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