dequeue() = %d %v\n", v, q)
	}
	for i := 1; i <= 2; i++ {
		q.Enqueue(i)
		fmt.Printf("Enqueue(%d)    %v\n", i, q)
	}
}

初期          [_ _ _] read=0 write=0 len=0 dropped=0
Enqueue(1)    [1 _ _] read=0 write=1 len=1 dropped=0
Enqueue(2)    [1 2 _] read=0 write=2 len=2 dropped=0
Enqueue(3)    [1 2 3] read=0 write=0 len=3 dropped=0
Enqueue(4)    [4 2 3] read=1 write=1 len=3 dropped=1
Enqueue(5)    [4 5 3] read=2 write=2 len=3 dropped=2
Dequeue() = 3 [4 5 _] read=0 write=2 len=2 dropped=2
Dequeue() = 4 [_ 5 _] read=1 write=2 len=1 dropped=2
Enqueue(1)    [_ 5 1] read=1 write=0 len=2 dropped=2
Enqueue(2)    [2 5 1] read=1 write=1 len=3 dropped=2