/*
Copyright 2015 The Kubernetes Authors.

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 scheduler

import (
	"container/heap"
	"sync"
	"time"

	"k8s.io/apimachinery/pkg/util/sets"
	"k8s.io/client-go/util/flowcontrol"

	"k8s.io/klog"
)

const (
	// NodeHealthUpdateRetry controls the number of retries of writing
	// node health update.
	NodeHealthUpdateRetry = 5
	// NodeEvictionPeriod controls how often NodeController will try to
	// evict Pods from non-responsive Nodes.
	NodeEvictionPeriod = 100 * time.Millisecond
	// EvictionRateLimiterBurst is the burst value for all eviction rate
	// limiters
	EvictionRateLimiterBurst = 1
)

// TimedValue is a value that should be processed at a designated time.
type TimedValue struct {
	Value string
	// UID could be anything that helps identify the value
	UID       interface{}
	AddedAt   time.Time
	ProcessAt time.Time
}

// now is used to test time
var now = time.Now

// TimedQueue is a priority heap where the lowest ProcessAt is at the front of the queue
type TimedQueue []*TimedValue

// Len is the length of the queue.
func (h TimedQueue) Len() int { return len(h) }

// Less returns true if queue[i] < queue[j].
func (h TimedQueue) Less(i, j int) bool { return h[i].ProcessAt.Before(h[j].ProcessAt) }

// Swap swaps index i and j.
func (h TimedQueue) Swap(i, j int) { h[i], h[j] = h[j], h[i] }

// Push a new TimedValue on to the queue.
func (h *TimedQueue) Push(x interface{}) {
	*h = append(*h, x.(*TimedValue))
}

// Pop the lowest ProcessAt item.
func (h *TimedQueue) Pop() interface{} {
	old := *h
	n := len(old)
	x := old[n-1]
	*h = old[0 : n-1]
	return x
}

// UniqueQueue is a FIFO queue which additionally guarantees that any
// element can be added only once until it is removed.
type UniqueQueue struct {
	lock  sync.Mutex
	queue TimedQueue
	set   sets.String
}

// Add a new value to the queue if it wasn't added before, or was
// explicitly removed by the Remove call. Returns true if new value
// was added.
func (q *UniqueQueue) Add(value TimedValue) bool {
	q.lock.Lock()
	defer q.lock.Unlock()

	if q.set.Has(value.Value) {
		return false
	}
	heap.Push(&q.queue, &value)
	q.set.Insert(value.Value)
	return true
}

// Replace replaces an existing value in the queue if it already
// exists, otherwise it does nothing. Returns true if the item was
// found.
func (q *UniqueQueue) Replace(value TimedValue) bool {
	q.lock.Lock()
	defer q.lock.Unlock()

	for i := range q.queue {
		if q.queue[i].Value != value.Value {
			continue
		}
		heap.Remove(&q.queue, i)
		heap.Push(&q.queue, &value)
		return true
	}
	return false
}

// RemoveFromQueue the value from the queue, but keeps it in the set,
// so it won't be added second time. Returns true if something was
// removed.
func (q *UniqueQueue) RemoveFromQueue(value string) bool {
	q.lock.Lock()
	defer q.lock.Unlock()

	if !q.set.Has(value) {
		return false
	}
	for i, val := range q.queue {
		if val.Value == value {
			heap.Remove(&q.queue, i)
			return true
		}
	}
	return false
}

// Remove the value from the queue, so Get() call won't return it, and
// allow subsequent addition of the given value. If the value is not
// present does nothing and returns false.
func (q *UniqueQueue) Remove(value string) bool {
	q.lock.Lock()
	defer q.lock.Unlock()

	if !q.set.Has(value) {
		return false
	}
	q.set.Delete(value)
	for i, val := range q.queue {
		if val.Value == value {
			heap.Remove(&q.queue, i)
			return true
		}
	}
	return true
}

// Get returns the oldest added value that wasn't returned yet.
func (q *UniqueQueue) Get() (TimedValue, bool) {
	q.lock.Lock()
	defer q.lock.Unlock()
	if len(q.queue) == 0 {
		return TimedValue{}, false
	}
	result := heap.Pop(&q.queue).(*TimedValue)
	q.set.Delete(result.Value)
	return *result, true
}

