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// -*- C++ -*-

// Copyright (C) 2005, 2006, 2009 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/>.

// Copyright (C) 2004 Ami Tavory and Vladimir Dreizin, IBM-HRL.

// Permission to use, copy, modify, sell, and distribute this software

// is hereby granted without fee, provided that the above copyright

// notice appears in all copies, and that both that copyright notice

// and this permission notice appear in supporting documentation. None

// of the above authors, nor IBM Haifa Research Laboratories, make any

// representation about the suitability of this software for any

// purpose. It is provided "as is" without express or implied

// warranty.

/**

 * @file ranged_hash_example.cpp

 * A basic example showing how to write a ranged-hash functor.

 */

/**

 * In some cases it is beneficial to write a hash function which determines

 * hash values based on the size of the container object.

 * The example shows an example of a string-hashing function which

 * uses a fast method for hashing strings when the number of strings

 * in the container object is small, and a slower but more careful method

 * for hashing strings when the number of strings in the container object

 * is large.

 */

#include <functional>

#include <cassert>

#include <string>

#include <ext/pb_ds/assoc_container.hpp>

#include <ext/pb_ds/hash_policy.hpp>

using namespace std;

using namespace __gnu_pbds;

/**

 * A (somewhat simplistic) ranged-hash function for strings.

 * It uses the size of the container object to determine

 * the hashing method. For smaller sizes it uses a simple hash function;

 * for larger sizes it uses a more complicated hash function.

 */

class simple_string_ranged_hash_fn

  : public unary_function<string, size_t>

{

public:

  typedef size_t size_type;

  simple_string_ranged_hash_fn() : m_container_size(0) { }

  // Called to notify that the size has changed.

  void

  notify_resized(size_t size)

  { m_container_size = size; }

  // Called for hashing a string into a size_t in a given range.

  size_t

  operator()(const string& r_string)

  {

    /*

     *  This (simplified) hash algorithm decides that if there are

     *  fewer than 100 strings in the container it will hash

     *  a string by summing its characters; otherwise, it will

     *  perform a more complicated operation in order to produce

     *  hash values with fewer collisions.

     */

    string::const_iterator it = r_string.begin();

    size_t hash = 0;

    if (m_container_size < 100)

      {

        // For this size, perform an accumulate type of operation.

        while (it != r_string.end())

          hash += static_cast<size_t>(*it++);

      }

    else

      {

        // For this size, perform a different operation.

        while (it != r_string.end())

          {

            hash += static_cast<size_t>(*it++);

            hash *= 5;

          }

      }

    // The function must, by whatever means, return a size in the

    // range 0 to m_container_size.

    return hash % m_container_size;

  }

  // Swaps content.

  void

  swap(simple_string_ranged_hash_fn& other)

  {

    std::swap(m_container_size, other.m_container_size);

  }

private:

  // Records the size of the container object.

  size_t m_container_size;

};

int

main()

{

  // A collision-chaining hash table storing strings.

  typedef

    cc_hash_table<

    string,

    null_type,

    null_type,

    equal_to<string>,

    simple_string_ranged_hash_fn>

    set_t;

  // Note that in the above, the library determines a resize policy

  // appropriate for modulo-based range hashing.

  set_t h;

  // Use the table normally.

  h.insert("Hello, ");

  h.insert("world");

  assert(h.size() == 2);

  assert(h.find("Hello, ") != h.end());

  assert(h.find("world") != h.end());

  assert(h.find("Goodbye, oh cruel world!") == h.end());

  return 0;

}