From d17b4aa60068a6b0f082ec8e483d90d6e8a216b5 Mon Sep 17 00:00:00 2001
From: stgatilov <stgatilov@gmail.com>
Date: Sat, 11 May 2024 12:10:26 +0200
Subject: [PATCH 1/3] Added unit test test_hash_function_default_integer
 against bad hash function on uint64_t.

It constructs a set of distinct keys with zero lower half, then computes hash function for all of them, and checks that the hashes are mostly different in lower bits.
---
 tests/robin_set_tests.cpp | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/tests/robin_set_tests.cpp b/tests/robin_set_tests.cpp
index e68b514..68ac68f 100644
--- a/tests/robin_set_tests.cpp
+++ b/tests/robin_set_tests.cpp
@@ -171,4 +171,23 @@ BOOST_AUTO_TEST_CASE(test_erase_fast) {
   BOOST_CHECK(set.size() == 0);
 }
 
+BOOST_AUTO_TEST_CASE(test_hash_function_default_integer) {
+  // Brief quality check for default integer hash function.
+  // Most importantly, make sure it is not identity function like std::hash on GCC.
+  // See: https://github.com/Tessil/robin-map/issues/73
+  const std::size_t nb_values = 1 << 18;
+  const std::size_t shift = 32;
+
+  using Set = tsl::robin_set<uint64_t>;
+  Set set;
+
+  for (uint64_t i = 0; i < nb_values; i++) {
+    uint64_t hash = set.hash_function()(uint64_t(i) << shift);
+    set.insert(hash & 0xFFFFFF);
+  }
+
+  double imageRatio = double(set.size()) / nb_values;
+  BOOST_CHECK(imageRatio >= 0.9);
+}
+
 BOOST_AUTO_TEST_SUITE_END()

From ff5adb9bbe1bc1d3df0e7c66eaa1f94d0d1f521a Mon Sep 17 00:00:00 2001
From: stgatilov <stgatilov@gmail.com>
Date: Sat, 11 May 2024 16:06:07 +0200
Subject: [PATCH 2/3] Added tsl::hash<T> wrapper which adds MurmurHash2
 finalizer in case libstd-c++ or libc++ is used.

---
 include/tsl/robin_growth_policy.h | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/include/tsl/robin_growth_policy.h b/include/tsl/robin_growth_policy.h
index b100bfc..6cb6b81 100644
--- a/include/tsl/robin_growth_policy.h
+++ b/include/tsl/robin_growth_policy.h
@@ -413,4 +413,35 @@ class prime_growth_policy {
 }  // namespace rh
 }  // namespace tsl
 
+namespace tsl {
+
+/**
+ * Wrapper around std::hash which adds hash finalizer missing in libstd-c++ and
+ * libc++. It should be used with power_of_two_growth_policy.
+ */
+template <class Key>
+struct hash : public std::hash<Key> {
+  std::size_t operator()(const Key& key) const {
+    std::size_t h = std::hash<Key>::operator()(key);
+
+#if defined(__GLIBCXX__) || defined(_LIBCPP_VERSION)
+#if SIZE_MAX >= ULLONG_MAX
+    // 64-bit finalizer from MurmurHash2
+    h ^= h >> 47;
+    h *= 0xc6a4a7935bd1e995ull;
+    h ^= h >> 47;
+#else
+    // 32-bit finalizer from MurmurHash2
+    h ^= h >> 13;
+    h *= 0x5bd1e995u;
+    h ^= h >> 15;
+#endif
+#endif
+
+    return h;
+  }
+};
+
+}  // namespace tsl
+
 #endif

From cd34e3e8af2afd1f41da49d3ac03d0d968ff5dfc Mon Sep 17 00:00:00 2001
From: stgatilov <stgatilov@gmail.com>
Date: Sat, 11 May 2024 16:06:39 +0200
Subject: [PATCH 3/3] Replaced default hash function from std::hash to
 tsl::hash everywhere.

---
 include/tsl/robin_hash.h | 2 +-
 include/tsl/robin_map.h  | 4 ++--
 include/tsl/robin_set.h  | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/tsl/robin_hash.h b/include/tsl/robin_hash.h
index 03d23a9..d60b21a 100644
--- a/include/tsl/robin_hash.h
+++ b/include/tsl/robin_hash.h
@@ -403,7 +403,7 @@ class robin_hash : private Hash, private KeyEqual, private GrowthPolicy {
         is_power_of_two_policy<GrowthPolicy>::value) &&
        // Don't store the hash for primitive types with default hash.
        (!std::is_arithmetic<key_type>::value ||
-        !std::is_same<Hash, std::hash<key_type>>::value));
+        !std::is_same<Hash, tsl::hash<key_type>>::value));
 
   /**
    * Only use the stored hash on lookup if we are explicitly asked. We are not
diff --git a/include/tsl/robin_map.h b/include/tsl/robin_map.h
index b594810..1ce02f8 100644
--- a/include/tsl/robin_map.h
+++ b/include/tsl/robin_map.h
@@ -83,7 +83,7 @@ namespace tsl {
  * insert, invalidate the iterators.
  *  - erase: always invalidate the iterators.
  */
-template <class Key, class T, class Hash = std::hash<Key>,
+template <class Key, class T, class Hash = tsl::hash<Key>,
           class KeyEqual = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<Key, T>>,
           bool StoreHash = false,
@@ -803,7 +803,7 @@ class robin_map {
  * Same as `tsl::robin_map<Key, T, Hash, KeyEqual, Allocator, StoreHash,
  * tsl::rh::prime_growth_policy>`.
  */
-template <class Key, class T, class Hash = std::hash<Key>,
+template <class Key, class T, class Hash = tsl::hash<Key>,
           class KeyEqual = std::equal_to<Key>,
           class Allocator = std::allocator<std::pair<Key, T>>,
           bool StoreHash = false>
diff --git a/include/tsl/robin_set.h b/include/tsl/robin_set.h
index e115007..03c9c78 100644
--- a/include/tsl/robin_set.h
+++ b/include/tsl/robin_set.h
@@ -83,7 +83,7 @@ namespace tsl {
  * insert, invalidate the iterators.
  *  - erase: always invalidate the iterators.
  */
-template <class Key, class Hash = std::hash<Key>,
+template <class Key, class Hash = tsl::hash<Key>,
           class KeyEqual = std::equal_to<Key>,
           class Allocator = std::allocator<Key>, bool StoreHash = false,
           class GrowthPolicy = tsl::rh::power_of_two_growth_policy<2>>
@@ -657,7 +657,7 @@ class robin_set {
  * Same as `tsl::robin_set<Key, Hash, KeyEqual, Allocator, StoreHash,
  * tsl::rh::prime_growth_policy>`.
  */
-template <class Key, class Hash = std::hash<Key>,
+template <class Key, class Hash = tsl::hash<Key>,
           class KeyEqual = std::equal_to<Key>,
           class Allocator = std::allocator<Key>, bool StoreHash = false>
 using robin_pg_set = robin_set<Key, Hash, KeyEqual, Allocator, StoreHash,