__inline__ uint8_t _crc_ibutton_update(
	uint8_t __crc,
	uint8_t __data)

Optimized Dallas (now Maxim) iButton 8-bit CRC calculation.

Polynomial: x^8 + x^5 + x^4 + 1 (0x8C) Initial value: 0x0

See http://www.maxim-ic.com/appnotes.cfm/appnote_number/27

The following is the equivalent functionality written in C.

uint8_t
_crc_ibutton_update(uint8_t crc, uint8_t data)
{
    uint8_t i;

    crc = crc ^ data;
    for (i = 0; i < 8; i++)
    {
        if (crc & 0x01)
            crc = (crc >> 1) ^ 0x8C;
        else
            crc >>= 1;
    }

    return crc;
}