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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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