package cache

import (
	"runtime"
	"sync"
	"time"
)

// Cache is an in-memory cache similar to memcached that is suitable for applications
// running on a single machine. Any object can be stored, for a given duration or forever,
// and the cache can be used safely by multiple goroutines.
//
// Installation:
//     goinstall github.com/pmylund/go-cache
//
// Usage:
//     // Create a cache with a default expiration time of 5 minutes, and which purges
//     // expired items every 30 seconds
//     c := cache.New(5*time.Minute, 30*time.Second)
//
//     // Set the value of the key "foo" to "bar", with the default expiration time
//     c.Set("foo", "bar", 0)
//
//     // Set the value of the key "baz" to "yes", with no expiration time (the item
//     // won't be removed until it is re-set, or removed using c.Delete("baz")
//     c.Set("baz", "yes", -1)
//
//     // Get the string associated with the key "foo" from the cache
//     foo, found := c.Get("foo")
//     if found {
//             fmt.Println(foo)
//     }
//
//     // Since Go is statically typed, and cache values can be anything, type assertion
//     // is needed when values are being passed to functions that don't take arbitrary types,
//     // (i.e. interface{}). The simplest way to do this for values which will only be passed
//     // once is:
//     foo, found := c.Get("foo")
//     if found {
//             MyFunction(foo.(string))
//     }
//
//     // This gets tedious if the value is used several times in the same function. You
//     // might do either of the following instead:
//     if x, found := c.Get("foo"); found {
//             foo := x.(string)
//             ...
//     }
//     // or
//     var foo string
//     if x, found := c.Get("foo"); found {
//             foo = x.(string)
//     }
//     // foo can then be passed around freely as a string
//
//     // Want performance? Store pointers!
//     c.Set("foo", &MyStruct, 0)
//     if x, found := c.Get("foo"); found {
//             foo := x.(*MyStruct)
//             ...
//     }
//
// If you store a reference type like a pointer, slice, map or channel, you do not need to
// run Set if you modify the underlying data. The cache does not serialize its data, so if
// you modify a struct whose pointer you've stored in the cache, retrieving that pointer
// with Get will point you to the same data:
//
//     foo := &MyStruct{Num: 1}
//     c.Set("foo", foo, 0)
//     ...
//     x, _ := c.Get("foo")
//     foo := x.(MyStruct)
//     fmt.Println(foo.Num)
//     ...
//     foo.Num++
//     ...
//     x, _ := c.Get("foo")
//     foo := x.(MyStruct)
//     foo.Println(foo.Num)
//
// will print:
//     1
//     2

type Cache struct {
	*cache
	// If this is confusing, see the comment at the bottom of the New() function
}

type cache struct {
	DefaultExpiration time.Duration
	Items             map[string]*Item
	mu                *sync.Mutex
	janitor           *janitor
}

type Item struct {
	Object     interface{}
	Expires    bool
	Expiration *time.Time
}

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

// Adds an item to the cache. If the duration is 0, the cache's default expiration time
// is used. If it is -1, the item never expires.
func (c *cache) Set(key string, x interface{}, d time.Duration) {
	c.mu.Lock()
	defer c.mu.Unlock()

	var e *time.Time
	expires := true
	if d == 0 {
		d = c.DefaultExpiration
	}
	if d == -1 {
		expires = false
	} else {
		t := time.Now().Add(d)
		e = &t
	}
	c.Items[key] = &Item{
		Object: x,
		Expires: expires,
		Expiration: e,
	}
}

// Gets an item from the cache.
func (c *cache) Get(key string) (interface{}, bool) {
	c.mu.Lock()
	defer c.mu.Unlock()

	item, found := c.Items[key]
	if !found {
		return nil, false
	}
	if item.Expired() {
		delete(c.Items, key)
		return nil, false
	}
	return item.Object, true
}

// Deletes an item from the cache. Does nothing if the item does not exist in the cache.
func (c *cache) Delete(key string) {
	c.mu.Lock()
	defer c.mu.Unlock()

	delete(c.Items, key)
}

// Deletes all expired items from the cache.
func (c *cache) DeleteExpired() {
	c.mu.Lock()
	defer c.mu.Unlock()

	for k, v := range c.Items {
		if v.Expired() {
			delete(c.Items, k)
		}
	}

}

// Deletes all items in the cache
func (c *cache) Flush() {
	c.mu.Lock()
	defer c.mu.Unlock()

	c.Items = map[string]*Item{}
}

// Returns true if the item has expired.
func (i *Item) Expired() bool {
	if i.Expiration == nil {
		return false
	}
	return i.Expiration.Before(time.Now())
}


func (j *janitor) Run(c *cache) {
	j.stop = make(chan bool)
	tick := time.Tick(j.Interval)
	for {
		select {
		case <-tick:
			c.DeleteExpired()
		case <-j.stop:
			return
		}
	}
}

func (j *janitor) Stop() {
	j.stop <- true
}

func stopJanitor(c *Cache) {
	c.janitor.Stop()
}

// Returns a new cache with a given default expiration duration and default cleanup
// interval. If the expiration duration is less than 1, the items in the cache never expire
// and have to be deleted manually. If the cleanup interval is less than one, expired
// items are not deleted from the cache before their next lookup or before calling
// DeleteExpired.
func New(de, ci time.Duration) *Cache {
	if de == 0 {
		de = -1
	}
	c := &cache{
		DefaultExpiration: de,
		Items: map[string]*Item{},
		mu: &sync.Mutex{},
	}
	if ci > 0 {
		j := &janitor{
			Interval: ci,
		}
		c.janitor = j
		go j.Run(c)
	}
	// This trick ensures that the janitor goroutine (which--granted it was enabled--is
        // running DeleteExpired on c forever, if it was enabled) 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 is collected.
	C := &Cache{c}
	if ci > 0 {
		runtime.SetFinalizer(C, stopJanitor)
	}
	return C
}