package metrics

import (
	"sync"
	"time"
)

// minuteCounter counts events in each clock minute and reports the count of
// the last full minute. It is safe for concurrent use.
type minuteCounter struct {
	mu  sync.Mutex
	now func() time.Time

	// minute is the start of the minute that count belongs to.
	minute time.Time
	count  int
	// lastMinuteCount is the count of the minute just before minute.
	lastMinuteCount int
}

// add counts one event in the current minute.
func (c *minuteCounter) add() {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.moveToCurrentMinute()
	c.count++
}

// lastFullMinute returns the number of events in the minute that ended most
// recently.
func (c *minuteCounter) lastFullMinute() int {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.moveToCurrentMinute()

	return c.lastMinuteCount
}

// moveToCurrentMinute starts a new count when the clock has moved into
// another minute. The count it ends becomes the last full minute's count when
// its minute is the one just before the current minute. Otherwise the last
// full minute had no events. The caller holds mu.
func (c *minuteCounter) moveToCurrentMinute() {
	minute := c.now().Truncate(time.Minute)
	if minute.Equal(c.minute) {
		return
	}

	if minute.Sub(c.minute) == time.Minute {
		c.lastMinuteCount = c.count
	} else {
		c.lastMinuteCount = 0
	}

	c.minute = minute
	c.count = 0
}