[libc] wcsncat implementation

libc/config/linux/x86_64/entrypoints.txt

@@ -371,6 +371,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.wchar.wmemcmp
     libc.src.wchar.wmempcpy
     libc.src.wchar.wmemcpy
+    libc.src.wchar.wcsncat
     libc.src.wchar.wcscpy
 
     # sys/uio.h entrypoints

libc/include/wchar.yaml

@@ -80,6 +80,14 @@ functions:
       - type: __restrict wchar_t *
       - type: const __restrict wchar_t *
       - type: size_t
+  - name: wcsncat
+    standards:
+      - stdc
+    return_type: wchar_t *
+    arguments: 
+      - type: __restrict wchar_t *
+      - type: const __restrict wchar_t *
+      - type: size_t
   - name: wcscpy
     standards:
       - stdc

libc/src/wchar/CMakeLists.txt

@@ -105,6 +105,18 @@ add_entrypoint_object(
     libc.src.__support.wctype_utils
 )
 
+add_entrypoint_object(
+  wcsncat
+  SRCS
+    wcsncat.cpp
+  HDRS
+    wcsncat.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.hdr.wchar_macros
+    libc.src.string.string_utils
+)
+
 add_entrypoint_object(
   wmempcpy
   SRCS

libc/src/wchar/wcsncat.cpp

@@ -0,0 +1,31 @@
+//===-- Implementation of wcsncat -----------------------------------------===//
+//
+// 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 "src/wchar/wcsncat.h"
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/string/string_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(wchar_t *, wcsncat,
+                   (wchar_t *__restrict s1, const wchar_t *__restrict s2,
+                    size_t n)) {
+  size_t size = internal::string_length(s1);
+  size_t i = 0;
+  for (; s2[i] && i < n; ++i)
+    s1[size + i] = s2[i];
+  // Appending null character to the end of the result.
+  s1[size + i] = L'\0';
+  return s1;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/wchar/wcsncat.h

@@ -0,0 +1,23 @@
+//===-- Implementation header for wcsncat ---------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_WCHAR_WCSNCAT_H
+#define LLVM_LIBC_SRC_WCHAR_WCSNCAT_H
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+wchar_t *wcsncat(wchar_t *__restrict s1, const wchar_t *__restrict s2,
+                 size_t n);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_WCHAR_WCSNCAT_H

libc/test/src/wchar/CMakeLists.txt

@@ -105,6 +105,16 @@ 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');
+}