/*
 *     Copyright 2020 The Dragonfly 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 cache

import (
	"encoding/gob"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"runtime"
	"sync"
	"time"
)

type Item struct {
	Object     any
	Expiration int64
}

// Returns true if the item has expired.
func (item Item) Expired() bool {
	if item.Expiration == 0 {
		return false
	}
	return time.Now().UnixNano() > item.Expiration
}

const (
	// For use with functions that take an expiration time.
	NoExpiration time.Duration = -1

	// For use with functions that take an expiration time. Equivalent to
	// passing in the same expiration duration as was given to New() or
	// NewFrom() when the cache was created (e.g. 5 minutes.)
	DefaultExpiration time.Duration = 0

	// For use with functions that do not clean up.
	NoCleanup time.Duration = -1
)

type Cache interface {
	Set(k string, x any, d time.Duration)
	SetDefault(k string, x any)
	Add(k string, x any, d time.Duration) error
	Get(k string) (any, bool)
	GetWithExpiration(k string) (any, time.Time, bool)
	Delete(k string)
	DeleteExpired()
	Keys() []string
	OnEvicted(f func(string, any))
	Save(w io.Writer) (err error)
	SaveFile(fname string) error
	Load(r io.Reader) error
	LoadFile(fname string) error
	Items() map[string]Item
	ItemCount() int
	Flush()
}

type cache struct {
	defaultExpiration time.Duration
	items             map[string]Item
	mu                sync.RWMutex
	onEvicted         func(string, any)
	janitor           *janitor
}

// Add an item to the cache, replacing any existing item. If the duration is 0
// (DefaultExpiration), the cache's default expiration time is used. If it is -1
// (NoExpiration), the item never expires.
func (c *cache) Set(k string, x any, d time.Duration) {
	// "Inlining" of set
	var e int64
	if d == DefaultExpiration {
		d = c.defaultExpiration
	}
	if d > 0 {
		e = time.Now().Add(d).UnixNano()
	}
	c.mu.Lock()
	c.items[k] = Item{
		Object:     x,
		Expiration: e,
	}
	// TODO: Calls to mu.Unlock are currently not deferred because defer
	// adds ~200 ns (as of go1.)
	c.mu.Unlock()
}

func (c *cache) set(k string, x any, d time.Duration) {
	var e int64
	if d == DefaultExpiration {
		d = c.defaultExpiration
	}
	if d > 0 {
		e = time.Now().Add(d).UnixNano()
	}
	c.items[k] = Item{
		Object:     x,
		Expiration: e,
	}
}

// Add an item to the cache, replacing any existing item, using the default
// expiration.
func (c *cache) SetDefault(k string, x any) {
	c.Set(k, x, DefaultExpiration)
}

// Add an item to the cache only if an item doesn't already exist for the given
// key, or if the existing item has expired. Returns an error otherwise.
func (c *cache) Add(k string, x any, d time.Duration) error {
	c.mu.Lock()
	_, found := c.get(k)
	if found {
		c.mu.Unlock()
		return fmt.Errorf("Item %s already exists", k)
	}
	c.set(k, x, d)
	c.mu.Unlock()
	return nil
}

// Get an item from the cache. Returns the item or nil, and a bool indicating
// whether the key was found.
func (c *cache) Get(k string) (any, bool) {
	c.mu.RLock()
	// "Inlining" of get and Expired
	item, found := c.items[k]
	if !found {
		c.mu.RUnlock()
		return nil, false
	}

	c.mu.RUnlock()
	return item.Object, true
}

// GetWithExpiration returns an item and its expiration time from the cache.
// It returns the item or nil, the expiration time if one is set (if the item
// never expires a zero value for time.Time is returned), and a bool indicating
// whether the key was found.
func (c *cache) GetWithExpiration(k string) (any, time.Time, bool) {
	c.mu.RLock()
	// "Inlining" of get and Expired
	item, found := c.items[k]
	if !found {
		c.mu.RUnlock()
		return nil, time.Time{}, false
	}

	if item.Expiration > 0 {
		if time.Now().UnixNano() > item.Expiration {
			c.mu.RUnlock()
			return nil, time.Time{}, false
		}

		// Return the item and the expiration time
		c.mu.RUnlock()
		return item.Object, time.Unix(0, item.Expiration), true
	}

	// If expiration <= 0 (i.e. no expiration time set) then return the item
	// and a zeroed time.Time
	c.mu.RUnlock()
	return item.Object, time.Time{}, true
}

func (c *cache) get(k string) (any, bool) {
	item, found := c.items[k]
	if !found {
		return nil, false
	}
	// "Inlining" of Expired
	if item.Expiration > 0 {
		if time.Now().UnixNano() > item.Expiration {
			return nil, false
		}
	}
	return item.Object, true
}

// Delete an item from the cache. Does nothing if the key is not in the cache.
func (c *cache) Delete(k string) {
	c.mu.Lock()
	v, evicted := c.delete(k)
	c.mu.Unlock()
	if evicted {
		c.onEvicted(k, v)
	}
}

func (c *cache) delete(k string) (any, bool) {
	if c.onEvicted != nil {
		if v, found := c.items[k]; found {
			delete(c.items, k)
			return v.Object, true
		}
	}
	delete(c.items, k)
	return nil, false
}

type keyAndValue struct {
	key   string
	value any
}

// Delete all expired items from the cache.
func (c *cache) DeleteExpired() {
	var evictedItems []keyAndValue
	now := time.Now().UnixNano()
	c.mu.Lock()
	for k, v := range c.items {
		// "Inlining" of expired
		if v.Expiration > 0 && now > v.Expiration {
			ov, evicted := c.delete(k)
			if evicted {
				evictedItems = append(evictedItems, keyAndValue{k, ov})
			}
		}
	}
	c.mu.Unlock()
	for _, v := range evictedItems {
		c.onEvicted(v.key, v.value)
	}
}

