IdHashCRC.pas contains constants, classes, and methods needed to implement 16- and 32-bit versions of the Cyclic Redundancy Check (CRC) algorithm used to detect errors in data storage or transmission.

Cyclic Redundancy Check classes detect small changes in blocks of data, and is often used in error-correcting protocols. CRC values are often included in data transmissions, and the CRC algorithm is reapplied to received data to detect error conditions.

In Indy, CRC calculations are performed using a pre-computed table containing all 256 possible combinations of the CRC value, and uses a 2-byte exclusive-OR operation for the 1-byte lookup value to calculate the CRC result.