/*

* Copyright (c) 2003-2009 Nokia Corporation and/or its subsidiary(-ies).

* All rights reserved.

* This component and the accompanying materials are made available

* under the terms of the License "Eclipse Public License v1.0"

* which accompanies this distribution, and is available

* at the URL "http://www.eclipse.org/legal/epl-v10.html".

*

* Initial Contributors:

* Nokia Corporation - initial contribution.

*

* Contributors:

*

* Description: 

*

*/

#include <bigint.h>

#include "windowslider.h"

#include "../common/inlines.h"

TWindowSlider::TWindowSlider(const TInteger& aExp, TUint aWindowSize)

	: iExp(aExp), iSize(aWindowSize)

	{

	if(iSize == 0)

		{

		TUint expLen = iExp.BitCount();

		//These numbers are more or less arbitrary and can be tuned for empirical

		//performance results if desired.  It's a trade off between amount of

		//precomputation (more if larger iSize) and number of iterations

		//(more if smaller iSize).  The current defaults were obtained

		//from crypto++

		if( expLen <= 17 )

			iSize = 1;

		else if( expLen <= 24 )

			iSize = 2;

		else if( expLen <= 70 )

			iSize = 3;

		else if( expLen <= 197 )

			iSize = 4;

		else if( expLen <= 539 )

			iSize = 5;

		else if( expLen <= 1434 )

			iSize = 6;

		else 

			iSize = 7;

		}

	assert(iSize>=1 && iSize<=7);

	}

void TWindowSlider::FindNextWindow(TUint aBegin)

	{

	assert(iExp.Bit(aBegin)); //initial bit must be 1

	TInt end = aBegin;

	TUint temp = 0;

	iValue = 0;

	TUint j = 0;

	for(TInt i=aBegin; i>=0 && j<iSize; i--, j++)

		{

		TUint saveMask = WORD_BITS - j - 1;

		temp |= iExp.Bit(i) << saveMask;

		if( temp & (1 << saveMask) )

			{

			end = i;

			iValue |= temp; //last valid substring

			}

		}

	iLength = aBegin - end + 1;

	iValue >>= (WORD_BITS - iLength);

	}