75 lines
2.5 KiB
C++
75 lines
2.5 KiB
C++
// Copyright (C) 2010-2015 Free Software Foundation, Inc.
|
|
//
|
|
// This file is part of the GNU ISO C++ Library. This library is free
|
|
// software; you can redistribute it and/or modify it under the
|
|
// terms of the GNU General Public License as published by the
|
|
// Free Software Foundation; either version 3, or (at your option)
|
|
// any later version.
|
|
//
|
|
// This library is distributed in the hope that it will be useful,
|
|
// but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
// GNU General Public License for more details.
|
|
//
|
|
// You should have received a copy of the GNU General Public License
|
|
// along with this library; see the file COPYING3. If not see
|
|
// <http://www.gnu.org/licenses/>.
|
|
|
|
// This file uses the chi^2 test to measure the quality of a hash
|
|
// function, by computing the uniformity with which it distributes a set
|
|
// of N strings into k buckets (where k is significantly greater than N).
|
|
//
|
|
// Each bucket has B[i] strings in it. The expected value of each bucket
|
|
// for a uniform distribution is z = N/k, so
|
|
// chi^2 = Sum_i (B[i] - z)^2 / z.
|
|
//
|
|
// We check whether chi^2 is small enough to be consistent with the
|
|
// hypothesis of a uniform distribution. If F(chi^2, k-1) is close to
|
|
// 0 (where F is the cumulative probability distribution), we can
|
|
// reject that hypothesis. So we don't want F to be too small, which
|
|
// for large k, means we want chi^2 to be not too much larger than k.
|
|
//
|
|
// We use the chi^2 test for several sets of strings. Any non-horrible
|
|
// hash function should do well with purely random strings. A really
|
|
// good hash function will also do well with more structured sets,
|
|
// including ones where the strings differ by only a few bits.
|
|
|
|
#include <algorithm>
|
|
#include <cstdlib>
|
|
#include <cstdio>
|
|
#include <fstream>
|
|
#include <functional>
|
|
#include <iostream>
|
|
#include <iterator>
|
|
#include <string>
|
|
#include <unordered_set>
|
|
#include <vector>
|
|
#include <testsuite_hooks.h>
|
|
|
|
#ifndef SAMPLES
|
|
#define SAMPLES 300000
|
|
#endif
|
|
|
|
template <typename Container>
|
|
double
|
|
chi2_hash(const Container& c, long buckets)
|
|
{
|
|
std::vector<int> counts(buckets);
|
|
std::hash<std::string> hasher;
|
|
double elements = 0;
|
|
for (auto i = c.begin(); i != c.end(); ++i)
|
|
{
|
|
++counts[hasher(*i) % buckets];
|
|
++elements;
|
|
}
|
|
|
|
const double z = elements / buckets;
|
|
double sum = 0;
|
|
for (long i = 0; i < buckets; ++i)
|
|
{
|
|
double delta = counts[i] - z;
|
|
sum += delta*delta;
|
|
}
|
|
return sum/z;
|
|
}
|