// Package bps is a simple tool for keeping track of the rate of bytes
// transmitted
package bps

import (
	"fmt"
	"io/ioutil"
	"sync"
	"time"
)

const (
	minResolution = 3
)

// BPS keeps track of state for byte counts
//
// Instantiate a BPS then feed bytes either via BPS.Add, or writing to it. When
// the accumulated values are needed call BPS.Cur. When the BPS is no loner
// needed call BPS.Close
type BPS struct {
	sync.RWMutex

	quit   chan interface{}
	closed chan interface{}

	interval time.Duration
	dt       time.Duration

	// curBs bytes read for this dt
	curBs int64

	// buckets contains an entry for bytes read for each dt of time in interval
	buckets []int64

	// timeI keys into buckets for the current point in time
	timeI int
}

// New Returns a populated and ready to use BPS. interval is the amount of time
// (for example 60 seconds) over which to track byte flow (bytes/second for the
// last interval), and resolution is used in the following calculation:
//
// dt = interval / resolution (s)
//
// where the dt is the temporal resolution of the updates (add or remove
// information every dt).
func New(interval time.Duration, resolution uint) (*BPS, error) {
	if resolution < minResolution {
		return nil, fmt.Errorf("resolution must be larger than %d", minResolution)
	}

	dtns := interval.Nanoseconds() / int64(resolution)
	dt := time.Duration(dtns) * time.Nanosecond
	r := &BPS{
		interval: interval,
		dt:       dt,
		quit:     make(chan interface{}),
		closed:   make(chan interface{}),
		buckets:  make([]int64, resolution),
	}
	go r.run()
	return r, nil
}

func (b *BPS) runLoop() {
	t := time.NewTicker(b.dt)
	for {
		select {
		case <-t.C:
			b.Lock()
			b.buckets[b.timeI] = b.curBs
			b.curBs = 0
			b.timeI = (b.timeI + 1) % int(len(b.buckets))
			b.Unlock()
		case <-b.quit:
			return
		}
	}
}

// Run is a method of BPS that must be started in a goroutine in order
// for things to be functional.
func (b *BPS) run() {
	b.runLoop()
	close(b.closed)
}

// Write implements io.Writer so that one can simply write bytes to the struct.
func (b *BPS) Write(p []byte) (int, error) {
	n, err := ioutil.Discard.Write(p)
	b.Add(int64(n))
	return n, err
}

// Add adds i to the current time bucket.
func (b *BPS) Add(i int64) {
	b.Lock()
	b.curBs += i
	b.buckets[b.timeI] = b.curBs
	b.Unlock()
}

// Rate returns the current rate (bytes / second).
func (b *BPS) Rate() float64 {
	b.Lock()
	var total int64 = 0
	for _, b := range b.buckets {
		total += b
	}
	b.Unlock()
	return float64(total) / b.interval.Seconds()
}

// HumanRate returns a human-friendly (e.g. 23.3MB/s) rate.
func (b *BPS) HumanRate() string {
	return Bytes(uint64(b.Rate())) + "/s"
}

// Close cleans up and shuts down a BPS.
func (b *BPS) Close() {
	close(b.quit)
	<-b.closed
}