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Adler-32 is a checksum algorithm which was invented by Mark Adler. Compared to a cyclic redundancy check of the same length it trades reliability for speed. Compared to its original form (Fletcher-16), Adler-32 is more reliable than Fletcher-16. However, Adler-32 is slightly less reliable than Fletcher-32
It is a modification of the Fletcher checksum.
An Adler-32 checksum is obtained by calculating two 16-bit checksums A and B and concatenating their bits into a 32-bit integer. A is the sum of all bytes in the string plus one, and B is the sum of the individual values of A from each step.
At the beginning of an Adler-32 run, A is initialized to 1, B to 0. The sums are done modulo 65521 (the largest prime number smaller than 216). The bytes are stored in network order (big endian), B occupying the two most significant bytes.