Make NELEMS type-safe

Change-Id: I867271665b3c595dc40ac63e93e68c87dc213d08

native/jni/NativeFileList.mk

@@ -101,4 +101,5 @@ LATIN_IME_CORE_SRC_FILES := \
         time_keeper.cpp)
 
 LATIN_IME_CORE_TEST_FILES := \
+    defines_test.cpp \
     utils/autocorrection_threshold_utils_test.cpp

native/jni/src/defines.h

@@ -35,7 +35,13 @@
 // Must be equal to ProximityInfo.MAX_PROXIMITY_CHARS_SIZE in Java
 #define MAX_PROXIMITY_CHARS_SIZE 16
 #define ADDITIONAL_PROXIMITY_CHAR_DELIMITER_CODE 2
-#define NELEMS(x) (sizeof(x) / sizeof((x)[0]))
+
+// TODO: Use size_t instead of int.
+// Disclaimer: You will see a compile error if you use this macro against a variable-length array.
+// Sorry for the inconvenience. It isn't supported.
+template <typename T, int N>
+char (&ArraySizeHelper(T (&array)[N]))[N];
+#define NELEMS(x) (sizeof(ArraySizeHelper(x)))
 
 AK_FORCE_INLINE static int intArrayToCharArray(const int *const source, const int sourceSize,
         char *dest, const int destSize) {

native/jni/tests/defines_test.cpp

@@ -0,0 +1,34 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "defines.h"
+
+#include <gtest/gtest.h>
+
+namespace latinime {
+namespace {
+
+TEST(DefinesTest, NELEMSForFixedLengthArray) {
+    const size_t SMALL_ARRAY_SIZE = 1;
+    const size_t LARGE_ARRAY_SIZE = 100;
+    int smallArray[SMALL_ARRAY_SIZE];
+    int largeArray[LARGE_ARRAY_SIZE];
+    EXPECT_EQ(SMALL_ARRAY_SIZE, NELEMS(smallArray));
+    EXPECT_EQ(LARGE_ARRAY_SIZE, NELEMS(largeArray));
+}
+
+}  // namespace
+}  // namespace latinime