package monitor

import (
	"sync"
	"time"
)

// SpeedTracker calculates transfer rates by diffing byte counters.
type SpeedTracker struct {
	mu       sync.Mutex
	lastIn   int64
	lastOut  int64
	lastTime time.Time
	downRate float64
	upRate   float64
}

// NewSpeedTracker creates a new speed tracker.
func NewSpeedTracker() *SpeedTracker {
	return &SpeedTracker{}
}

// Update records new byte counters and calculates rates.
func (s *SpeedTracker) Update(inBytes, outBytes int64) {
	s.mu.Lock()
	defer s.mu.Unlock()

	now := time.Now()
	if !s.lastTime.IsZero() {
		dt := now.Sub(s.lastTime).Seconds()
		if dt > 0 {
			s.downRate = float64(inBytes-s.lastIn) / dt
			s.upRate = float64(outBytes-s.lastOut) / dt
			if s.downRate < 0 {
				s.downRate = 0
			}
			if s.upRate < 0 {
				s.upRate = 0
			}
		}
	}
	s.lastIn = inBytes
	s.lastOut = outBytes
	s.lastTime = now
}

// Rates returns the current download and upload rates in bytes/sec.
func (s *SpeedTracker) Rates() (down, up float64) {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.downRate, s.upRate
}