package stats

import (
	"runtime"
	"time"
)

// CpuStats calculates cpu usage percentage
type CpuStats struct {
	prevCpuTime float64
	prevTime    time.Time

	totalCpus int
}

// NewCpuStats returns a cpu stats calculator
func NewCpuStats() *CpuStats {
	numCpus := runtime.NumCPU()
	return &CpuStats{totalCpus: numCpus}
}

// Percent calculates the cpu usage percentage based on the current cpu usage
// and the previous cpu usage where usage is given as time in nanoseconds spend
// in the cpu
func (c *CpuStats) Percent(cpuTime float64) float64 {
	now := time.Now()

	if c.prevCpuTime == 0.0 {
		// invoked first time
		c.prevCpuTime = cpuTime
		c.prevTime = now
		return 0.0
	}

	timeDelta := now.Sub(c.prevTime).Nanoseconds()
	ret := c.calculatePercent(c.prevCpuTime, cpuTime, timeDelta)
	c.prevCpuTime = cpuTime
	c.prevTime = now
	return ret

}

func (c *CpuStats) calculatePercent(t1, t2 float64, timeDelta int64) float64 {
	vDelta := t2 - t1
	if timeDelta <= 0 || vDelta <= 0.0 {
		return 0.0
	}

	overall_percent := (vDelta / float64(timeDelta)) * 100.0
	return overall_percent
}