abstract class DrupalCacheArray implements ArrayAccess {

  /**
   * A cid to pass to cache_set() and cache_get().
   */
  protected $cid;

  /**
   * A bin to pass to cache_set() and cache_get().
   */
  protected $bin;

  /**
   * An array of keys to add to the cache at the end of the request.
   */
  protected $keysToPersist = array();

  /**
   * Storage for the data itself.
   */
  protected $storage = array();

  /**
   * Constructs a DrupalCacheArray object.
   *
   * @param $cid
   *   The cid for the array being cached.
   * @param $bin
   *   The bin to cache the array.
   */
  public function __construct($cid, $bin) {
    $this->cid = $cid;
    $this->bin = $bin;

    if ($cached = cache_get($this->cid, $this->bin)) {
      $this->storage = $cached->data;
    }
  }

  /**
   * Implements ArrayAccess::offsetExists().
   */
  public function offsetExists($offset) {
    return $this->offsetGet($offset) !== NULL;
  }

  /**
   * Implements ArrayAccess::offsetGet().
   */
  public function offsetGet($offset) {
    if (isset($this->storage[$offset]) || array_key_exists($offset, $this->storage)) {
      return $this->storage[$offset];
    }
    else {
      return $this->resolveCacheMiss($offset);
    }
  }

  /**
   * Implements ArrayAccess::offsetSet().
   */
  public function offsetSet($offset, $value) {
    $this->storage[$offset] = $value;
  }

  /**
   * Implements ArrayAccess::offsetUnset().
   */
  public function offsetUnset($offset) {
    unset($this->storage[$offset]);
  }

  /**
   * Flags an offset value to be written to the persistent cache.
   *
   * If a value is assigned to a cache object with offsetSet(), by default it
   * will not be written to the persistent cache unless it is flagged with this
   * method. This allows items to be cached for the duration of a request,
   * without necessarily writing back to the persistent cache at the end.
   *
   * @param $offset
   *   The array offset that was requested.
   * @param $persist
   *   Optional boolean to specify whether the offset should be persisted or
   *   not, defaults to TRUE. When called with $persist = FALSE the offset will
   *   be unflagged so that it will not be written at the end of the request.
   */
  protected function persist($offset, $persist = TRUE) {
    $this->keysToPersist[$offset] = $persist;
  }

  /**
   * Resolves a cache miss.
   *
   * When an offset is not found in the object, this is treated as a cache
   * miss. This method allows classes implementing the interface to look up
   * the actual value and allow it to be cached.
   *
   * @param $offset
   *   The offset that was requested.
   *
   * @return
   *   The value of the offset, or NULL if no value was found.
   */
  abstract protected function resolveCacheMiss($offset);

  /**
   * Writes a value to the persistent cache immediately.
   *
   * @param $data
   *   The data to write to the persistent cache.
   * @param $lock
   *   Whether to acquire a lock before writing to cache.
   */
  protected function set($data, $lock = TRUE) {
    // Lock cache writes to help avoid stampedes.
    // To implement locking for cache misses, override __construct().
    $lock_name = $this->cid . ':' . $this->bin;
    if (!$lock || lock_acquire($lock_name)) {
      if ($cached = cache_get($this->cid, $this->bin)) {
        $data = $cached->data + $data;
      }
      cache_set($this->cid, $data, $this->bin);
      if ($lock) {
        lock_release($lock_name);
      }
    }
  }

  /**
   * Destructs the DrupalCacheArray object.
   */
  public function __destruct() {
    $data = array();
    foreach ($this->keysToPersist as $offset => $persist) {
      if ($persist) {
        $data[$offset] = $this->storage[$offset];
      }
    }
    if (!empty($data)) {
      $this->set($data);
    }
  }
}