index_idmap.go: go binding added

IndexIDMap was missing from the go bindings, even though is is important
to wrap a flat index in if addWithID functionality is required.

index_idmap.go

@@ -0,0 +1,26 @@
+package faiss
+
+/*
+#include <faiss/c_api/MetaIndexes_c.h>
+*/
+import "C"
+import (
+	"errors"
+)
+
+type IndexIDMapWrapper struct {
+	Index
+	cpointer **C.FaissIndex
+}
+
+func NewIndexIDMap(index *IndexFlat) (*IndexIDMapWrapper, error) {
+	var indexMapPointer *C.FaissIndexIDMap
+	var pointerToIndexMapPointer **C.FaissIndexIDMap
+	pointerToIndexMapPointer = &indexMapPointer
+	wrapper := IndexIDMapWrapper{cpointer: &indexMapPointer}
+	if C.faiss_IndexIDMap_new(pointerToIndexMapPointer, index.cPtr()) != 0 {
+		return nil, errors.New("Error occurred while initializing IndexIDMapWrapper")
+	}
+	wrapper.Index = &faissIndex{idx: *wrapper.cpointer}
+	return &wrapper, nil
+}