[util] Handle the negative zero value in the NumericHash

Two equal numbers must produce the same hash value.
35bb6ef6de3ccbb3b5cacd09950a21063814b499

2 files changed, 30 insertions, 0 deletions

diff --git a/util/digest/numeric.h b/util/digest/numeric.h
index 1e9c74ebbb..791bce55a9 100644
--- a/util/digest/numeric.h
+++ b/util/digest/numeric.h
@@ -3,6 +3,8 @@
 #include <util/generic/typelist.h>
 #include <util/system/defaults.h>
 
+#include <type_traits>
+
 /*
  * original url (now dead): http://www.cris.com/~Ttwang/tech/inthash.htm
  * copy: https://gist.github.com/badboy/6267743
@@ -70,6 +72,11 @@ static constexpr T IntHash(T t) noexcept {
  */
 template <class T>
 static constexpr size_t NumericHash(T t) noexcept {
+    if constexpr (std::is_floating_point<T>::value) {
+        if (t == T(0)) { // the negative zero is equal to the positive zero, but has different bitwise representation
+            t = T(0);    // make sure that the hash will be the same for both kind of zeros
+        }
+    }
     using TCvt = TFixedWidthUnsignedInt<T>;
 
     union Y_HIDDEN {
diff --git a/util/generic/hash_ut.cpp b/util/generic/hash_ut.cpp
index f8bf4e8eac..78fc5657ce 100644
--- a/util/generic/hash_ut.cpp
+++ b/util/generic/hash_ut.cpp
@@ -62,6 +62,7 @@ class THashTest: public TTestBase {
     UNIT_TEST(TestHSetInsertInitializerList);
     UNIT_TEST(TestTupleHash);
     UNIT_TEST(TestStringHash);
+    UNIT_TEST(TestFloatingPointHash);
     UNIT_TEST_SUITE_END();
 
     using hmset = THashMultiSet<char, hash<char>, TEqualTo<char>>;
@@ -115,6 +116,7 @@ protected:
     void TestHSetInsertInitializerList();
     void TestTupleHash();
     void TestStringHash();
+    void TestFloatingPointHash();
 };
 
 UNIT_TEST_SUITE_REGISTRATION(THashTest);
@@ -1334,3 +1336,24 @@ void THashTest::TestStringHash() {
     UNIT_ASSERT_VALUES_EQUAL(ComputeHash(TStringBuf("hehe")), expected);  // TStringBuf
     UNIT_ASSERT_VALUES_EQUAL(ComputeHash<const char*>("hehe"), expected); // const char*
 }
+
+template <class TFloat>
+static void TestFloatingPointHashImpl() {
+    const TFloat f = 0;
+    Y_ASSERT(f == -f);
+    THashSet<TFloat> set;
+    set.insert(f);
+    UNIT_ASSERT_C(set.contains(-f), TypeName<TFloat>());
+    UNIT_ASSERT_VALUES_EQUAL_C(ComputeHash(f), ComputeHash(-f), TypeName<TFloat>());
+    for (int i = 0; i < 5; ++i) {
+        set.insert(-TFloat(i));
+        set.insert(+TFloat(i));
+    }
+    UNIT_ASSERT_VALUES_EQUAL_C(set.size(), 9, TypeName<TFloat>());
+}
+
+void THashTest::TestFloatingPointHash() {
+    TestFloatingPointHashImpl<float>();
+    TestFloatingPointHashImpl<double>();
+    // TestFloatingPointHashImpl<long double>();
+}