Changing type mapping cache to be a dictionary

src/compiler/checker.ts

@@ -3869,14 +3869,13 @@ module ts {
             // If this type has already been instantiated using this mapper, returned the cached result. This guards against
             // infinite instantiations of cyclic types, e.g. "var x: { a: T, b: typeof x };"
             if (mapper.mappings) {
-                for (let mapping of mapper.mappings) {
-                    if (mapping.type === type) {
-                        return mapping.result;
-                    }
+                let cached = <ObjectType>mapper.mappings[type.id];
+                if (cached) {
+                    return cached;
                 }
             }
             else {
-                mapper.mappings = [];
+                mapper.mappings = {};
             }
             // Instantiate the given type using the given mapper and cache the result
             let result = <ResolvedType>createObjectType(TypeFlags.Anonymous, type.symbol);
@@ -3888,7 +3887,7 @@ module ts {
             let numberIndexType = getIndexTypeOfType(type, IndexKind.Number);
             if (stringIndexType) result.stringIndexType = instantiateType(stringIndexType, mapper);
             if (numberIndexType) result.numberIndexType = instantiateType(numberIndexType, mapper);
-            mapper.mappings.push({ type, result });
+            mapper.mappings[type.id] = result;
             return result;
         }
 

src/compiler/types.ts

@@ -1586,15 +1586,10 @@ module ts {
         Number,
     }
 
-    export interface TypeMapping {
-        type: Type;
-        result: Type;
-    }
-
     /* @internal */
     export interface TypeMapper {
         (t: TypeParameter): Type;
-        mappings?: TypeMapping[];  // Type mapping cache
+        mappings?: Map<Type>;  // Type mapping cache
     }
 
     /* @internal */