Move snapshot building function into ot

The function for building a snapshot from ops is useful, and has no
dependencies on `Backend`. This change moves it into the `ot` module,
where it will be a bit more discoverable and can be reused.

lib/backend.js

@@ -709,38 +709,9 @@ Backend.prototype._fetchSnapshotByTimestamp = function (collection, id, timestam
 };
 
 Backend.prototype._buildSnapshotFromOps = function (id, startingSnapshot, ops, callback) {
-  var type = null;
-  var data;
-  var fetchedVersion = 0;
-
-  if (startingSnapshot) {
-    type = types.map[startingSnapshot.type];
-    if (!type) return callback({ code: 4008, message: 'Unknown type' });
-    data = startingSnapshot.data;
-    fetchedVersion = startingSnapshot.v;
-  }
-
-  for (var index = 0; index < ops.length; index++) {
-    var op = ops[index];
-
-    fetchedVersion = op.v + 1;
-
-    if (op.create) {
-      type = types.map[op.create.type];
-      if (!type) return callback({ code: 4008, message: 'Unknown type' });
-      data = type.create(op.create.data);
-    } else if (op.del) {
-      data = undefined;
-      type = null;
-    } else {
-      data = type.apply(data, op.op);
-    }
-  }
-
-  type = type ? type.uri : null;
-
-  var snapshot = new Snapshot(id, fetchedVersion, type, data, null);
-  callback(null, snapshot);
+  var snapshot = startingSnapshot || new Snapshot(id, 0, null, undefined, null);
+  var error = ot.applyOps(snapshot, ops);
+  callback(error, snapshot);
 };
 
 function pluckIds(snapshots) {

lib/ot.js

@@ -149,3 +149,38 @@ exports.transform = function(type, op, appliedOp) {
 
   if (op.v != null) op.v++;
 };
+
+/**
+ * Apply an array of ops to the provided snapshot.
+ *
+ * @param snapshot - a Snapshot object which will be mutated by the provided ops
+ * @param ops - an array of ops to apply to the snapshot
+ * @returns an error object if applicable
+ */
+exports.applyOps = function (snapshot, ops) {
+  var type = null;
+
+  if (snapshot.type) {
+    type = types[snapshot.type];
+    if (!type) return { code: 4008, message: 'Unknown type' };
+  }
+
+  for (var index = 0; index < ops.length; index++) {
+    var op = ops[index];
+
+    snapshot.v = op.v + 1;
+
+    if (op.create) {
+      type = types[op.create.type];
+      if (!type) return { code: 4008, message: 'Unknown type' };
+      snapshot.data = type.create(op.create.data);
+      snapshot.type = type.uri;
+    } else if (op.del) {
+      snapshot.data = undefined;
+      type = null;
+      snapshot.type = null;
+    } else {
+      snapshot.data = type.apply(snapshot.data, op.op);
+    }
+  }
+};