/*
   Copyright 2014 CoreOS, Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package stats

import (
	"sync"
	"time"
)

const (
	queueCapacity = 200
)

// RequestStats represent the stats for a request.
// It encapsulates the sending time and the size of the request.
type RequestStats struct {
	SendingTime time.Time
	Size        int
}

type statsQueue struct {
	items        [queueCapacity]*RequestStats
	size         int
	front        int
	back         int
	totalReqSize int
	rwl          sync.RWMutex
}

func (q *statsQueue) Len() int {
	return q.size
}

func (q *statsQueue) ReqSize() int {
	return q.totalReqSize
}

// FrontAndBack gets the front and back elements in the queue
// We must grab front and back together with the protection of the lock
func (q *statsQueue) frontAndBack() (*RequestStats, *RequestStats) {
	q.rwl.RLock()
	defer q.rwl.RUnlock()
	if q.size != 0 {
		return q.items[q.front], q.items[q.back]
	}
	return nil, nil
}

// Insert function insert a RequestStats into the queue and update the records
func (q *statsQueue) Insert(p *RequestStats) {
	q.rwl.Lock()
	defer q.rwl.Unlock()

	q.back = (q.back + 1) % queueCapacity

	if q.size == queueCapacity { //dequeue
		q.totalReqSize -= q.items[q.front].Size
		q.front = (q.back + 1) % queueCapacity
	} else {
		q.size++
	}

	q.items[q.back] = p
	q.totalReqSize += q.items[q.back].Size

}

// Rate function returns the package rate and byte rate
func (q *statsQueue) Rate() (float64, float64) {
	front, back := q.frontAndBack()

	if front == nil || back == nil {
		return 0, 0
	}

	if time.Now().Sub(back.SendingTime) > time.Second {
		q.Clear()
		return 0, 0
	}

	sampleDuration := back.SendingTime.Sub(front.SendingTime)

	pr := float64(q.Len()) / float64(sampleDuration) * float64(time.Second)

	br := float64(q.ReqSize()) / float64(sampleDuration) * float64(time.Second)

	return pr, br
}

// Clear function clear up the statsQueue
func (q *statsQueue) Clear() {
	q.rwl.Lock()
	defer q.rwl.Unlock()
	q.back = -1
	q.front = 0
	q.size = 0
	q.totalReqSize = 0
}