libstdc++-v3/testsuite/ext/pb_ds/example/ranged_hash.cc

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2005-2023 Free Software Foundation, Inc.
   4 //
   5 // This file is part of the GNU ISO C++ Library.  This library is free
   6 // software; you can redistribute it and/or modify it under the terms
   7 // of the GNU General Public License as published by the Free Software
   8 // Foundation; either version 3, or (at your option) any later
   9 // version.
  10
  11 // This library is distributed in the hope that it will be useful, but
  12 // WITHOUT ANY WARRANTY; without even the implied warranty of
  13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 // General Public License for more details.
  15
  16 // You should have received a copy of the GNU General Public License
  17 // along with this library; see the file COPYING3.  If not see
  18 // <http://www.gnu.org/licenses/>.
  19
  20
  21 // Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.
  22
  23 // Permission to use, copy, modify, sell, and distribute this software
  24 // is hereby granted without fee, provided that the above copyright
  25 // notice appears in all copies, and that both that copyright notice
  26 // and this permission notice appear in supporting documentation. None
  27 // of the above authors, nor IBM Haifa Research Laboratories, make any
  28 // representation about the suitability of this software for any
  29 // purpose. It is provided "as is" without express or implied
  30 // warranty.
  31
  32 /**
  33  * @file ranged_hash_example.cpp
  34  * A basic example showing how to write a ranged-hash functor.
  35  */
  36
  37 /**
  38  * In some cases it is beneficial to write a hash function which determines
  39  * hash values based on the size of the container object.
  40  * The example shows an example of a string-hashing function which
  41  * uses a fast method for hashing strings when the number of strings
  42  * in the container object is small, and a slower but more careful method
  43  * for hashing strings when the number of strings in the container object
  44  * is large.
  45  */
  46
  47 #include <functional>
  48 #include <cassert>
  49 #include <string>
  50 #include <ext/pb_ds/assoc_container.hpp>
  51 #include <ext/pb_ds/hash_policy.hpp>
  52
  53 using namespace std;
  54 using namespace __gnu_pbds;
  55
  56 /**
  57  * A (somewhat simplistic) ranged-hash function for strings.
  58  * It uses the size of the container object to determine
  59  * the hashing method. For smaller sizes it uses a simple hash function;
  60  * for larger sizes it uses a more complicated hash function.
  61  */
  62 class simple_string_ranged_hash_fn
  63 {
  64 public:
  65   typedef size_t size_type;
  66
  67   simple_string_ranged_hash_fn() : m_container_size(0) { }
  68
  69   // Called to notify that the size has changed.
  70   void
  71   notify_resized(size_t size)
  72   { m_container_size = size; }
  73
  74   // Called for hashing a string into a size_t in a given range.
  75   size_t
  76   operator()(const string& r_string)
  77   {
  78     /*
  79      *  This (simplified) hash algorithm decides that if there are
  80      *  fewer than 100 strings in the container it will hash
  81      *  a string by summing its characters; otherwise, it will
  82      *  perform a more complicated operation in order to produce
  83      *  hash values with fewer collisions.
  84      */
  85     string::const_iterator it = r_string.begin();
  86     size_t hash = 0;
  87     if (m_container_size < 100)
  88       {
  89         // For this size, perform an accumulate type of operation.
  90         while (it != r_string.end())
  91           hash += static_cast<size_t>(*it++);
  92       }
  93     else
  94       {
  95         // For this size, perform a different operation.
  96         while (it != r_string.end())
  97           {
  98             hash += static_cast<size_t>(*it++);
  99             hash *= 5;
 100           }
 101       }
 102
 103     // The function must, by whatever means, return a size in the
 104     // range 0 to m_container_size.
 105     return hash % m_container_size;
 106   }
 107
 108   // Swaps content.
 109   void
 110   swap(simple_string_ranged_hash_fn& other)
 111   {
 112     std::swap(m_container_size, other.m_container_size);
 113   }
 114
 115 private:
 116   // Records the size of the container object.
 117   size_t m_container_size;
 118 };
 119
 120 int
 121 main()
 122 {
 123   // A collision-chaining hash table storing strings.
 124   typedef
 125     cc_hash_table<
 126     string,
 127     null_type,
 128     null_type,
 129     equal_to<string>,
 130     simple_string_ranged_hash_fn>
 131     set_t;
 132
 133   // Note that in the above, the library determines a resize policy
 134   // appropriate for modulo-based range hashing.
 135   set_t h;
 136
 137   // Use the table normally.
 138   h.insert("Hello, ");
 139   h.insert("world");
 140
 141   assert(h.size() == 2);
 142
 143   assert(h.find("Hello, ") != h.end());
 144   assert(h.find("world") != h.end());
 145
 146   assert(h.find("Goodbye, oh cruel world!") == h.end());
 147
 148   return 0;
 149 }
 150