Fix issue #158 - imperfect hash for tuple of small integers

CMakeLists.txt

@@ -145,6 +145,7 @@ if (PHMAP_BUILD_EXAMPLES)
     target_include_directories(ex_serialize PUBLIC $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/../cereal/include>)
     add_executable(ex_hash_std examples/hash_std.cc phmap.natvis)
     add_executable(ex_hash_value examples/hash_value.cc phmap.natvis)
+    add_executable(ex_hash examples/hash.cc phmap.natvis)
     add_executable(ex_two_files examples/f1.cc examples/f2.cc phmap.natvis)
     add_executable(ex_insert_bench examples/insert_bench.cc phmap.natvis)
     add_executable(ex_knucleotide examples/knucleotide.cc phmap.natvis)

parallel_hashmap/phmap_utils.h

@@ -286,6 +286,13 @@ struct Hash<double> : public phmap_unary_function<double, size_t>
 
 #endif
 
+#if defined(_MSC_VER)
+#   define PHMAP_HASH_ROTL32(x, r) _rotl(x,r)
+#else
+#   define PHMAP_HASH_ROTL32(x, r) (x << r) | (x >> (32 - r))
+#endif
+
+
 template <class H, int sz> struct Combiner
 {
     H operator()(H seed, size_t value);
@@ -293,17 +300,57 @@ template <class H, int sz> struct Combiner
 
 template <class H> struct Combiner<H, 4>
 {
-    H operator()(H seed, size_t value)
+    H operator()(H h1, size_t k1)
     {
-        return seed ^ (value + 0x9e3779b9 + (seed << 6) + (seed >> 2));
+#if 1
+        // Copyright 2005-2014 Daniel James.
+        // Distributed under the Boost Software License, Version 1.0. (See accompanying
+        // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+        
+        const uint32_t c1 = 0xcc9e2d51;
+        const uint32_t c2 = 0x1b873593;
+
+        k1 *= c1;
+        k1 = PHMAP_HASH_ROTL32(k1,15);
+        k1 *= c2;
+
+        h1 ^= k1;
+        h1 = PHMAP_HASH_ROTL32(h1,13);
+        h1 = h1*5+0xe6546b64;
+
+        return h1;
+#else
+        return h1 ^ (k1 + 0x9e3779b9 + (h1 << 6) + (h1 >> 2));
+#endif
     }
 };
 
 template <class H> struct Combiner<H, 8>
 {
-    H operator()(H seed, size_t value)
+    H operator()(H h, size_t k)
     {
-        return seed ^ (value + size_t(0xc6a4a7935bd1e995) + (seed << 6) + (seed >> 2));
+#if 1
+        // Copyright 2005-2014 Daniel James.
+        // Distributed under the Boost Software License, Version 1.0. (See accompanying
+        // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+        const uint64_t m = (uint64_t(0xc6a4a793) << 32) + 0x5bd1e995;
+        const int r = 47;
+
+        k *= m;
+        k ^= k >> r;
+        k *= m;
+
+        h ^= k;
+        h *= m;
+
+        // Completely arbitrary number, to prevent 0's
+        // from hashing to 0.
+        h += 0xe6546b64;
+
+        return h;
+#else
+        return h ^ (k + size_t(0xc6a4a7935bd1e995) + (h << 6) + (h >> 2));
+#endif
     }
 };
 
@@ -347,21 +394,22 @@ struct Hash<std::pair<T1, T2>> {
 template<class... T> 
 struct Hash<std::tuple<T...>> {
     size_t operator()(std::tuple<T...> const& t) const noexcept {
-        return _hash_helper(t);
+        size_t seed = 0;
+        return _hash_helper(seed, t);
     }
 
 private:
-    template<size_t I = 0, class ...P>
-        typename std::enable_if<I == sizeof...(P), size_t>::type
-        _hash_helper(const std::tuple<P...> &) const noexcept { return 0; }
+    template<size_t I = 0, class TUP>
+    typename std::enable_if<I == std::tuple_size<TUP>::value, size_t>::type
+    _hash_helper(size_t seed, const TUP &) const noexcept { return seed; }
 
-    template<size_t I = 0, class ...P>
-    typename std::enable_if<I < sizeof...(P), size_t>::type
-    _hash_helper(const std::tuple<P...> &t) const noexcept {
+    template<size_t I = 0, class TUP>
+    typename std::enable_if<I < std::tuple_size<TUP>::value, size_t>::type
+    _hash_helper(size_t seed, const TUP &t) const noexcept {
         const auto &el = std::get<I>(t);
         using el_type = typename std::remove_cv<typename std::remove_reference<decltype(el)>::type>::type;
-        return Combiner<size_t, sizeof(size_t)>()(
-            phmap::Hash<el_type>()(el),  _hash_helper<I + 1>(t));
+        seed = Combiner<size_t, sizeof(size_t)>()(seed, phmap::Hash<el_type>()(el));
+        return _hash_helper<I + 1>(seed, t);
     }
 };