Simple LRU implementation for ArrayCache

README.md

@@ -74,15 +74,30 @@ or not the item has been removed from cache.
 
 ### ArrayCache
 
-The `ArrayCache` provides an in-memory implementation of the
-[`CacheInterface`](#cacheinterface).
+The `ArrayCache` provides an in-memory implementation of the [`CacheInterface`](#cacheinterface).
 
 ```php
 $cache = new ArrayCache();
 
 $cache->set('foo', 'bar');
 ```
 
+Its constructor accepts an optional `?int $limit` parameter to limit the
+maximum number of entries to store in the LRU cache. If you add more
+entries to this instance, it will automatically take care of removing
+the one that was least recently used (LRU).
+
+For example, this snippet will overwrite the first value and only store
+the last two entries:
+
+```php
+$cache = new ArrayCache(2);
+
+$cache->set('foo', '1');
+$cache->set('bar', '2');
+$cache->set('baz', '3');
+```
+
 ## Common usage
 
 ### Fallback get

src/ArrayCache.php

@@ -6,20 +6,67 @@
 
 class ArrayCache implements CacheInterface
 {
+    private $limit;
     private $data = array();
 
+    /**
+     * The `ArrayCache` provides an in-memory implementation of the [`CacheInterface`](#cacheinterface).
+     *
+     * ```php
+     * $cache = new ArrayCache();
+     *
+     * $cache->set('foo', 'bar');
+     * ```
+     *
+     * Its constructor accepts an optional `?int $limit` parameter to limit the
+     * maximum number of entries to store in the LRU cache. If you add more
+     * entries to this instance, it will automatically take care of removing
+     * the one that was least recently used (LRU).
+     *
+     * For example, this snippet will overwrite the first value and only store
+     * the last two entries:
+     *
+     * ```php
+     * $cache = new ArrayCache(2);
+     *
+     * $cache->set('foo', '1');
+     * $cache->set('bar', '2');
+     * $cache->set('baz', '3');
+     * ```
+     *
+     * @param int|null $limit maximum number of entries to store in the LRU cache
+     */
+    public function __construct($limit = null)
+    {
+        $this->limit = $limit;
+    }
+
     public function get($key)
     {
         if (!isset($this->data[$key])) {
             return Promise\resolve();
         }
 
-        return Promise\resolve($this->data[$key]);
+        // remove and append to end of array to keep track of LRU info
+        $value = $this->data[$key];
+        unset($this->data[$key]);
+        $this->data[$key] = $value;
+
+        return Promise\resolve($value);
     }
 
     public function set($key, $value)
     {
+        // unset before setting to ensure this entry will be added to end of array
+        unset($this->data[$key]);
         $this->data[$key] = $value;
+
+        // ensure size limit is not exceeded or remove first entry from array
+        if ($this->limit !== null && count($this->data) > $this->limit) {
+            reset($this->data);
+            unset($this->data[key($this->data)]);
+        }
+
         return Promise\resolve(true);
     }
 

tests/ArrayCacheTest.php

@@ -82,4 +82,52 @@ public function removeShouldRemoveKey()
                 $this->expectCallableNever()
             );
     }
+
+    public function testLimitSizeToZeroDoesNotStoreAnyData()
+    {
+        $this->cache = new ArrayCache(0);
+
+        $this->cache->set('foo', 'bar');
+
+        $this->cache->get('foo')->then($this->expectCallableOnceWith(null));
+    }
+
+    public function testLimitSizeToOneWillOnlyReturnLastWrite()
+    {
+        $this->cache = new ArrayCache(1);
+
+        $this->cache->set('foo', '1');
+        $this->cache->set('bar', '2');
+
+        $this->cache->get('foo')->then($this->expectCallableOnceWith(null));
+        $this->cache->get('bar')->then($this->expectCallableOnceWith('2'));
+    }
+
+    public function testOverwriteWithLimitedSizeWillUpdateLRUInfo()
+    {
+        $this->cache = new ArrayCache(2);
+
+        $this->cache->set('foo', '1');
+        $this->cache->set('bar', '2');
+        $this->cache->set('foo', '3');
+        $this->cache->set('baz', '4');
+
+        $this->cache->get('foo')->then($this->expectCallableOnceWith('3'));
+        $this->cache->get('bar')->then($this->expectCallableOnceWith(null));
+        $this->cache->get('baz')->then($this->expectCallableOnceWith('4'));
+    }
+
+    public function testGetWithLimitedSizeWillUpdateLRUInfo()
+    {
+        $this->cache = new ArrayCache(2);
+
+        $this->cache->set('foo', '1');
+        $this->cache->set('bar', '2');
+        $this->cache->get('foo')->then($this->expectCallableOnceWith('1'));
+        $this->cache->set('baz', '3');
+
+        $this->cache->get('foo')->then($this->expectCallableOnceWith('1'));
+        $this->cache->get('bar')->then($this->expectCallableOnceWith(null));
+        $this->cache->get('baz')->then($this->expectCallableOnceWith('3'));
+    }
 }

tests/TestCase.php

@@ -26,6 +26,17 @@ protected function expectCallableOnce()
         return $mock;
     }
 
+    protected function expectCallableOnceWith($param)
+    {
+        $mock = $this->createCallableMock();
+        $mock
+            ->expects($this->once())
+            ->method('__invoke')
+            ->with($param);
+
+        return $mock;
+    }
+
     protected function expectCallableNever()
     {
         $mock = $this->createCallableMock();