[libc] wcscat implementation (#142243)

Implemented wcscat and tests.

---------

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(
+  wcscat_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcscat_test.cpp
+  DEPENDS
+    libc.src.wchar.wcscat
+)
+  
 add_libc_test(
   wcsstr_test
   SUITE

libc/test/src/wchar/wcscat_test.cpp

@@ -0,0 +1,47 @@
+//===-- 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/wcscat.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcWCSCatTest, EmptyDest) {
+  // Dest should be fully replaced with src.
+  wchar_t dest[4] = {L'\0'};
+  const wchar_t *src = L"abc";
+  LIBC_NAMESPACE::wcscat(dest, src);
+  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(LlvmLibcWCSCatTest, NonEmptyDest) {
+  // Src should be appended on to dest.
+  wchar_t dest[7] = {L'x', L'y', L'z', L'\0'};
+  const wchar_t *src = L"abc";
+  LIBC_NAMESPACE::wcscat(dest, src);
+  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');
+}
+
+TEST(LlvmLibcWCSCatTest, EmptySrc) {
+  // Dest should remain intact.
+  wchar_t dest[4] = {L'x', L'y', L'z', L'\0'};
+  const wchar_t *src = L"";
+  LIBC_NAMESPACE::wcscat(dest, src);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'y');
+  ASSERT_TRUE(dest[2] == L'z');
+  ASSERT_TRUE(dest[3] == L'\0');
+}