/*
Copyright 2015 The Kubernetes Authors All rights reserved.

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 util

import (
	"math"
	"sync"
	"time"
)

type backoffEntry struct {
	backoff    time.Duration
	lastUpdate time.Time
}

type Backoff struct {
	sync.Mutex
	Clock           Clock
	defaultDuration time.Duration
	maxDuration     time.Duration
	perItemBackoff  map[string]*backoffEntry
}

func NewBackOff(initial, max time.Duration) *Backoff {
	return &Backoff{
		perItemBackoff:  map[string]*backoffEntry{},
		Clock:           RealClock{},
		defaultDuration: initial,
		maxDuration:     max,
	}
}

// Get the current backoff Duration
func (p *Backoff) Get(id string) time.Duration {
	p.Lock()
	defer p.Unlock()
	var delay time.Duration
	entry, ok := p.perItemBackoff[id]
	if ok {
		delay = entry.backoff
	}
	return delay
}

// move backoff to the next mark, capping at maxDuration
func (p *Backoff) Next(id string, eventTime time.Time) {
	p.Lock()
	defer p.Unlock()
	entry, ok := p.perItemBackoff[id]
	if !ok || hasExpired(eventTime, entry.lastUpdate, p.maxDuration) {
		entry = p.initEntryUnsafe(id)
	} else {
		delay := entry.backoff * 2 // exponential
		entry.backoff = time.Duration(math.Min(float64(delay), float64(p.maxDuration)))
	}
	entry.lastUpdate = p.Clock.Now()
}

// Returns True if the elapsed time since eventTime is smaller than the current backoff window
func (p *Backoff) IsInBackOffSince(id string, eventTime time.Time) bool {
	p.Lock()
	defer p.Unlock()
	entry, ok := p.perItemBackoff[id]
	if !ok {
		return false
	}
	if hasExpired(eventTime, entry.lastUpdate, p.maxDuration) {
		return false
	}
	return p.Clock.Now().Sub(eventTime) < entry.backoff
}

// Garbage collect records that have aged past maxDuration. Backoff users are expected
// to invoke this periodically.
func (p *Backoff) GC() {
	p.Lock()
	defer p.Unlock()
	now := p.Clock.Now()
	for id, entry := range p.perItemBackoff {
		if now.Sub(entry.lastUpdate) > p.maxDuration*2 {
			// GC when entry has not been updated for 2*maxDuration
			delete(p.perItemBackoff, id)
		}
	}
}

// Take a lock on *Backoff, before calling initEntryUnsafe
func (p *Backoff) initEntryUnsafe(id string) *backoffEntry {
	entry := &backoffEntry{backoff: p.defaultDuration}
	p.perItemBackoff[id] = entry
	return entry
}

// After 2*maxDuration we restart the backoff factor to the begining
func hasExpired(eventTime time.Time, lastUpdate time.Time, maxDuration time.Duration) bool {
	return eventTime.Sub(lastUpdate) > maxDuration*2 // consider stable if it's ok for twice the maxDuration
}