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// Definition of _Hash_bytes. -*- C++ -*-
// Copyright (C) 2010, 2011 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
// This file defines Hash_bytes, a primitive used for defining hash
// functions. Based on public domain MurmurHashUnaligned2, by Austin
// Appleby. http://murmurhash.googlepages.com/
// This file also defines _Fnv_hash_bytes, another primitive with
// exactly the same interface but using a different hash algorithm,
// Fowler / Noll / Vo (FNV) Hash (type FNV-1a). The Murmur hash
// function apears to be better in both speed and hash quality, and
// FNV is provided primarily for backward compatibility.
#include <bits/hash_bytes.h>
namespace
{
inline std::size_t
unaligned_load(const char* p)
{
std::size_t result;
__builtin_memcpy(&result, p, sizeof(result));
return result;
}
#if __SIZEOF_SIZE_T__ == 8
// Loads n bytes, where 1 <= n < 8.
inline std::size_t
load_bytes(const char* p, int n)
{
std::size_t result = 0;
--n;
do
result = (result << 8) + static_cast<unsigned char>(p[n]);
while (--n >= 0);
return result;
}
inline std::size_t
shift_mix(std::size_t v)
{ return v ^ (v >> 47);}
#endif
}
namespace std
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
#if __SIZEOF_SIZE_T__ == 4
// Implementation of Murmur hash for 32-bit size_t.
size_t
_Hash_bytes(const void* ptr, size_t len, size_t seed)
{
const size_t m = 0x5bd1e995;
size_t hash = seed ^ len;
const char* buf = static_cast<const char*>(ptr);
// Mix 4 bytes at a time into the hash.
while(len >= 4)
{
size_t k = unaligned_load(buf);
k *= m;
k ^= k >> 24;
k *= m;
hash *= m;
hash ^= k;
buf += 4;
len -= 4;
}
// Handle the last few bytes of the input array.
switch(len)
{
case 3:
hash ^= static_cast<unsigned char>(buf[2]) << 16;
case 2:
hash ^= static_cast<unsigned char>(buf[1]) << 8;
case 1:
hash ^= static_cast<unsigned char>(buf[0]);
hash *= m;
};
// Do a few final mixes of the hash.
hash ^= hash >> 13;
hash *= m;
hash ^= hash >> 15;
return hash;
}
// Implementation of FNV hash for 32-bit size_t.
size_t
_Fnv_hash_bytes(const void* ptr, size_t len, size_t hash)
{
const char* cptr = static_cast<const char*>(ptr);
for (; len; --len)
{
hash ^= static_cast<size_t>(*cptr++);
hash *= static_cast<size_t>(16777619UL);
}
return hash;
}
#elif __SIZEOF_SIZE_T__ == 8
// Implementation of Murmur hash for 64-bit size_t.
size_t
_Hash_bytes(const void* ptr, size_t len, size_t seed)
{
static const size_t mul = (0xc6a4a793UL << 32UL) + 0x5bd1e995UL;
const char* const buf = static_cast<const char*>(ptr);
// Remove the bytes not divisible by the sizeof(size_t). This
// allows the main loop to process the data as 64-bit integers.
const int len_aligned = len & ~0x7;
const char* const end = buf + len_aligned;
size_t hash = seed ^ (len * mul);
for (const char* p = buf; p != end; p += 8)
{
const size_t data = shift_mix(unaligned_load(p) * mul) * mul;
hash ^= data;
hash *= mul;
}
if ((len & 0x7) != 0)
{
const size_t data = load_bytes(end, len & 0x7);
hash ^= data;
hash *= mul;
}
hash = shift_mix(hash) * mul;
hash = shift_mix(hash);
return hash;
}
// Implementation of FNV hash for 64-bit size_t.
size_t
_Fnv_hash_bytes(const void* ptr, size_t len, size_t hash)
{
const char* cptr = static_cast<const char*>(ptr);
for (; len; --len)
{
hash ^= static_cast<size_t>(*cptr++);
hash *= static_cast<size_t>(1099511628211ULL);
}
return hash;
}
#else
// Dummy hash implementation for unusual sizeof(size_t).
size_t
_Hash_bytes(const void* ptr, size_t len, size_t seed)
{
size_t hash = seed;
const char* cptr = reinterpret_cast<const char*>(ptr);
for (; len; --len)
hash = (hash * 131) + *cptr++;
return hash;
}
size_t
_Fnv_hash_bytes(const void* ptr, size_t len, size_t seed)
{ return _Hash_bytes(ptr, len, seed); }
#endif /* __SIZEOF_SIZE_T__ */
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
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