Least Recently Used (LRU) cache algorithm
A finite key-value cache using the Least Recently Used (LRU) cache algorithm where the most recently used objects are keept in cache while less recently used items are purged.
This implementation is compatible with most JavaScript environments (including ye olde browser) and is very efficient.
Terminology & design
-
Based on a doubly-linked list for low complexity random shuffling of entries.
-
The cache object iself has a "head" (least recently used entry) and a
"tail" (most recently used entry).
-
The "head" and "tail" are "entries" -- an entry might have a "newer" and
an "older" entry (doubly-linked, "older" being close to "head" and "newer"
being closer to "tail").
-
Key lookup is done through a key-entry mapping native object, which on most
platforms mean O(1)
complexity. This comes at a very low memory cost (for
storing two extra pointers for each entry).
Fancy ASCII art illustration of the general design:
entry entry entry entry
______ ______ ______ ______
| head |.newer => | |.newer => | |.newer => | tail |
| A | | B | | C | | D |
|______| <= older.|______| <= older.|______| <= older.|______|
removed <-- <-- <-- <-- <-- <-- <-- <-- <-- <-- <-- added
Example
var c = new LRUCache(3);
c.put('adam', 29);
c.put('john', 26);
c.put('angela', 24);
c.toString(); // -> "adam:29 < john:26 < angela:24"
c.get('john'); // -> 26
// Now 'john' is the most recently used entry, since we just requested it
c.toString(); // -> "adam:29 < angela:24 < john:26"
c.put('zorro', 141); // -> {key:adam, value:29}
// Because we only have room for 3 entries, put-ing 'zorro' purged 'adam'
c.toString(); // -> "angela:24 < john:26 < zorro:141"
API
An entry is a simple Object
with at least two members: {key:Object, value:Object}
. An entry might also have a newer
member which points to a newer entry, and/or a older
member pointing to an older entry.
Included in lru-core.js and lru.js:
new LRUCache(Number limit) -> LRUCache instance
Creates a new cache object which will hold up to limit
entries.
LRUCache.prototype.size -> Number
Current number of entries. Read-only.
LRUCache.prototype.limit <-> Number
Maximum number of items this cache will keep.
LRUCache.prototype.put (Object key, Object value) -> Object entry
Put value
into the cache associated with key
.
Returns an entry which was removed (to make room for the new entry) or undefined
if there was enough space for the new entry.
Note: The returned entry does not include any (strong) references to other entries (i.e. there is no older
or newer
members). This design makes garbage collection predictable.
LRUCache.prototype.get (Object key) -> Object value
Retrieve value for, and register recent use of, key
. Returns the value associated with key
or undefined
if not in the cache.
LRUCache.prototype.shift () -> Object entry
Remove the least recently used (oldest) entry. Returns the removed entry, or undefined
if the cache was empty.
If you need to perform any form of finalization of purged items, this is a good place to do it. Simply override/replace this function:
var c = new LRUCache(123);
c.shift = function() {
var entry = LRUCache.prototype.shift.call(this);
doSomethingWith(entry);
return entry;
}
The returned entry must not include any strong references to other entries. See note in the documentation of LRUCache.prototype.put (Object key, Object value) -> Object entry
.
Included in lru.js only
LRUCache.prototype.find (Object key) -> Object entry
Check if key
is in the cache without registering recent use. Feasible if
you do not want to chage the state of the cache, but only "peek" at it.
Returns the entry associated with key
if found, otherwise undefined
is
returned.
Note: The entry returned is managed by the cache (until purged) and thus contains members with strong references which might be altered at any time by the cache object. You should look at the returned entry as being immutable.
LRUCache.prototype.set (key, value) -> Object oldValue
Update the value of entry with key
or put
a new entry. Returns the old value, or undefined if the cache was empty.
LRUCache.prototype.remove (key) -> Object value
Remove entry key
from cache and return its value. Returns undefined
if key
is not found.
LRUCache.prototype.removeAll () -> LRUCache instance
Removes all entries and return itself.
LRUCache.prototype.keys () -> Array keys
Return an array containing all keys of entries in arbitrary order.
LRUCache.prototype.forEach (fun, [Object context, Boolean desc | true])
Call fun
for each entry. Starting with the newest entry if desc
is a true
value, otherwise starts with the oldest (head) enrty and moves towards the
tail.
Returns nothing (undefined
).
fun
is called with 3 arguments in the context context
:
fun.call(context, Object key, Object value, LRUCache self)
Example which prints "key: value" starting with the most recent entry:
cache.forEach(function(key, value) {
puts(key+': '+value);
}, true);
LRUCache.prototype.toJSON () -> Array representation
Returns an array of object (for use by JSON.stringify
) of the form:
[
{key:"key1", value:"value1"},
{key:"key2", value:"value2"},
{key:"key3", value:"value3"}
]
LRUCache.prototype.toString () -> String representation
Returns a string representation in the format:
key1:value1 < key2:value2 < key3:value3
Oldest (head) on the left hand side and newer entries to the right hand side.
Factorising a minimal implementation
As this code is most suitable for embedding, here is a shortlist of the essential parts (prototype functions) needed for a minimal implementation. All other functions, not mentioned here, are simply ancillary.
- LRUCache -- the constructor is naturally a good thing to keep ;)
- LRUCache.prototype.put -- handles appending and chaining.
- LRUCache.prototype.shift -- used by put to "purge" an old entry.
- LRUCache.prototype.get -- fetches a cached entry and registers that entry as being recently used.
To include only the minimal code above, require lru-core.js instead of lru.js
MIT license
Copyright (c) 2014 Rasmus Andersson http://hunch.se/, Ben Woosley
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