add bsearch for arrays

Arrays can already be sorted with json_object_array_sort() which uses
qsort() of the standard C library. This adds a counterpart using the
bsearch() from C.

arraylist.c

@@ -91,8 +91,14 @@ array_list_add(struct array_list *arr, void *data)
 void
 array_list_sort(struct array_list *arr, int(*sort_fn)(const void *, const void *))
 {
-  qsort(arr->array, arr->length, sizeof(arr->array[0]),
+  qsort(arr->array, arr->length, sizeof(arr->array[0]), sort_fn);
-	(int (*)(const void *, const void *))sort_fn);
+}
+
+void* array_list_bsearch( const void **key, struct array_list *arr,
+		int (*sort_fn)(const void *, const void *) )
+{
+	return bsearch( key, arr->array, arr->length, sizeof(arr->array[0]),
+			sort_fn );
 }
 
 int

arraylist.h

@@ -49,6 +49,11 @@ array_list_length(struct array_list *al);
 extern void
 array_list_sort(struct array_list *arr, int(*compar)(const void *, const void *));
 
+extern void* array_list_bsearch( const void **key,
+		struct array_list *arr,
+		int (*sort_fn)(const void *, const void *) );
+
+
 #ifdef __cplusplus
 }
 #endif

json_object.c

@@ -889,6 +889,23 @@ void json_object_array_sort(struct json_object *jso, int(*sort_fn)(const void *,
 	array_list_sort(jso->o.c_array, sort_fn);
 }
 
+struct json_object* json_object_array_bsearch(
+		const struct json_object *key,
+		const struct json_object *jso,
+		int (*sort_fn)(const void *, const void *) )
+{
+	struct json_object **result;
+
+	result = (struct json_object **) array_list_bsearch(
+			(const void **) &key, jso->o.c_array, sort_fn );
+
+	if ( result == NULL ) {
+		return NULL;
+	} else {
+		return *result;
+	}
+}
+
 int json_object_array_length(struct json_object *jso)
 {
 	return array_list_length(jso->o.c_array);

json_object.h

@@ -402,6 +402,25 @@ extern int json_object_array_length(struct json_object *obj);
 */
 extern void json_object_array_sort(struct json_object *jso, int(*sort_fn)(const void *, const void *));
 
+/** Binary search a sorted array for a specified key object.
+ *
+ * It depends on your compare function what's sufficient as a key.
+ * Usually you create some dummy object with the parameter compared in
+ * it, to identify the right item you're actually looking for.
+ *
+ * @see json_object_array_sort() for hints on the compare function.
+ *
+ * @param key a dummy json_object with the right key
+ * @param jso the array object we're searching
+ * @param sort_fn the sort/compare function
+ *
+ * @return the wanted json_object instance
+ */
+extern struct json_object* json_object_array_bsearch(
+		const struct json_object *key,
+		const struct json_object *jso,
+		int (*sort_fn)(const void *, const void *) );
+
 /** Add an element to the end of a json_object of type json_type_array
  *
  * The reference count will *not* be incremented. This is to make adding