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// { dg-options "-std=gnu++11" }
// { dg-options "-DNTESTS=1 -DNSTRINGS=100 -DSTRSIZE=21 -std=gnu++11" { target simulator } }
// 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/>.
#include <cstdlib>
#include <unordered_set>
#include <string>
#include <functional>
#include <vector>
#include <testsuite_hooks.h>
using namespace std;
#ifndef NTESTS
#define NTESTS 5
#endif
#ifndef NSTRINGS
#define NSTRINGS 200
#endif
#ifndef STRSIZE
#define STRSIZE 42
#endif
const unsigned int num_quality_tests = NTESTS;
const unsigned int num_strings_for_quality_tests = NSTRINGS;
const unsigned int string_size = STRSIZE;
vector<string>
random_strings(unsigned int n, unsigned int len)
{
string s(len, '\0');
unordered_set<string> result_set;
while (result_set.size() < n)
{
result_set.insert(s);
unsigned int tmp = rand();
tmp %= len * 256;
s[tmp / 256] = tmp % 256;
}
return vector<string>(result_set.begin(), result_set.end());
}
double
score_from_varying_position(string s, unsigned int index)
{
bool test __attribute__((unused)) = true;
unsigned int bits_in_hash_code = sizeof(size_t) * 8;
// We'll iterate through all 256 vals for s[index], leaving the rest
// of s fixed. Then, for example, out of the 128 times that
// s[index] has its 3rd bit equal to 0 we would like roughly half 1s
// and half 0s in bit 9 of the hash codes.
//
// Bookkeeping: Conceptually we want a 3D array of ints. We want to
// count the number of times each output position (of which there are
// bits_in_hash_code) is 1 for each bit position within s[index] (of
// which there are 8) and value of that bit (of which there are 2).
const unsigned int jj = 2;
const unsigned int kk = jj * bits_in_hash_code;
const unsigned int array_size = 8 * kk;
vector<int> ones(array_size, 0);
for (int i = 0; i < 256; i++)
{
s[index] = i;
size_t h = hash<string>()(s);
for (int j = 0; h != 0; j++, h >>= 1)
{
if (h & 1)
{
for (int k = 0; k < 8; k++)
++ones[k * kk + j * jj + ((i >> k) & 1)];
}
}
}
// At most, the innermost statement in the above loop nest can
// execute 256 * bits_in_hash_code * 8 times. If the hash is good,
// it'll execute about half that many times, with a pretty even
// spread across the elements of ones[].
VERIFY( 256 * bits_in_hash_code * 8 / array_size == 128 );
int max_ones_possible = 128;
int good = 0, bad = 0;
for (int bit = 0; bit <= 1; bit++)
{
for (unsigned int j = 0; j < bits_in_hash_code; j++)
{
for (int bitpos = 0; bitpos < 8; bitpos++)
{
int z = ones[bitpos * kk + j * jj + bit];
if (z <= max_ones_possible / 6
|| z >= max_ones_possible * 5 / 6)
{
// The hash function screwed up, or was just unlucky,
// as 128 flips of a perfect coin occasionally yield
// far from 64 heads.
bad++;
}
else
good++;
}
}
}
return good / (double)(good + bad);
}
double
score_from_varying_position(const vector<string>& v, unsigned int index)
{
double score = 0;
for (unsigned int i = 0; i < v.size(); i++)
score += score_from_varying_position(v[i], index);
return score / v.size();
}
double
quality_test(unsigned int num_strings, unsigned int string_size)
{
// Construct random strings.
vector<string> v = random_strings(num_strings, string_size);
double sum_of_scores = 0;
for (unsigned int i = 0; i < string_size; i++)
sum_of_scores += score_from_varying_position(v, i);
// A good hash function should have a score very close to 1, and a bad
// hash function will have a score close to 0.
return sum_of_scores / string_size;
}
void
quality_test()
{
bool test __attribute__((unused)) = true;
srand(137);
double sum_of_scores = 0;
for (unsigned int i = 0; i < num_quality_tests; i++)
{
double score = quality_test(num_strings_for_quality_tests,
string_size);
sum_of_scores += score;
VERIFY( score > 0.99 );
}
if (num_quality_tests > 1)
{
double mean_quality = sum_of_scores / num_quality_tests;
VERIFY( mean_quality > 0.9999 );
}
}
int
main()
{
quality_test();
return 0;
}
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