[libc] Adds implementation for memrchr.

Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D84469

2 files changed, 124 insertions, 0 deletions

diff --git a/libc/test/src/string/CMakeLists.txt b/libc/test/src/string/CMakeLists.txt
index be43cc912b5a..a116effef271 100644
--- a/libc/test/src/string/CMakeLists.txt
+++ b/libc/test/src/string/CMakeLists.txt
@@ -82,6 +82,16 @@ add_libc_unittest(
     libc.src.string.strnlen
 )
 
+add_libc_unittest(
+  memrchr_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    memrchr_test.cpp
+  DEPENDS
+    libc.src.string.memrchr
+)
+
 # Tests all implementations that can run on the host.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)
diff --git a/libc/test/src/string/memrchr_test.cpp b/libc/test/src/string/memrchr_test.cpp
new file mode 100644
index 000000000000..5f5f7a0d0182
--- /dev/null
+++ b/libc/test/src/string/memrchr_test.cpp
@@ -0,0 +1,114 @@
+//===-- Unittests for memrchr ---------------------------------------------===//
+//
+// 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/string/memrchr.h"
+#include "utils/UnitTest/Test.h"
+#include <stddef.h>
+
+// A helper function that calls memrchr and abstracts away the explicit cast for
+// readability purposes.
+const char *call_memrchr(const void *src, int c, size_t size) {
+  return reinterpret_cast<const char *>(__llvm_libc::memrchr(src, c, size));
+}
+
+TEST(MemRChrTest, FindsCharacterAfterNullTerminator) {
+  // memrchr should continue searching after a null terminator.
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', '\0', 'b', 'c', 'd', '\0'};
+  // Should return 'b', 'c', 'd', '\0' even when after null terminator.
+  ASSERT_STREQ(call_memrchr(src, 'b', size), "bcd");
+}
+
+TEST(MemRChrTest, FindsCharacterInNonNullTerminatedCollection) {
+  const size_t size = 3;
+  const unsigned char src[size] = {'a', 'b', 'c'};
+  // Should return 'b', 'c'.
+  const char *ret = call_memrchr(src, 'b', size);
+  ASSERT_EQ(ret[0], 'b');
+  ASSERT_EQ(ret[1], 'c');
+}
+
+TEST(MemRChrTest, FindsFirstCharacter) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return original array since 'a' is the first character.
+  ASSERT_STREQ(call_memrchr(src, 'a', size), "abcde");
+}
+
+TEST(MemRChrTest, FindsMiddleCharacter) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return characters after (and including) 'c'.
+  ASSERT_STREQ(call_memrchr(src, 'c', size), "cde");
+}
+
+TEST(MemRChrTest, FindsLastCharacterThatIsNotNullTerminator) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return 'e' and null-terminator.
+  ASSERT_STREQ(call_memrchr(src, 'e', size), "e");
+}
+
+TEST(MemRChrTest, FindsNullTerminator) {
+  const size_t size = 6;
+  const unsigned char src[size] = {'a', 'b', 'c', 'd', 'e', '\0'};
+  // Should return null terminator.
+  ASSERT_STREQ(call_memrchr(src, '\0', size), "");
+}
+
+TEST(MemRChrTest, CharacterNotWithinStringShouldReturnNullptr) {
+  const size_t size = 4;
+  const unsigned char src[size] = {'1', '2', '3', '?'};
+  // Since 'z' is not within 'characters', should return nullptr.
+  ASSERT_STREQ(call_memrchr(src, 'z', size), nullptr);
+}
+
+TEST(MemRChrTest, CharacterNotWithinSizeShouldReturnNullptr) {
+  const unsigned char src[5] = {'1', '2', '3', '4', '\0'};
+  // Since '4' is not within the first 2 characters, this should return nullptr.
+  const size_t size = 2;
+  ASSERT_STREQ(call_memrchr(src, '4', size), nullptr);
+}
+
+TEST(MemRChrTest, ShouldFindLastOfDuplicates) {
+  size_t size = 12; // 11 characters + null terminator.
+  const char *dups = "abc1def1ghi";
+  // 1 is duplicated in 'dups', but it should find the last copy.
+  ASSERT_STREQ(call_memrchr(dups, '1', size), "1ghi");
+
+  const char *repeated = "XXXXX";
+  size = 6; // 5 characters + null terminator.
+  // Should return the last X with the null terminator.
+  ASSERT_STREQ(call_memrchr(repeated, 'X', size), "X");
+}
+
+TEST(MemRChrTest, EmptyStringShouldOnlyMatchNullTerminator) {
+  const size_t size = 1; // Null terminator.
+  const char *empty_string = "";
+  // Null terminator should match.
+  ASSERT_STREQ(call_memrchr(empty_string, '\0', size), "");
+  // All other characters should not match.
+  ASSERT_STREQ(call_memrchr(empty_string, 'A', size), nullptr);
+  ASSERT_STREQ(call_memrchr(empty_string, '9', size), nullptr);
+  ASSERT_STREQ(call_memrchr(empty_string, '?', size), nullptr);
+}
+
+TEST(MemRChrTest, SignedCharacterFound) {
+  char c = -1;
+  const size_t size = 1;
+  char src[size] = {c};
+  const char *actual = call_memrchr(src, c, size);
+  // Should find the last character 'c'.
+  ASSERT_EQ(actual[0], c);
+}
+
+TEST(MemRChrTest, ZeroLengthShouldReturnNullptr) {
+  const unsigned char src[4] = {'a', 'b', 'c', '\0'};
+  // This will iterate over exactly zero characters, so should return nullptr.
+  ASSERT_STREQ(call_memrchr(src, 'd', 0), nullptr);
+}