lfu cache

A cache object that deletes the least-frequently-used items. A direct port of lru-cache but using this algorithm.

Installation:

``javascript
npm install node-lfu-cache --save
`

Usage:

`javascript
var LFU = require('node-lfu-cache');
var options = {
max: 500,
length: function (n, key) {
return n * 2 + key.length;
},
dispose: function (key, n) {
n.close();
},
maxAge: 1000 60 60
};
var cache = LFU(options);
var otherCache = LFU(50); // sets just the max size

cache.set('key', 'value');
cache.get('key'); // "value"

// non-string keys ARE fully supported
var someObject = {};

cache.set(someObject, 'a value');
cache.set('[object Object]', 'a different value');
assert.equal(cache.get(someObject), 'a value');

cache.reset(); // empty the cache
`

If you put more stuff in it, then items will fall out.

If you try to put an oversized thing in it, then it'll fall out right
away.

Options

* max The maximum size of the cache, checked by applying the length
function to all values in the cache. Not setting this is kind of
silly, since that's the whole purpose of this lib, but it defaults
to Infinity.
* maxAge Maximum age in ms. Items are not pro-actively pruned out
as they age, but if you try to get an item that is too old, it'll
drop it and return undefined instead of giving it to you.
* length Function that is used to calculate the length of stored
items. If you're storing strings or buffers, then you probably want
to do something like function(n, key){return n.length}. The default is
function(){return 1}, which is fine if you want to store max
like-sized things. The item is passed as the first argument, and
the key is passed as the second argumnet.
* dispose Function that is called on items when they are dropped
from the cache. This can be handy if you want to close file
descriptors or do other cleanup tasks when items are no longer
accessible. Called with key, value. It's called before
actually removing the item from the internal cache, so if you want
to immediately put it back in, you'll have to do that in a
nextTick or setTimeout callback or it won't do anything.
* stale By default, if you set a maxAge, it'll only actually pull
stale items out of the cache when you get(key). (That is, it's
not pre-emptively doing a setTimeout or anything.) If you set
stale:true, it'll return the stale value before deleting it. If
you don't set this, then it'll return undefined when you try to
get a stale entry, as if it had already been deleted.
* noDisposeOnSet By default, if you set a dispose() method, then
it'll be called whenever a set() operation overwrites an existing
key. If you set this option, dispose() will only be called when a
key falls out of the cache, not when it is overwritten.

API

* set(key, value, maxAge)
* get(key) => value

Both of these will update the "recently used"-ness of the key.
They do what you think. maxAge is optional and overrides the
cache maxAge option if provided.

If the key is not found, get() will return undefined.

The key and val can be any value.

* peek(key)

Returns the key value (or undefined if not found) without
updating the "recently used"-ness of the key.

(If you find yourself using this a lot, you might be using the
wrong sort of data structure, but there are some use cases where
it's handy.)

* del(key)

Deletes a key out of the cache.

* reset()

Clear the cache entirely, throwing away all values.

* has(key)

Check if a key is in the cache, without updating the recent-ness
or deleting it for being stale.

* forEach(function(value,key,cache), [thisp])

Just like Array.prototype.forEach. Iterates over all the keys
in the cache, in order of frequent-ness. (Ie, more frequently used
items are iterated over first.)

* rforEach(function(value,key,cache), [thisp])

The same as cache.forEach(...) but items are iterated over in
reverse order. (ie, least frequently used items are iterated over
first.)

* keys()

Return an array of the keys in the cache.

* values()

Return an array of the values in the cache.

* length

Return total length of objects in cache taking into account
length options function.

* itemCount

Return total quantity of objects currently in cache. Note, that
stale (see options) items are returned as part of this item
count.

* dump()

Return an array of the cache entries ready for serialization and usage
with 'destinationCache.load(arr).

* load(cacheEntriesArray)

Loads another cache entries array, obtained with sourceCache.dump(),
into the cache. The destination cache is reset before loading new entries

* prune()

Manually iterates over the entire cache proactively pruning old entries

* pop()

Removes and returns the least frequently used item, i.e. the one that would be deleted next.