crypto/weakcrypto/source/bigint/windowslider.cpp
author Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com>
Tue, 31 Aug 2010 17:00:08 +0300
branchRCL_3
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permissions -rw-r--r--
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/*
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* Copyright (c) 2003-2009 Nokia Corporation and/or its subsidiary(-ies).
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* All rights reserved.
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* This component and the accompanying materials are made available
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* under the terms of the License "Eclipse Public License v1.0"
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* which accompanies this distribution, and is available
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* at the URL "http://www.eclipse.org/legal/epl-v10.html".
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*
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* Initial Contributors:
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* Nokia Corporation - initial contribution.
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*
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* Contributors:
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*
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* Description: 
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*
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*/
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#include <bigint.h>
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#include "windowslider.h"
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#include "../common/inlines.h"
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TWindowSlider::TWindowSlider(const TInteger& aExp, TUint aWindowSize)
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	: iExp(aExp), iSize(aWindowSize)
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	{
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	if(iSize == 0)
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		{
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		TUint expLen = iExp.BitCount();
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		//These numbers are more or less arbitrary and can be tuned for empirical
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		//performance results if desired.  It's a trade off between amount of
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		//precomputation (more if larger iSize) and number of iterations
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		//(more if smaller iSize).  The current defaults were obtained
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		//from crypto++
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		if( expLen <= 17 )
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			iSize = 1;
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		else if( expLen <= 24 )
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			iSize = 2;
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		else if( expLen <= 70 )
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			iSize = 3;
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		else if( expLen <= 197 )
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			iSize = 4;
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		else if( expLen <= 539 )
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			iSize = 5;
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		else if( expLen <= 1434 )
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			iSize = 6;
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		else 
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			iSize = 7;
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		}
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	assert(iSize>=1 && iSize<=7);
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	}
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void TWindowSlider::FindNextWindow(TUint aBegin)
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	{
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	assert(iExp.Bit(aBegin)); //initial bit must be 1
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	TInt end = aBegin;
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	TUint temp = 0;
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	iValue = 0;
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	TUint j = 0;
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	for(TInt i=aBegin; i>=0 && j<iSize; i--, j++)
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		{
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		TUint saveMask = WORD_BITS - j - 1;
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		temp |= iExp.Bit(i) << saveMask;
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		if( temp & (1 << saveMask) )
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			{
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			end = i;
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			iValue |= temp; //last valid substring
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			}
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		}
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	iLength = aBegin - end + 1;
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	iValue >>= (WORD_BITS - iLength);
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	}
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