introduce hash_combine (#39511)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/39511

changelog: [internal]

C++ standard library doesn't offer a way to combine hashes together. This is implementation for uses in React Native code.

Reviewed By: javache

Differential Revision: D49349539

fbshipit-source-id: 1e1f47585969181d5910f5bbc5b6ebfffa12e96b

packages/react-native/ReactCommon/react/utils/hash_combine.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <functional>
+
+namespace facebook::react {
+
+template <
+    typename T,
+    typename... Rest,
+    bool Enabled = !std::is_same<T, const char*>::value,
+    typename = typename std::enable_if<Enabled>::type>
+void hash_combine(std::size_t& seed, const T& v, const Rest&... rest) {
+  seed ^= std::hash<T>{}(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
+  (hash_combine(seed, rest), ...);
+}
+
+template <typename T, typename... Args>
+std::size_t hash_combine(const T& v, const Args&... args) {
+  std::size_t seed = 0;
+  hash_combine<T, Args...>(seed, v, args...);
+  return seed;
+}
+
+} // namespace facebook::react

packages/react-native/ReactCommon/react/utils/tests/hash_combineTests.cpp

@@ -0,0 +1,82 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <gtest/gtest.h>
+#include <react/utils/hash_combine.h>
+
+struct Person {
+  std::string firstName;
+  std::string lastName;
+};
+
+namespace std {
+template <>
+struct hash<Person> {
+  size_t operator()(const Person& person) const {
+    return facebook::react::hash_combine(person.firstName, person.lastName);
+  }
+};
+} // namespace std
+
+namespace facebook::react {
+
+TEST(hash_combineTests, testIntegerTemplating) {
+  std::size_t seed = 0;
+  hash_combine(seed, 1);
+
+  auto hashedValue = hash_combine(1);
+  EXPECT_EQ(hashedValue, seed);
+
+  EXPECT_NE(hash_combine(1), hash_combine(2));
+}
+
+TEST(hash_combineTests, testIntegerCombinationsHashing) {
+  std::size_t seed = 0;
+  hash_combine(seed, 1, 2);
+
+  auto hashedValue = hash_combine(1, 2);
+  EXPECT_EQ(hashedValue, seed);
+
+  EXPECT_NE(hash_combine(1, 2), hash_combine(2, 1));
+}
+
+TEST(hash_combineTests, testContiniousIntegerHashing) {
+  std::size_t seed = 0;
+
+  for (int i = 1; i <= 200; ++i) {
+    auto previousSeed = seed;
+    hash_combine(seed, i);
+    EXPECT_NE(seed, previousSeed);
+  }
+}
+
+TEST(hash_combineTests, testStrings) {
+  std::size_t seed = 0;
+  hash_combine<std::string>(seed, "react");
+
+  auto hashedValue = hash_combine<std::string>("react");
+  EXPECT_EQ(hashedValue, seed);
+
+  EXPECT_NE(
+      hash_combine<std::string>("react"),
+      hash_combine<std::string>("react native"));
+}
+
+TEST(hash_combineTests, testCustomTypes) {
+  auto person1 = Person{"John", "Doe"};
+  auto person2 = Person{"Jane", "Doe"};
+
+  std::size_t seed = 0;
+  hash_combine(seed, person1);
+
+  auto hashedValue = hash_combine(person1);
+  EXPECT_EQ(hashedValue, seed);
+
+  EXPECT_NE(hash_combine(person1), hash_combine(person2));
+}
+
+} // namespace facebook::react