68 lines
2.1 KiB
C++
68 lines
2.1 KiB
C++
/*
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open source routing machine
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Copyright (C) Dennis Luxen, others 2010
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU AFFERO General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU Affero General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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or see http://www.gnu.org/licenses/agpl.txt.
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*/
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#ifndef FASTXORHASH_H_
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#define FASTXORHASH_H_
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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 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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std::vector<unsigned char> table1;
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std::vector<unsigned char> table2;
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public:
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XORFastHash() {
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table1.resize(1 << 16);
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table2.resize(1 << 16);
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for(unsigned i = 0; i < (1 << 16); ++i) {
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table1[i] = i; table2[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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unsigned short operator()(const unsigned originalValue) const {
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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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#endif /* FASTXORHASH_H_ */
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