// Copyright 2019 Yunion
//
// 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 cache

import (
	"sync"
	"time"

	"yunion.io/x/log"
	"yunion.io/x/pkg/util/clock"
)

// ExpirationCache implements the store interface
//	1. All entries are automatically time stamped on insert
//		a. The key is computed based off the original item/keyFunc
//		b. The value inserted under that key is the timestamped item
//	2. Expiration happens lazily on read based on the expiration policy
//      a. No item can be inserted into the store while we're expiring
//		   *any* item in the cache.
//	3. Time-stamps are stripped off unexpired entries before return
// Note that the ExpirationCache is inherently slower than a normal
// threadSafeStore because it takes a write lock every time it checks if
// an item has expired.
type ExpirationCache struct {
	cacheStorage     ThreadSafeStore
	keyFunc          KeyFunc
	clock            clock.Clock
	expirationPolicy ExpirationPolicy
	// expirationLock is a write lock used to guarantee that we don't clobber
	// newly inserted objects because of a stale expiration timestamp comparison
	expirationLock sync.Mutex
}

// ExpirationPolicy dictates when an object expires. Currently only abstracted out
// so unittests don't rely on the system clock.
type ExpirationPolicy interface {
	IsExpired(obj *timestampedEntry) bool
}

// TTLPolicy implements a ttl based ExpirationPolicy.
type TTLPolicy struct {
	//	 >0: Expire entries with an age > ttl
	//	<=0: Don't expire any entry
	Ttl time.Duration

	// Clock used to calculate ttl expiration
	Clock clock.Clock
}

// IsExpired returns true if the given object is older than the ttl, or it can't
// determine its age.
func (p *TTLPolicy) IsExpired(obj *timestampedEntry) bool {
	return p.Ttl > 0 && p.Clock.Since(obj.timestamp) > p.Ttl
}

// timestampedEntry is the only type allowed in a ExpirationCache.
type timestampedEntry struct {
	obj       interface{}
	timestamp time.Time
}

// getTimestampedEntry returns the timestampedEntry stored under the given key.
func (c *ExpirationCache) getTimestampedEntry(key string) (*timestampedEntry, bool) {
	item, _ := c.cacheStorage.Get(key)
	if tsEntry, ok := item.(*timestampedEntry); ok {
		return tsEntry, true
	}
	return nil, false
}

// getOrExpire retrieves the object from the timestampedEntry if and only if it hasn't
// already expired. It holds a write lock across deletion.
func (c *ExpirationCache) getOrExpire(key string) (interface{}, bool) {
	// Prevent all inserts from the time we deem an item as "expired" to when we
	// delete it, so an un-expired item doesn't sneak in under the same key, just
	// before the Delete.
	c.expirationLock.Lock()
	defer c.expirationLock.Unlock()
	timestampedItem, exists := c.getTimestampedEntry(key)
	if !exists {
		return nil, false
	}
	if c.expirationPolicy.IsExpired(timestampedItem) {
		log.V(4).Infof("Entry %v: %+v has expired", key, timestampedItem.obj)
		c.cacheStorage.Delete(key)
		return nil, false
	}
	return timestampedItem.obj, true
}

// GetByKey returns the item stored under the key, or sets exists=false.
func (c *ExpirationCache) GetByKey(key string) (interface{}, bool, error) {
	obj, exists := c.getOrExpire(key)
	return obj, exists, nil
}

// Get returns unexpired items. It purges the cache of expired items in the
// process.
func (c *ExpirationCache) Get(obj interface{}) (interface{}, bool, error) {
	key, err := c.keyFunc(obj)
	if err != nil {
		return nil, false, KeyError{obj, err}
	}
	obj, exists := c.getOrExpire(key)
	return obj, exists, nil
}

// List retrieves a list of unexpired items. It purges the cache of expired
// items in the process.
func (c *ExpirationCache) List() []interface{} {
	items := c.cacheStorage.List()

	list := make([]interface{}, 0, len(items))
	for _, item := range items {
		obj := item.(*timestampedEntry).obj
		if key, err := c.keyFunc(obj); err != nil {
			list = append(list, obj)
		} else if obj, exists := c.getOrExpire(key); exists {
			list = append(list, obj)
		}
	}
	return list
}

// ListKeys returns a list of all keys in the expiration cache.
func (c *ExpirationCache) ListKeys() []string {
	return c.cacheStorage.ListKeys()
}

// Add timestamps an item and inserts it into the cache, overwriting entries
// that might exist under the same key.
func (c *ExpirationCache) Add(obj interface{}) error {
	c.expirationLock.Lock()
	defer c.expirationLock.Unlock()

	key, err := c.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	c.cacheStorage.Add(key, &timestampedEntry{obj, c.clock.Now()})
	return nil
}

// Update has not been implemented yet for lack of a use case, so this method
// simply calls `Add`. This effectively refreshes the timestamp.
func (c *ExpirationCache) Update(obj interface{}) error {
	return c.Add(obj)
}

// Delete removes an item from the cache.
func (c *ExpirationCache) Delete(obj interface{}) error {
	c.expirationLock.Lock()
	defer c.expirationLock.Unlock()
	key, err := c.keyFunc(obj)
	if err != nil {
		return KeyError{obj, err}
	}
	c.cacheStorage.Delete(key)
	return nil
}

// Replace will convert all items in the given list to TimestampedEntries
// before attempting the replace operation. The replace operation will
// delete the contents of the ExpirationCache `c`.
func (c *ExpirationCache) Replace(list []interface{}, resourceVersion string) error {
	c.expirationLock.Lock()
	defer c.expirationLock.Unlock()
	items := map[string]interface{}{}
	ts := c.clock.Now()
	for _, item := range list {
		key, err := c.keyFunc(item)
		if err != nil {
			return KeyError{item, err}
		}
		items[key] = &timestampedEntry{item, ts}
	}
	c.cacheStorage.Replace(items, resourceVersion)
	return nil
}

// Resync will touch all objects to put them into the processing queue
func (c *ExpirationCache) Resync() error {
	return c.cacheStorage.Resync()
}

// NewTTLStore creates and returns a ExpirationCache with a TTLPolicy
func NewTTLStore(keyFunc KeyFunc, ttl time.Duration) Store {
	return &ExpirationCache{
		cacheStorage:     NewThreadSafeStore(Indexers{}, Indices{}),
		keyFunc:          keyFunc,
		clock:            clock.RealClock{},
		expirationPolicy: &TTLPolicy{ttl, clock.RealClock{}},
	}
}