source: box/trunk/lib/crypto/RollingChecksum.cpp @ 217

// --------------------------------------------------------------------------
//
// File
//              Name:    RollingChecksum.cpp
//              Purpose: A simple rolling checksum over a block of data
//              Created: 6/12/03
//
// --------------------------------------------------------------------------

#include "Box.h"
#include "RollingChecksum.h"

#include "MemLeakFindOn.h"

// --------------------------------------------------------------------------
//
// Function
//              Name:    RollingChecksum::RollingChecksum(const void *, unsigned int)
//              Purpose: Constructor -- does initial computation of the checksum.
//              Created: 6/12/03
//
// --------------------------------------------------------------------------
RollingChecksum::RollingChecksum(const void * const data, const unsigned int Length)
        : a(0),
          b(0)
{
        const uint8_t *block = (const uint8_t *)data;
        for(unsigned int x = Length; x >= 1; --x)
        {
                a += (*block);
                b += x * (*block);
                
                ++block;
        }
}

// --------------------------------------------------------------------------
//
// Function
//              Name:    RollingChecksum::RollForwardSeveral(uint8_t*, uint8_t*, unsigned int, unsigned int)
//              Purpose: Move the checksum forward a block, given a pointer to the first byte of the current block,
//                               and a pointer just after the last byte of the current block and the length of the block and of the skip.
//              Created: 7/14/05
//
// --------------------------------------------------------------------------
void RollingChecksum::RollForwardSeveral(const uint8_t * const StartOfThisBlock, const uint8_t * const LastOfNextBlock, const unsigned int Length, const unsigned int Skip)
{
        // IMPLEMENTATION NOTE: Everything is implicitly mod 2^16 -- uint16_t's will overflow nicely.
        unsigned int i;
        uint16_t sumBegin=0, j,k;

        for(i=0; i < Skip; i++)
        {
                j = StartOfThisBlock[i];
                k = LastOfNextBlock[i];
                sumBegin += j;
                a += (k - j);
                b += a;
        }

        b -= Length * sumBegin;
}