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//-----------------------------------------------------------------------------
// MurmurHash2 was written by Austin Appleby, and is placed in the public
// domain. The author hereby disclaims copyright to this source code.
//
// This was modified as to possibilitate it's usage incrementally.
// Those modifications are also placed in the public domain, and the author of
// such modifications hereby disclaims copyright to this source code.
#include "MurmurHash2.h"
//-----------------------------------------------------------------------------
// 'm' and 'r' are mixing constants generated offline.
// They're not really 'magic', they just happen to work well.
const uint32_t m = 0x5bd1e995;
const int r = 24;
uint32_t MurmurHash2(std::ifstream&& file_stream, std::size_t buffer_size, std::function<bool(char)> filter_out)
{
auto* buffer = new char[buffer_size];
char data[4];
int read = 0;
uint32_t size = 0;
// We need the size without the filtered out characters before actually calculating the hash,
// to setup the initial value for the hash.
do {
file_stream.read(buffer, buffer_size);
read = file_stream.gcount();
for (int i = 0; i < read; i++) {
if (!filter_out(buffer[i]))
size += 1;
}
} while (!file_stream.eof());
file_stream.clear();
file_stream.seekg(0, file_stream.beg);
int index = 0;
// This forces a seed of 1.
IncrementalHashInfo info{ (uint32_t)1 ^ size, (uint32_t)size };
do {
file_stream.read(buffer, buffer_size);
read = file_stream.gcount();
for (int i = 0; i < read; i++) {
char c = buffer[i];
if (filter_out(c))
continue;
data[index] = c;
index = (index + 1) % 4;
// Mix 4 bytes at a time into the hash
if (index == 0)
FourBytes_MurmurHash2(reinterpret_cast<unsigned char*>(&data), info);
}
} while (!file_stream.eof());
// Do one last bit shuffle in the hash
FourBytes_MurmurHash2(reinterpret_cast<unsigned char*>(&data), info);
delete[] buffer;
file_stream.close();
return info.h;
}
void FourBytes_MurmurHash2(const unsigned char* data, IncrementalHashInfo& prev)
{
if (prev.len >= 4) {
// Not the final mix
uint32_t k = *reinterpret_cast<const uint32_t*>(data);
k *= m;
k ^= k >> r;
k *= m;
prev.h *= m;
prev.h ^= k;
prev.len -= 4;
} else {
// The final mix
// Handle the last few bytes of the input array
switch (prev.len) {
case 3:
prev.h ^= data[2] << 16;
case 2:
prev.h ^= data[1] << 8;
case 1:
prev.h ^= data[0];
prev.h *= m;
};
// Do a few final mixes of the hash to ensure the last few
// bytes are well-incorporated.
prev.h ^= prev.h >> 13;
prev.h *= m;
prev.h ^= prev.h >> 15;
prev.len = 0;
}
}
//-----------------------------------------------------------------------------
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