[libc] wcsncpy implementation (#142237)

Implemented wcsncpy and tests for the function.

---------

Co-authored-by: Sriya Pratipati <sriyap@google.com>

libc/test/src/wchar/CMakeLists.txt

@@ -135,6 +135,16 @@ add_libc_test(
     libc.src.wchar.wmemcpy
 )
 
+add_libc_test(
+  wcsncpy_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcsncpy_test.cpp
+  DEPENDS
+    libc.src.wchar.wcsncpy
+)
+
 add_libc_test(
   wcscat_test
   SUITE

libc/test/src/wchar/wcsncpy_test.cpp

@@ -0,0 +1,66 @@
+//===-- Unittests for wcsncpy ---------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/types/wchar_t.h"
+#include "src/wchar/wcsncpy.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcWCSNCpyTest, CopyZero) {
+  // Dest should remain unchanged.
+  wchar_t dest[3] = {L'a', L'b', L'\0'};
+  const wchar_t *src = L"x";
+  LIBC_NAMESPACE::wcsncpy(dest, src, 0);
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyFullIntoEmpty) {
+  // Dest should be the exact same as src.
+  wchar_t dest[15];
+  const wchar_t *src = L"aaaaabbbbccccc";
+  LIBC_NAMESPACE::wcsncpy(dest, src, 15);
+  for (int i = 0; i < 15; i++)
+    ASSERT_TRUE(dest[i] == src[i]);
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyPartial) {
+  // First two characters of dest should be the first two characters of src.
+  wchar_t dest[] = {L'a', L'b', L'c', L'd', L'\0'};
+  const wchar_t *src = L"1234";
+  LIBC_NAMESPACE::wcsncpy(dest, src, 2);
+  ASSERT_TRUE(dest[0] == L'1');
+  ASSERT_TRUE(dest[1] == L'2');
+  ASSERT_TRUE(dest[2] == L'c');
+  ASSERT_TRUE(dest[3] == L'd');
+  ASSERT_TRUE(dest[4] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyNullTerminator) {
+  // Null terminator should copy into dest.
+  wchar_t dest[] = {L'a', L'b', L'c', L'd', L'\0'};
+  const wchar_t src[] = {L'\0', L'y'};
+  LIBC_NAMESPACE::wcsncpy(dest, src, 1);
+  ASSERT_TRUE(dest[0] == L'\0');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'c');
+  ASSERT_TRUE(dest[3] == L'd');
+  ASSERT_TRUE(dest[4] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyPastSrc) {
+  // Copying past src should fill with null terminator.
+  wchar_t dest[] = {L'a', L'b', L'c', L'd', L'\0'};
+  const wchar_t src[] = {L'x', L'\0'};
+  LIBC_NAMESPACE::wcsncpy(dest, src, 4);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_TRUE(dest[2] == L'\0');
+  ASSERT_TRUE(dest[3] == L'\0');
+  ASSERT_TRUE(dest[4] == L'\0');
+}