CRC.cpp

#include <windows.h>

#define CRC32_POLYNOMIAL     0x04C11DB7

unsigned long CRCTable[256];

DWORD Reflect(DWORD ref, char ch)
{
    DWORD value = 0;

    // Swap bit 0 for bit 7
    // bit 1 for bit 6, etc.
    for (int i = 1; i < (ch + 1); i++)
    {
        if (ref & 1)
            value |= 1 << (ch - i);
        ref >>= 1;
    }
    return value;
}

void CRC_BuildTable()
{
    DWORD crc;

    for (int i = 0; i <= 255; i++)
    {
        crc = Reflect(i, 8) << 24;
        for (int j = 0; j < 8; j++)
            crc = (crc << 1) ^ (crc & (1 << 31) ? CRC32_POLYNOMIAL : 0);

        CRCTable[i] = Reflect(crc, 32);
    }
}

DWORD CRC_Calculate(DWORD crc, void* buffer, DWORD count)
{
    BYTE* p;
    DWORD orig = crc;

    p = (BYTE*)buffer;
    while (count--)
        crc = (crc >> 8) ^ CRCTable[(crc & 0xFF) ^ *p++];

    return crc ^ orig;
}

DWORD CRC_CalculatePalette(DWORD crc, void* buffer, DWORD count)
{
    BYTE* p;
    DWORD orig = crc;

    p = (BYTE*)buffer;
    while (count--)
    {
        crc = (crc >> 8) ^ CRCTable[(crc & 0xFF) ^ *p++];
        crc = (crc >> 8) ^ CRCTable[(crc & 0xFF) ^ *p++];

        p += 6;
    }

    return crc ^ orig;
}