Add mapping function to get type from source and layer

helper/type_mapping.js

@@ -76,11 +76,25 @@ var LAYER_ALIASES = {
 
 var LAYER_WITH_ALIASES_TO_TYPE = extend({}, LAYER_ALIASES, LAYER_TO_TYPE);
 
+/**
+ * Calculate the set-style intersection of two arrays
+ */
+var intersection = function intersection(set1, set2) {
+  return set2.filter(function(value) {
+    return set1.indexOf(value) !== -1;
+  });
+};
+
+var sourceAndLayerToType = function sourceAndLayerToType(source, layer) {
+  return intersection(SOURCE_TO_TYPE[source], LAYER_WITH_ALIASES_TO_TYPE[layer]);
+};
+
 module.exports = {
   types_list: ALL_TYPES,
   type_to_source: TYPE_TO_SOURCE,
   type_to_layer: TYPE_TO_LAYER,
   source_to_type: SOURCE_TO_TYPE,
   layer_to_type: LAYER_TO_TYPE,
-  layer_with_aliases_to_type: LAYER_WITH_ALIASES_TO_TYPE
+  layer_with_aliases_to_type: LAYER_WITH_ALIASES_TO_TYPE,
+  source_and_layer_to_type: sourceAndLayerToType
 };