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// -*- C++ -*-
// Copyright (C) 2005-2018 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 hash_shift_mask_example.cpp
* An example showing how to write a range-hasing functor.
*/
/**
* In some rare cases, advance knowledge of the distribution of keys allows
* writing more efficient hash-related policies.
* In the rather simplistic case of the example, it is known in advance that
* all keys have 0 two lowest bits. The example shows how to write
* a range-hashing function disregarding the two lowest bits of the hash value.
*/
#include <functional>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/hash_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
// A simple hash functor. hash could serve instead of this functor,
// but it is not yet standard everywhere.
struct simple_int_hash : public unary_function<int, size_t>
{
inline size_t
operator()(int i) const
{ return i; }
};
// A range-hashing function which shifts 2 bits right and then masks.
class shift_two_mask_range_hashing : private direct_mask_range_hashing<>
{
public:
typedef size_t size_type;
// Swaps with a different instant.
void
swap(shift_two_mask_range_hashing& other)
{ direct_mask_range_hashing<>::swap(other); }
// Called by the container when internally resized.
void
notify_resized(size_type size)
{ direct_mask_range_hashing<>::notify_resized(size); }
// Given a hash value, returns a number in the range of the internal
// size of the container.
inline size_type
operator()(size_type hash) const
{ return direct_mask_range_hashing<>::operator()(hash >> 2); }
};
int
main()
{
// A collision-chaining hash table mapping ints to chars.
typedef
cc_hash_table<
int,
char,
// Hash function.
simple_int_hash,
equal_to<int>,
// Range hashing function.
shift_two_mask_range_hashing>
map_t;
map_t h;
// Use normally.
h[16] = 'a';
h[256] = 'e';
h[4] = 'z';
return 0;
}
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