89 lines
2.7 KiB
C++
89 lines
2.7 KiB
C++
#ifndef XOR_FAST_HASH_HPP
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#define XOR_FAST_HASH_HPP
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#include <algorithm>
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#include <vector>
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/*
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This is an implementation of Tabulation hashing, which has suprising properties like
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universality.
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The space requirement is 2*2^16 = 256 kb of memory, which fits into L2 cache.
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Evaluation boils down to 10 or less assembly instruction on any recent X86 CPU:
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1: movq table2(%rip), %rdx
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2: movl %edi, %eax
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3: movzwl %di, %edi
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4: shrl $16, %eax
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5: movzwl %ax, %eax
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6: movzbl (%rdx,%rax), %eax
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7: movq table1(%rip), %rdx
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8: xorb (%rdx,%rdi), %al
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9: movzbl %al, %eax
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10: ret
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*/
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class XORFastHash
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{ // 65k entries
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std::vector<unsigned short> table1;
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std::vector<unsigned short> table2;
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public:
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XORFastHash()
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{
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table1.resize(2 << 16);
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table2.resize(2 << 16);
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for (unsigned i = 0; i < (2 << 16); ++i)
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{
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table1[i] = static_cast<unsigned short>(i);
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table2[i] = static_cast<unsigned short>(i);
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}
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std::random_shuffle(table1.begin(), table1.end());
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std::random_shuffle(table2.begin(), table2.end());
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}
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inline unsigned short operator()(const unsigned originalValue) const
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{
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unsigned short lsb = ((originalValue)&0xffff);
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unsigned short msb = (((originalValue) >> 16) & 0xffff);
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return table1[lsb] ^ table2[msb];
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}
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};
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class XORMiniHash
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{ // 256 entries
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std::vector<unsigned char> table1;
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std::vector<unsigned char> table2;
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std::vector<unsigned char> table3;
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std::vector<unsigned char> table4;
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public:
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XORMiniHash()
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{
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table1.resize(1 << 8);
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table2.resize(1 << 8);
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table3.resize(1 << 8);
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table4.resize(1 << 8);
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for (unsigned i = 0; i < (1 << 8); ++i)
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{
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table1[i] = static_cast<unsigned char>(i);
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table2[i] = static_cast<unsigned char>(i);
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table3[i] = static_cast<unsigned char>(i);
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table4[i] = static_cast<unsigned char>(i);
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}
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std::random_shuffle(table1.begin(), table1.end());
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std::random_shuffle(table2.begin(), table2.end());
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std::random_shuffle(table3.begin(), table3.end());
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std::random_shuffle(table4.begin(), table4.end());
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}
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unsigned char operator()(const unsigned originalValue) const
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{
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unsigned char byte1 = ((originalValue)&0xff);
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unsigned char byte2 = ((originalValue >> 8) & 0xff);
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unsigned char byte3 = ((originalValue >> 16) & 0xff);
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unsigned char byte4 = ((originalValue >> 24) & 0xff);
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return table1[byte1] ^ table2[byte2] ^ table3[byte3] ^ table4[byte4];
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}
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};
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#endif // XOR_FAST_HASH_HPP
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