Ichthyostega
974c670d41
to make them stand out more prominently, some entity comments where started with a line of starts. Unfortunately, doxygen (and javadoc) only recogise comments which are started exactly with /** This caused quite some comments to be ignored by doxygen. Credits to Hendrik Boom for spotting this problem! A workaround is to end the line of stars with *//**
113 lines
3.5 KiB
C++
113 lines
3.5 KiB
C++
/*
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HashGenerator(Test) - hash value generation details
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Copyright (C) Lumiera.org
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2011, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of
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the License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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* *****************************************************/
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#include "lib/test/run.hpp"
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#include "lib/util.hpp"
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#include <boost/functional/hash.hpp>
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#include <boost/lexical_cast.hpp>
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#include <iostream>
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#include <string>
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#include <map>
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using boost::lexical_cast;
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using util::contains;
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using std::string;
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using std::cout;
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using std::endl;
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namespace lib {
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namespace test{
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/***********************************************************************//**
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* @test cover various detail aspects regarding hash value generation
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* - weakness of boost::hash
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*
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* @see HashIndexed_test
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* @see EntryID_test
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*/
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class HashGenerator_test : public Test
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{
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virtual void
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run (Arg)
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{
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demonstrate_boost_hash_weakness();
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}
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/** @test demonstrate a serious weakness of boost::hash for strings.
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* When hashing just the plain string representation of integers,
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* we get collisions already with small numbers below 100000.
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* This is counter-intuitive, as the generated hash values
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* are 17 digits long and could span much wider scale.
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*
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* This problem is especially dangerous when storing objects keyed
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* by a string-id, which is generated from running numbers.
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*/
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void
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demonstrate_boost_hash_weakness ()
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{
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typedef boost::hash<string> BoostStringHasher;
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typedef std::map<size_t, string> StringsTable;
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BoostStringHasher hashFunction;
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StringsTable hashValues;
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string prefix = "Entry.";
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uint collisions(0);
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for (uint i=0; i<100000; ++i)
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{
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string candidate = prefix + lexical_cast<string> (i);
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size_t hashVal = hashFunction(candidate);
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if (contains (hashValues, hashVal))
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{
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++collisions;
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string other = hashValues[hashVal];
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cout << "Duplicate at "<< i << endl;
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cout << "existing--->" << other << endl;
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cout << "new-------->" << candidate << endl;
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size_t exHash = hashFunction(other);
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size_t newHash = hashFunction(candidate);
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cout << "hash-ex---->" << exHash << endl;
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cout << "hash_new--->" << newHash << endl;
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}
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hashValues[hashVal] = candidate;
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}
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CHECK (0 < collisions, "boost::hash for strings is expected to produce collisions");
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}
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};
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/** Register this test class... */
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LAUNCHER (HashGenerator_test, "unit common");
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}} // namespace lib::test
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