Parallel Computation
Another common optimization uses a lookup table indexed by highest order coefficients of rem to perform the inner loop over 8 bits in fewer steps. A 256-entry lookup table is a particularly common choice, although using a 16-entry table twice per byte is very compact and still faster than the bit at a time version. This replaces the inner loop over j with
- Code fragment 6: Cores of table based division
One of the most commonly encountered CRC algorithms is known as CRC-32, used by (among others) Ethernet, FDDI, ZIP and other archive formats, and PNG image format. Its polynomial can be written msbit-first as 0x04C11DB7, or lsbit-first as 0xEDB88320. The W3C webpage on PNG includes an appendix with a short and simple table-driven implementation in C of CRC-32. You will note that the code corresponds to the lsbit-first byte-at-a-time pseudocode presented here, and the table is generated using the bit-at-a-time code.
Read more about this topic: Computation Of CRC
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