// Keys returns a sorted slice of all the keys in the cache.
func (c *cache) Keys() []string {
	c.mu.RLock()
	defer c.mu.RUnlock()
	keys := make([]string, len(c.items))
	var i int
	for k := range c.items {
		keys[i] = k
		i++
	}
	return keys
}

// Sets an (optional) function that is called with the key and value when an
// item is evicted from the cache. (Including when it is deleted manually, but
// not when it is overwritten.) Set to nil to disable.
func (c *cache) OnEvicted(f func(string, any)) {
	c.mu.Lock()
	c.onEvicted = f
	c.mu.Unlock()
}

// Write the cache's items (using Gob) to an io.Writer.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
func (c *cache) Save(w io.Writer) (err error) {
	enc := gob.NewEncoder(w)
	defer func() {
		if x := recover(); x != nil {
			err = fmt.Errorf("Error registering item types with Gob library")
		}
	}()
	c.mu.RLock()
	defer c.mu.RUnlock()
	for _, v := range c.items {
		gob.Register(v.Object)
	}
	err = enc.Encode(&c.items)
	return
}

// Save the cache's items to the given filename, creating the file if it
// doesn't exist, and overwriting it if it does.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
func (c *cache) SaveFile(fname string) error {
	dir := filepath.Dir(fname)
	if err := os.MkdirAll(dir, os.ModePerm); err != nil {
		return err
	}

	fp, err := os.Create(fname)
	if err != nil {
		return err
	}

	defer fp.Close()

	return c.Save(fp)
}

// Add (Gob-serialized) cache items from an io.Reader, excluding any items with
// keys that already exist (and haven't expired) in the current cache.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
func (c *cache) Load(r io.Reader) error {
	dec := gob.NewDecoder(r)
	items := map[string]Item{}
	err := dec.Decode(&items)
	if err == nil {
		c.mu.Lock()
		defer c.mu.Unlock()
		for k, v := range items {
			ov, found := c.items[k]
			if !found || ov.Expired() {
				c.items[k] = v
			}
		}
	}
	return err
}

// Load and add cache items from the given filename, excluding any items with
// keys that already exist in the current cache.
//
// NOTE: This method is deprecated in favor of c.Items() and NewFrom() (see the
// documentation for NewFrom().)
func (c *cache) LoadFile(fname string) error {
	fp, err := os.Open(fname)
	if err != nil {
		return err
	}
	err = c.Load(fp)
	if err != nil {
		fp.Close()
		return err
	}
	return fp.Close()
}

// Copies all unexpired items in the cache into a new map and returns it.
func (c *cache) Items() map[string]Item {
	c.mu.RLock()
	defer c.mu.RUnlock()
	m := make(map[string]Item, len(c.items))
	now := time.Now().UnixNano()
	for k, v := range c.items {
		// "Inlining" of Expired
		if v.Expiration > 0 {
			if now > v.Expiration {
				continue
			}
		}
		m[k] = v
	}
	return m
}

// Returns the number of items in the cache. This may include items that have
// expired, but have not yet been cleaned up.
func (c *cache) ItemCount() int {
	c.mu.RLock()
	n := len(c.items)
	c.mu.RUnlock()
	return n
}

// Delete all items from the cache.
func (c *cache) Flush() {
	c.mu.Lock()
	c.items = map[string]Item{}
	c.mu.Unlock()
}

type janitor struct {
	Interval time.Duration
	stop     chan bool
}

func (j *janitor) Run(c *cache) {
	ticker := time.NewTicker(j.Interval)
	for {
		select {
		case <-ticker.C:
			c.DeleteExpired()
		case <-j.stop:
			ticker.Stop()
			return
		}
	}
}

func stopJanitor(c *cache) {
	c.janitor.stop <- true
}

func runJanitor(c *cache, ci time.Duration) {
	j := &janitor{
		Interval: ci,
		stop:     make(chan bool),
	}
	c.janitor = j
	go j.Run(c)
}

func newCache(de time.Duration) *cache {
	if de == 0 {
		de = -1
	}
	c := &cache{
		defaultExpiration: de,
		items:             make(map[string]Item),
	}
	return c
}

func newCacheWithJanitor(de time.Duration, ci time.Duration) Cache {
	c := newCache(de)
	// This trick ensures that the janitor goroutine (which--granted it
	// was enabled--is running DeleteExpired on c forever) does not keep
	// the returned C object from being garbage collected. When it is
	// garbage collected, the finalizer stops the janitor goroutine, after
	// which c can be collected.
	if ci > 0 {
		runJanitor(c, ci)
		runtime.SetFinalizer(c, stopJanitor)
	}
	return c
}

// Return a new cache with a given default expiration duration and cleanup
// interval. If the expiration duration is less than one (or NoExpiration),
// the items in the cache never expire (by default), and must be deleted
// manually. If the cleanup interval is less than one, expired items are not
// deleted from the cache before calling c.DeleteExpired().
func New(defaultExpiration, cleanupInterval time.Duration) Cache {
	return newCacheWithJanitor(defaultExpiration, cleanupInterval)
}