use murmur2a hash

Author: grisumbras

include/boost/json/detail/digest.hpp

@@ -10,31 +10,122 @@
 #ifndef BOOST_JSON_DETAIL_DIGEST_HPP
 #define BOOST_JSON_DETAIL_DIGEST_HPP
 
+#include <boost/json/detail/config.hpp>
+
+#include <algorithm>
+#include <iterator>
+
 namespace boost {
 namespace json {
 namespace detail {
 
 // Calculate salted digest of string
+#if BOOST_JSON_ARCH == 64
+
 template<class ForwardIterator>
 std::size_t
 digest(
     ForwardIterator b,
     ForwardIterator e,
     std::size_t salt) noexcept
 {
-#if BOOST_JSON_ARCH == 64
-    std::uint64_t const prime = 0x100000001B3ULL;
-    std::uint64_t hash  = 0xcbf29ce484222325ULL;
+    std::size_t len = std::distance(b, e);
+
+    using state_type = std::uint64_t;
+    state_type const m = 0xc6a4a7935bd1e995ULL;
+    int const r = 47;
+    state_type hash = salt ^ (len * m);
+
+    constexpr std::size_t N = sizeof(state_type);
+    e = std::next( b, len & (std::size_t(-1) ^ 7) );
+    for( ; b != e; std::advance(b, N) )
+    {
+        state_type num;
+        std::copy_n( b, N, reinterpret_cast<unsigned char*>(&num) );
+
+        num *= m;
+        num ^= num >> r;
+        num *= m;
+
+        hash ^= num;
+        hash *= m;
+    }
+
+    state_type num = 0;
+    switch( len & 7 )
+    {
+    case 7: num |= state_type( *std::next(b, 6) ) << 48; // fall through
+    case 6: num |= state_type( *std::next(b, 5) ) << 40; // fall through
+    case 5: num |= state_type( *std::next(b, 4) ) << 32; // fall through
+    case 4: num |= state_type( *std::next(b, 3) ) << 24; // fall through
+    case 3: num |= state_type( *std::next(b, 2) ) << 16; // fall through
+    case 2: num |= state_type( *std::next(b, 1) ) << 8;  // fall through
+    case 1: num |= state_type( *std::next(b, 0) );       // fall through
+    default: break;
+    };
+
+    hash ^= num;
+    hash *= m;
+
+    hash ^= hash >> r;
+    hash *= m;
+    hash ^= hash >> r;
+
+    return hash;
+}
+
 #else
-    std::uint32_t const prime = 0x01000193UL;
-    std::uint32_t hash  = 0x811C9DC5UL;
-#endif
-    hash += salt;
-    for(; b != e; ++b)
-        hash = (*b ^ hash) * prime;
+
+template<class ForwardIterator>
+std::size_t
+digest(
+    ForwardIterator b,
+    ForwardIterator e,
+    std::size_t salt) noexcept
+{
+    std::size_t len = std::distance(b, e);
+
+    using state_type = std::uint32_t;
+    state_type const m = 0x5bd1e995;
+    int const r = 24;
+    state_type hash = salt ^ len;
+
+    constexpr std::size_t N = sizeof(state_type);
+    e = std::next( b, len & (std::size_t(-1) ^ 3) );
+    for( ; len >= N ; len -= N, std::advance(b, N) )
+    {
+        state_type num;
+        std::copy_n( b, N, reinterpret_cast<unsigned char*>(&num) );
+
+        num *= m;
+        num ^= num >> r;
+        num *= m;
+
+        hash *= m;
+        hash ^= num;
+    }
+
+    state_type num = 0;
+    switch( len & 3 )
+    {
+    case 3: num |= state_type( *std::next(b, 2) ) << 16; // fall through
+    case 2: num |= state_type( *std::next(b, 1) ) << 8;  // fall through
+    case 1: num |= state_type( *std::next(b, 0) );       // fall through
+    default: break;
+    };
+
+    hash ^= num;
+    hash *= m;
+
+    hash ^= hash >> 13;
+    hash *= m;
+    hash ^= hash >> 15;
+
     return hash;
 }
 
+#endif
+
 } // detail
 } // namespace json
 } // namespace boost