75 lines
2.5 KiB
C++
75 lines
2.5 KiB
C++
// Copyright (C) 2010-2020 Free Software Foundation, Inc.
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//
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// This file is part of the GNU ISO C++ Library. This library is free
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// software; you can redistribute it and/or modify it under the
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// terms of the GNU General Public License as published by the
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// Free Software Foundation; either version 3, or (at your option)
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// any later version.
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//
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// This library 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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//
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// You should have received a copy of the GNU General Public License
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// along with this library; see the file COPYING3. If not see
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// <http://www.gnu.org/licenses/>.
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// This file uses the chi^2 test to measure the quality of a hash
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// function, by computing the uniformity with which it distributes a set
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// of N strings into k buckets (where k is significantly greater than N).
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//
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// Each bucket has B[i] strings in it. The expected value of each bucket
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// for a uniform distribution is z = N/k, so
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// chi^2 = Sum_i (B[i] - z)^2 / z.
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//
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// We check whether chi^2 is small enough to be consistent with the
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// hypothesis of a uniform distribution. If F(chi^2, k-1) is close to
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// 0 (where F is the cumulative probability distribution), we can
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// reject that hypothesis. So we don't want F to be too small, which
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// for large k, means we want chi^2 to be not too much larger than k.
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//
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// We use the chi^2 test for several sets of strings. Any non-horrible
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// hash function should do well with purely random strings. A really
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// good hash function will also do well with more structured sets,
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// including ones where the strings differ by only a few bits.
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#include <algorithm>
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#include <cstdlib>
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#include <cstdio>
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#include <fstream>
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#include <functional>
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#include <iostream>
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#include <iterator>
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#include <string>
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#include <unordered_set>
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#include <vector>
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#include <testsuite_hooks.h>
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#ifndef SAMPLES
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#define SAMPLES 300000
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#endif
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template <typename Container>
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double
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chi2_hash(const Container& c, long buckets)
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{
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std::vector<int> counts(buckets);
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std::hash<std::string> hasher;
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double elements = 0;
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for (auto i = c.begin(); i != c.end(); ++i)
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{
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++counts[hasher(*i) % buckets];
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++elements;
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}
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const double z = elements / buckets;
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double sum = 0;
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for (long i = 0; i < buckets; ++i)
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{
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double delta = counts[i] - z;
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sum += delta*delta;
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}
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return sum/z;
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}
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