Question

Who knows how this function calculates checksum values?

My goal is to rewrite the FoxPro application on .NET, preserving the old database, but some values in the DB were calculated with this function.

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Question is closed.

I didn’t notice that FOXCHKSUM has been internal function in the project.

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But, anyway, thanx for aswers!

Answer 1

If is it interested to somebody – I found Foxpro algorithm that calc Crc16 and Crc32 with functions SYS(2107) and SYS(2007). (See answer of @Stuart DunkeId).

It is not really CRC, but some checksum calculation. Anyway that function return just the same result that foxpro analog.

Here is it on C#:

    public static uint Crc(byte[] bytes, uint seed = (uint)0, int lcBitLen = 16)
    {
        var laPower = new byte[] { 0x1, 0x2, 0x4, 0x8, 0x10, 0x20, 0x40, 0x80 };
        var lnBitLen = lcBitLen == 32 ? 0x8005 : 0x1021; // 1021 hex (16bit), 8005 hex (32bit)
        var lnCrc = seed == 0 ? 0xffffffff : seed;       // Reset for Each Text Block
        foreach (var byteVal in bytes)
        {
            for (var inLoop = 7; inLoop >= 0; inLoop--)
            {
                var testBit = ((lnCrc & 0x8000) == 0x8000 && (byteVal & laPower[inLoop]) != laPower[inLoop])
                              || ((lnCrc & 0x8000) != 0x8000 && (byteVal & laPower[inLoop]) == laPower[inLoop]);
                lnCrc = (lnCrc & 0x7fff)*2;
                if (testBit)
                    lnCrc = (uint)(lnCrc ^ lnBitLen);
            }
        }
        return lnCrc;
    }