Add countDocs method

This allows one to get the number of affected documents without actually
having to retrieve all the documents from the store.

This fixes #1

Author: heiglandreas

src/DocumentStore.php

@@ -136,4 +136,12 @@ public function getDoc(string $collectionName, string $docId): ?array;
      * @throws UnknownCollection
      */
     public function filterDocs(string $collectionName, Filter $filter, int $skip = null, int $limit = null, OrderBy $orderBy = null): \Traversable;
+
+    /**
+     * @param string $collectionName
+     * @param Filter $filter
+     * @return int The number of documents
+     * @throws UnknownCollection
+     */
+    public function countDocs(string $collectionName, Filter $filter): int;
 }

src/InMemoryDocumentStore.php

@@ -329,4 +329,21 @@ private function sort(&$docs, OrderBy $orderBy)
             return $docCmp($docA, $docB, $orderBy);
         });
     }
+
+    /**
+     * @inheritDoc
+     */
+    public function countDocs(string $collectionName, Filter $filter) : int
+    {
+        $this->assertHasCollection($collectionName);
+
+        $counter = 0;
+        foreach ($this->inMemoryConnection['documents'][$collectionName] as $docId => $doc) {
+            if ($filter->match($doc, $docId)) {
+                $counter++;
+            }
+        }
+
+        return $counter;
+    }
 }