HashValue is an overridden Word function in
TIdHashCRC16 that implements calculation of the 16-bit CRC value for a block of data. HashValue reimplements the inherited method to use the pre-computed table of 16-bit CRC values for all 256 possible character combinations that can be represented in a byte from the input data. HashValue calculates the CRC result by performing an exclusive OR for the current CRC value and the precomputed CRC value in the table for low nibble of the current input data.