Fletcher32

Fletcher32 is a 32-bit checksum algorithm that belongs to the Fletcher family of checksums. It is designed to detect accidental data corruption in blocks of data and is used as a lightweight method for error detection in various software and hardware contexts. Fletcher32 aims to provide better error detection than simpler checksums while remaining computationally efficient.

The algorithm uses two 16-bit accumulators, s1 and s2, initialized to zero. It processes the input data as a sequence of 16-bit words; if the total number of bytes is odd, the last byte is padded with a zero to form a final 16-bit word. For each word w, the updates are: s1 = (s1 + w) mod 65535, and s2 = (s2 + s1) mod 65535. After processing all words, the 32-bit checksum is formed by combining the two accumulators as (s2 << 16) | s1. The modulus 65535 (0xFFFF) is used to keep the sums within 16-bit bounds.

Fletcher32 is valued for its speed on typical processors and its ability to detect a wide range of error patterns, including several single-bit and burst errors. However, it is not as robust as cyclic redundancy checks (CRCs) against all error patterns, and it is not suitable for cryptographic integrity or security purposes.

Implementation notes often include handling endianness consistently and ensuring that odd-length data is padded appropriately. Fletcher32 is employed in various data integrity contexts where a balance between protection level and computational overhead is desired, serving as a practical alternative to more complex error-detection schemes in lightweight systems.