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// --------------------------------------------------------------------------
//
// File
// Name: RollingChecksum.h
// Purpose: A simple rolling checksum over a block of data
// Created: 6/12/03
//
// --------------------------------------------------------------------------
#ifndef ROLLINGCHECKSUM__H
#define ROLLINGCHECKSUM__H
// --------------------------------------------------------------------------
//
// Class
// Name: RollingChecksum
// Purpose: A simple rolling checksum over a block of data -- can move the block
// "forwards" in memory and get the next checksum efficiently.
//
// Implementation of http://rsync.samba.org/tech_report/node3.html
// Created: 6/12/03
//
// --------------------------------------------------------------------------
class RollingChecksum
{
public:
RollingChecksum(const void *data, unsigned int Length);
// --------------------------------------------------------------------------
//
// Function
// Name: RollingChecksum::RollForward(uint8_t, uint8_t, unsigned int)
// Purpose: Move the checksum forward a block, given the first byte of the current block,
// last byte of the next block (it's rolling forward to) and the length of the block.
// Created: 6/12/03
//
// --------------------------------------------------------------------------
inline void RollForward(uint8_t StartOfThisBlock, uint8_t LastOfNextBlock, unsigned int Length)
{
// IMPLEMENTATION NOTE: Everything is implicitly mod 2^16 -- uint16_t's will overflow nicely.
a -= StartOfThisBlock;
a += LastOfNextBlock;
b -= Length * StartOfThisBlock;
b += a;
}
// --------------------------------------------------------------------------
//
// Function
// Name: RollingChecksum::GetChecksum()
// Purpose: Returns the checksum
// Created: 6/12/03
//
// --------------------------------------------------------------------------
inline uint32_t GetChecksum()
{
return ((uint32_t)a) | (((uint32_t)b) << 16);
}
// Components, just in case they're handy
inline uint16_t GetComponent1() {return a;}
inline uint16_t GetComponent2() {return b;}
// --------------------------------------------------------------------------
//
// Function
// Name: RollingChecksum::GetComponentForHashing()
// Purpose: Return the 16 bit component used for hashing and/or quick checks
// Created: 6/12/03
//
// --------------------------------------------------------------------------
inline uint16_t GetComponentForHashing()
{
return b;
}
// --------------------------------------------------------------------------
//
// Function
// Name: RollingChecksum::ExtractHashingComponent(uint32_t)
// Purpose: Static. Given a full checksum, extract the component used in the hashing table.
// Created: 14/1/04
//
// --------------------------------------------------------------------------
static inline uint16_t ExtractHashingComponent(uint32_t Checksum)
{
return Checksum >> 16;
}
private:
uint16_t a;
uint16_t b;
};
#endif // ROLLINGCHECKSUM__H
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