Implement Pin#check method

DOCS.md

@@ -129,6 +129,7 @@ to pin attachments.
 * [Pin](#Pin)
     * [new Pin()](#new_Pin_new)
     * [.all()](#Pin+all) ⇒ <code>Promise</code>
+    * [.check()](#Pin+check) ⇒ <code>Promise</code>
 
 <a name="new_Pin_new"></a>
 
@@ -142,6 +143,14 @@ Pins attachments to local storage for which there are records in Redis.
 
 **Kind**: instance method of [<code>Pin</code>](#Pin)  
 **Returns**: <code>Promise</code> - Number of pinned attachments.  
+<a name="Pin+check"></a>
+
+### pin.check() ⇒ <code>Promise</code>
+Checks if IPFS objects for hashes found in Redis are stored locally.
+
+**Kind**: instance method of [<code>Pin</code>](#Pin)  
+**Returns**: <code>Promise</code> - Resolves with array of unpinned hashes or rejects with
+                  error. If array is empty, all hashes are pinned.  
 <a name="Cleaner"></a>
 
 ## Cleaner

README.md

@@ -148,6 +148,7 @@ to pin attachments.
 * [Pin](#Pin)
     * [new Pin()](#new_Pin_new)
     * [.all()](#Pin+all) ⇒ <code>Promise</code>
+    * [.check()](#Pin+check) ⇒ <code>Promise</code>
 
 <a name="new_Pin_new"></a>
 
@@ -161,6 +162,14 @@ Pins attachments to local storage for which there are records in Redis.
 
 **Kind**: instance method of [<code>Pin</code>](#Pin)  
 **Returns**: <code>Promise</code> - Number of pinned attachments.  
+<a name="Pin+check"></a>
+
+### pin.check() ⇒ <code>Promise</code>
+Checks if IPFS objects for hashes found in Redis are stored locally.
+
+**Kind**: instance method of [<code>Pin</code>](#Pin)  
+**Returns**: <code>Promise</code> - Resolves with array of unpinned hashes or rejects with
+                  error. If array is empty, all hashes are pinned.  
 <a name="Cleaner"></a>
 
 ## Cleaner

components/pin.js

@@ -47,6 +47,28 @@ class Pin {
       throw error;
     });
   }
+
+  /**
+   * Checks if IPFS objects for hashes found in Redis are stored locally.
+   * @return {Promise} Resolves with array of unpinned hashes or rejects with
+   *                   error. If array is empty, all hashes are pinned.
+   */
+  check() {
+    logging.info('Checking if all attachments are locally stored');
+
+    return Promise.join(
+      this.ipfs.refs.local(),
+      redis.keysAsync(this.prefix + '*')
+    ).spread((refs, keys) => {
+      let hashes = keys.map((key) => key.split('_')[1]);
+      logging.info(`Found ${hashes.length} attachments
+                    and ${refs.length} local objects`);
+
+      return hashes.filter(refs.includes);
+    }).catch((error) => {
+      throw error;
+    });
+  }
 }
 
 module.exports = Pin;