[libc] wcsncat implementation (#142431)

Implemented wcsncat and tests.

---------

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

libc/test/src/wchar/CMakeLists.txt

@@ -125,6 +125,16 @@ add_libc_test(
     libc.src.wchar.wmemcpy
 )
 
+add_libc_test(
+  wcsncat_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcsncat_test.cpp
+  DEPENDS
+    libc.src.wchar.wcsncat
+)
+
 add_libc_test(
   wcscpy_test
   SUITE

libc/test/src/wchar/wcsncat_test.cpp

@@ -0,0 +1,82 @@
+//===-- Unittests for wcscat ---------------------------------------------===//
+//
+// 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/wcsncat.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcWCSNCatTest, EmptyDest) {
+  wchar_t dest[4] = {L'\0'};
+  const wchar_t *src = L"abc";
+
+  // Start by copying nothing
+  LIBC_NAMESPACE::wcsncat(dest, src, 0);
+  ASSERT_TRUE(dest[0] == L'\0');
+
+  // Copying part of it.
+  LIBC_NAMESPACE::wcsncat(dest, src, 1);
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'\0');
+
+  // Resetting for the last test.
+  dest[0] = '\0';
+
+  // Copying all of it.
+  LIBC_NAMESPACE::wcsncat(dest, src, 3);
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'c');
+  ASSERT_TRUE(dest[3] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCatTest, NonEmptyDest) {
+  wchar_t dest[7] = {L'x', L'y', L'z', L'\0'};
+  const wchar_t *src = L"abc";
+
+  // Adding on only part of the string
+  LIBC_NAMESPACE::wcsncat(dest, src, 1);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'y');
+  ASSERT_TRUE(dest[2] == L'z');
+  ASSERT_TRUE(dest[3] == L'a');
+  ASSERT_TRUE(dest[4] == L'\0');
+
+  // Copying more without resetting
+  LIBC_NAMESPACE::wcsncat(dest, src, 2);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'y');
+  ASSERT_TRUE(dest[2] == L'z');
+  ASSERT_TRUE(dest[3] == L'a');
+  ASSERT_TRUE(dest[4] == L'a');
+  ASSERT_TRUE(dest[5] == L'b');
+  ASSERT_TRUE(dest[6] == L'\0');
+
+  // Setting end marker to make sure it overwrites properly.
+  dest[3] = L'\0';
+
+  // Copying all of it.
+  LIBC_NAMESPACE::wcsncat(dest, src, 3);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'y');
+  ASSERT_TRUE(dest[2] == L'z');
+  ASSERT_TRUE(dest[3] == L'a');
+  ASSERT_TRUE(dest[4] == L'b');
+  ASSERT_TRUE(dest[5] == L'c');
+  ASSERT_TRUE(dest[6] == L'\0');
+
+  // Check that copying still works when count > src length.
+  dest[0] = L'\0';
+  // And that it doesn't write beyond what is necessary.
+  dest[4] = L'Z';
+  LIBC_NAMESPACE::wcsncat(dest, src, 4);
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'c');
+  ASSERT_TRUE(dest[3] == L'\0');
+  ASSERT_TRUE(dest[4] == L'Z');
+}