libstdc++-v3/testsuite/ext/pb_ds/example/store_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 store_hash_example.cpp
  34  * An example showing how to store hash-values with
  35  *     each entry in a hash-based container.
  36  */
  37
  38 /**
  39  * This example shows how to configure a hash-based container to store
  40  * the hash value of each key along with each entry. This technique
  41  * is useful for complex keys (e.g., strings in this example), since
  42  * it lowers the cost of collisions. For simpler types (e.g., integers),
  43  * where the cost of comparing keys is of the same order as the cost
  44  * of comparing hash values, this technique adds unnecessary overhead.
  45  */
  46
  47 #include <functional>
  48 #include <string>
  49 #include <ext/pb_ds/assoc_container.hpp>
  50 #include <ext/pb_ds/hash_policy.hpp>
  51
  52 using namespace std;
  53 using namespace __gnu_pbds;
  54
  55 // A string hash functor.
  56 struct string_hash
  57 {
  58   size_t
  59   operator()(string str) const
  60   {
  61     string::const_iterator b = str.begin();
  62     string::const_iterator e = str.end();
  63
  64     size_t hash = 0;
  65     while (b != e)
  66       {
  67         hash *= 5;
  68         hash += static_cast<size_t>(*b);
  69         ++b;
  70       }
  71     return hash;
  72   }
  73 };
  74
  75 int main()
  76 {
  77   // A collision-chaining hash table mapping strings to ints.
  78   typedef
  79     cc_hash_table<
  80     string,
  81     int,
  82     string_hash,
  83     equal_to<string>,
  84     direct_mask_range_hashing<>,
  85     hash_standard_resize_policy<>,
  86     true>
  87     map_t;
  88
  89   map_t h;
  90
  91   // Use regularly.
  92   h["Hello, "] = 0;
  93   h["world"] = 1;
  94
  95   return 0;
  96 }
  97