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1 files changed, 218 insertions, 0 deletions

diff --git a/libstdc++-v3/testsuite/20_util/hash/chi2_quality.cc b/libstdc++-v3/testsuite/20_util/hash/chi2_quality.cc
new file mode 100644
index 000000000..8a388349b
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/hash/chi2_quality.cc
@@ -0,0 +1,218 @@
+// { dg-options "-std=gnu++0x" }
+
+// Use smaller statistics when running on simulators, so it takes less time.
+// { dg-options "-std=gnu++0x -DSAMPLES=10000" { target simulator } }
+
+// Copyright (C) 2010, 2011 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;
+  }
+
+// Tests chi^2 for a distribution of uniformly generated random strings.
+void
+test_uniform_random()
+{
+  bool test __attribute__((unused)) = true;
+  std::srand(137);
+  std::unordered_set<std::string> set;
+  std::string s;
+  const unsigned long N = SAMPLES;
+  const unsigned long k = N/100;
+  const unsigned int len = 25;
+  while (set.size() < N)
+    {
+      s.clear();
+      for (unsigned int i = 0; i < len; ++i)
+	s.push_back(rand() % 128);
+      set.insert(s);
+    }
+
+  double chi2 = chi2_hash(set, k);
+  VERIFY( chi2 < k*1.1 );
+}
+
+// Tests chi^2 for a distribution of strings that differ from each
+// other by only a few bits. We start with an arbitrary base string, and
+// flip three random bits for each member of the set.
+void
+test_bit_flip_set()
+{
+  bool test __attribute__((unused)) = true;
+  const unsigned long N = SAMPLES;
+  const unsigned long k = N/100;
+  const unsigned int len = 67;
+  const unsigned int bitlen = len * 8;
+  const unsigned int bits_to_flip = 3;
+  const char base[len+1] = "abcdefghijklmnopqrstuvwxyz"
+                           "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+                           "0123456789!@#$%";
+
+  std::unordered_set<std::string> set;
+  while (set.size() < N)
+    {
+      std::string s(base, base+len);
+      for (unsigned int i = 0; i < bits_to_flip; ++i)
+        {
+          int bit = rand() % bitlen;
+          s[bit/8] ^= (1 << (bit%8));
+        }
+      set.insert(s);
+    }
+
+  double chi2 = chi2_hash(set, k);
+  VERIFY( chi2 < k*1.1 );
+}
+
+// Tests chi^2 of a set of strings that all have a similar pattern,
+// intended to mimic some sort of ID string.
+void
+test_numeric_pattern_set()
+{
+  bool test __attribute__((unused)) = true;
+  const unsigned long N = SAMPLES;
+  const unsigned long k = N/100;
+  std::vector<std::string> set;
+  for (unsigned long i = 0; i < N; ++i)
+    {
+      long i1 = i % 100000;
+      long i2 = i / 100000;
+      char buf[16];
+      std::sprintf(buf, "XX-%05lu-%05lu", i1, i2);
+      set.push_back(buf);
+    }
+
+  double chi2 = chi2_hash(set, k);
+  VERIFY( chi2 < k*1.1 );
+}
+
+// Tests chi^2 for a set of strings that all consist of '1' and '0'.
+void
+test_bit_string_set()
+{
+  bool test __attribute__((unused)) = true;
+  const unsigned long N = SAMPLES;
+  const unsigned long k = N/100;
+  std::vector<std::string> set;
+  std::string s;
+  for (unsigned long i = 0; i < N; ++i)
+    {
+      s.clear();
+      for (unsigned int j = 0; j < sizeof(unsigned long) * 8; ++j)
+        {
+          const bool bit = (1UL << j) & i;
+          s.push_back(bit ? '1' : '0');
+        }
+      set.push_back(s);
+    }
+
+  double chi2 = chi2_hash(set, k);
+  VERIFY( chi2 < k*1.1 );
+}
+
+// Tests chi^2 for a set of words taken from a document written in English.
+void
+test_document_words()
+{
+  // That file is 187587 single-word lines.  To avoid a timeout, just skip
+  // this part, which would take up to 95% of the program runtime (with
+  // SAMPLES == 10000), if we're not supposed to run anywhere that long.
+#if SAMPLES >= 100000
+  bool test __attribute__((unused)) = true;
+  const std::string f_name = "thirty_years_among_the_dead_preproc.txt";
+  std::ifstream in(f_name);
+  VERIFY( in.is_open() );
+  std::vector<std::string> words;
+  words.assign(std::istream_iterator<std::string>(in),
+               std::istream_iterator<std::string>());
+  VERIFY( words.size() > 100000 );
+  std::sort(words.begin(), words.end());
+  auto it = std::unique(words.begin(), words.end());
+  words.erase(it, words.end());
+  VERIFY( words.size() > 5000 );
+
+  const unsigned long k = words.size() / 20;
+  double chi2 = chi2_hash(words, k);
+  VERIFY( chi2 < k*1.1 );
+#endif
+}
+
+int
+main()
+{
+  test_uniform_random();
+  test_bit_flip_set();
+  test_numeric_pattern_set();
+  test_bit_string_set();
+  test_document_words();
+  return 0;
+}