From bf86730a6288c75c714ccc854e80159e526aee9c Mon Sep 17 00:00:00 2001
From: Peter Dimov <pdimov@gmail.com>
Date: Tue, 18 Jan 2022 21:31:53 +0200
Subject: [PATCH] Add mix32_policy

---
 .../boost/unordered/detail/implementation.hpp | 44 +++++++++++++++++++
 test/unordered/mix_policy.cpp                 | 19 ++++++++
 2 files changed, 63 insertions(+)

diff --git a/include/boost/unordered/detail/implementation.hpp b/include/boost/unordered/detail/implementation.hpp
index 2de4c9a8..d99debc8 100644
--- a/include/boost/unordered/detail/implementation.hpp
+++ b/include/boost/unordered/detail/implementation.hpp
@@ -2035,6 +2035,45 @@ namespace boost {
         }
       };
 
+      template <typename SizeT> struct mix32_policy
+      {
+        template <typename Hash, typename T>
+        static inline SizeT apply_hash(Hash const& hf, T const& x)
+        {
+          SizeT key = hf(x);
+
+          // "Integer Hash Function", Thomas Wang, 1997
+          // https://gist.github.com/badboy/6267743
+          // http://web.archive.org/web/20071223173210/http://www.concentric.net/~Ttwang/tech/inthash.htm
+
+          key = ~key + (key << 15); // key = (key << 15) - key - 1;
+          key = key ^ (key >> 12);
+          key = key + (key << 2);
+          key = key ^ (key >> 4);
+          key = key * 2057; // key = (key + (key << 3)) + (key << 11);
+          key = key ^ (key >> 16);
+          return key;
+        }
+
+        static inline SizeT to_bucket(SizeT bucket_count, SizeT hash)
+        {
+          BOOST_ASSERT( boost::core::has_single_bit( bucket_count ) );
+          return hash & (bucket_count - 1);
+        }
+
+        static inline SizeT new_bucket_count(SizeT min)
+        {
+          if (min <= 4)
+            return 4;
+          return boost::core::bit_ceil(min);
+        }
+
+        static inline SizeT prev_bucket_count(SizeT max)
+        {
+          return boost::core::bit_floor(max);
+        }
+      };
+
       template <int digits, int radix> struct pick_policy_impl
       {
         typedef prime_policy<std::size_t> type;
@@ -2045,6 +2084,11 @@ namespace boost {
         typedef mix64_policy<std::size_t> type;
       };
 
+      template <> struct pick_policy_impl<32, 2>
+      {
+        typedef mix32_policy<std::size_t> type;
+      };
+
       template <typename T>
       struct pick_policy2
         : pick_policy_impl<std::numeric_limits<std::size_t>::digits,
diff --git a/test/unordered/mix_policy.cpp b/test/unordered/mix_policy.cpp
index 87cd7b8c..15308a1f 100644
--- a/test/unordered/mix_policy.cpp
+++ b/test/unordered/mix_policy.cpp
@@ -42,5 +42,24 @@ int main()
         }
     }
 
+    {
+        typedef boost::uint32_t SizeT;
+        typedef boost::unordered::detail::mix32_policy<SizeT> policy;
+
+        for( SizeT i = 1; i < 200; ++i )
+        {
+            test<policy>( i );
+        }
+
+        for( int i = 8; i < 32; ++i )
+        {
+            SizeT x = SizeT( 1 ) << i;
+
+            test<policy>( x - 1 );
+            test<policy>( x );
+            test<policy>( x + 1 );
+        }
+    }
+
     return boost::report_errors();
 }