counter.go

package metric

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

// Counter counts monotonically increasing values
type Counter interface {
	// Inc increments the counter by 1. Use Add to increment it by arbitrary
	// non-negative values.
	Inc()
	// Add adds the given value to the counter. It panics if the value is <
	// 0.
	Add(float64)
	// Value gets the current value of the counter.
	Value() float64
}

// NewCounter creates a new counter
func NewCounter() Counter {
	result := &counter{}
	return result
}

type counter struct {
	// valBits contains the bits of the represented float64 value, while
	// valInt stores values that are exact integers. Both have to go first
	// in the struct to guarantee alignment for atomic operations.
	// http://golang.org/pkg/sync/atomic/#pkg-note-BUG
	valBits uint64
	valInt  uint64
}

func (c *counter) Add(v float64) {
	if v < 0 {
		panic(errors.New("counter cannot decrease in value"))
	}

	ival := uint64(v)
	if float64(ival) == v {
		atomic.AddUint64(&c.valInt, ival)
		return
	}

	for {
		oldBits := atomic.LoadUint64(&c.valBits)
		newBits := math.Float64bits(math.Float64frombits(oldBits) + v)
		if atomic.CompareAndSwapUint64(&c.valBits, oldBits, newBits) {
			return
		}
	}
}

func (c *counter) Inc() {
	atomic.AddUint64(&c.valInt, 1)
}

func (c *counter) Value() float64 {
	fval := math.Float64frombits(atomic.LoadUint64(&c.valBits))
	ival := atomic.LoadUint64(&c.valInt)
	return fval + float64(ival)
}