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

// --------------------------------------------------------------------------
//
// 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 * const data, const 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(const uint8_t StartOfThisBlock, const uint8_t LastOfNextBlock, const 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::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 RollForwardSeveral(const uint8_t * const StartOfThisBlock, const uint8_t * const LastOfNextBlock, const unsigned int Length, const unsigned int Skip);

        // --------------------------------------------------------------------------
        //
        // Function
        //              Name:    RollingChecksum::GetChecksum()
        //              Purpose: Returns the checksum
        //              Created: 6/12/03
        //
        // --------------------------------------------------------------------------   
        inline uint32_t GetChecksum() const
        {
                return ((uint32_t)a) | (((uint32_t)b) << 16);
        }
        
        // Components, just in case they're handy
        inline uint16_t GetComponent1() const {return a;}
        inline uint16_t GetComponent2() const {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() const
        {
                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(const uint32_t Checksum)
        {
                return Checksum >> 16;
        }
        
private:
        uint16_t a;
        uint16_t b;
};

#endif // ROLLINGCHECKSUM__H