leverage reflect.MakeMapWithSize introduced in go1.9

decode.go

@@ -669,8 +669,7 @@ func objectIntoStruct(structPtr reflect.Value) func(*ObjectDecoder) error {
 func objectIntoMap(mapPtr reflect.Value) func(*ObjectDecoder) error {
 	return func(o *ObjectDecoder) error {
 		mapValue := mapPtr.Elem()
-		//TODO go1.9 - MakeMapWithSize
-		mapValue.Set(reflect.MakeMap(mapValue.Type()))
+		mapValue.Set(makeMap(mapValue.Type(), o.Len))
 		elemType := mapValue.Type().Elem()
 
 		for o.NextEntry() {
@@ -697,8 +696,7 @@ func objectAsInterface(o *ObjectDecoder) (interface{}, error) {
 	valType := elementTypeFor(o.ValType)
 	mapType := reflect.MapOf(stringType, valType)
 
-	//TODO go1.9 - MakeMapWithSize
-	mapValue := reflect.MakeMap(mapType)
+	mapValue := makeMap(mapType, o.Len)
 	for o.NextEntry() {
 		k, err := o.DecodeKey()
 		if err != nil {

decode_go18.go

@@ -0,0 +1,9 @@
+// +build !go1.9
+
+package ubjson
+
+import "reflect"
+
+func makeMap(typ reflect.Type, _ int) reflect.Value {
+	return reflect.MakeMap(typ)
+}

decode_go19.go

@@ -0,0 +1,12 @@
+// +build go1.9
+
+package ubjson
+
+import "reflect"
+
+func makeMap(typ reflect.Type, cap int) reflect.Value {
+	if cap < 0 {
+		cap = 0
+	}
+	return reflect.MakeMapWithSize(typ, cap)
+}