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// --------------------------------------------------------------------------
//
// 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;
}
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