// Head returns the oldest added value that wasn't returned yet
// without removing it.
func (q *UniqueQueue) Head() (TimedValue, bool) {
	q.lock.Lock()
	defer q.lock.Unlock()
	if len(q.queue) == 0 {
		return TimedValue{}, false
	}
	result := q.queue[0]
	return *result, true
}

// Clear removes all items from the queue and duplication preventing
// set.
func (q *UniqueQueue) Clear() {
	q.lock.Lock()
	defer q.lock.Unlock()
	if q.queue.Len() > 0 {
		q.queue = make(TimedQueue, 0)
	}
	if len(q.set) > 0 {
		q.set = sets.NewString()
	}
}

// RateLimitedTimedQueue is a unique item priority queue ordered by
// the expected next time of execution. It is also rate limited.
type RateLimitedTimedQueue struct {
	queue       UniqueQueue
	limiterLock sync.Mutex
	limiter     flowcontrol.RateLimiter
}

// NewRateLimitedTimedQueue creates new queue which will use given
// RateLimiter to oversee execution.
func NewRateLimitedTimedQueue(limiter flowcontrol.RateLimiter) *RateLimitedTimedQueue {
	return &RateLimitedTimedQueue{
		queue: UniqueQueue{
			queue: TimedQueue{},
			set:   sets.NewString(),
		},
		limiter: limiter,
	}
}

// ActionFunc takes a timed value and returns false if the item must
// be retried, with an optional time.Duration if some minimum wait
// interval should be used.
type ActionFunc func(TimedValue) (bool, time.Duration)

// Try processes the queue.Ends prematurely if RateLimiter forbids an
// action and leak is true. Otherwise, requeues the item to be
// processed. Each value is processed once if fn returns true,
// otherwise it is added back to the queue. The returned remaining is
// used to identify the minimum time to execute the next item in the
// queue. The same value is processed only once unless Remove is
// explicitly called on it (it's done by the cancelPodEviction
// function in NodeController when Node becomes Ready again) TODO:
// figure out a good way to do garbage collection for all Nodes that
// were removed from the cluster.
func (q *RateLimitedTimedQueue) Try(fn ActionFunc) {
	val, ok := q.queue.Head()
	q.limiterLock.Lock()
	defer q.limiterLock.Unlock()
	for ok {
		// rate limit the queue checking
		if !q.limiter.TryAccept() {
			klog.V(10).Infof("Try rate limited for value: %v", val)
			// Try again later
			break
		}

		now := now()
		if now.Before(val.ProcessAt) {
			break
		}

		if ok, wait := fn(val); !ok {
			val.ProcessAt = now.Add(wait + 1)
			q.queue.Replace(val)
		} else {
			q.queue.RemoveFromQueue(val.Value)
		}
		val, ok = q.queue.Head()
	}
}

// Add value to the queue to be processed. Won't add the same
// value(comparison by value) a second time if it was already added
// and not removed.
func (q *RateLimitedTimedQueue) Add(value string, uid interface{}) bool {
	now := now()
	return q.queue.Add(TimedValue{
		Value:     value,
		UID:       uid,
		AddedAt:   now,
		ProcessAt: now,
	})
}

// Remove Node from the Evictor. The Node won't be processed until
// added again.
func (q *RateLimitedTimedQueue) Remove(value string) bool {
	return q.queue.Remove(value)
}

// Clear removes all items from the queue
func (q *RateLimitedTimedQueue) Clear() {
	q.queue.Clear()
}

// SwapLimiter safely swaps current limiter for this queue with the
// passed one if capacities or qps's differ.
func (q *RateLimitedTimedQueue) SwapLimiter(newQPS float32) {
	q.limiterLock.Lock()
	defer q.limiterLock.Unlock()
	if q.limiter.QPS() == newQPS {
		return
	}
	var newLimiter flowcontrol.RateLimiter
	if newQPS <= 0 {
		newLimiter = flowcontrol.NewFakeNeverRateLimiter()
	} else {
		newLimiter = flowcontrol.NewTokenBucketRateLimiter(newQPS, EvictionRateLimiterBurst)

		// If we're currently waiting on limiter, we drain the new one - this is a good approach when Burst value is 1
		// TODO: figure out if we need to support higher Burst values and decide on the drain logic, should we keep:
		// - saturation (percentage of used tokens)
		// - number of used tokens
		// - number of available tokens
		// - something else
		if q.limiter.TryAccept() == false {
			newLimiter.TryAccept()
		}
	}
	q.limiter.Stop()
	q.limiter = newLimiter
}
