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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 stream 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 ...
When the data word is divided into 8-bit blocks, as in the example above, two 8-bit sums result and are combined into a 16-bit Fletcher checksum. Usually, the second sum will be multiplied by 256 and added to the simple checksum, effectively stacking the sums side-by-side in a 16-bit word with the simple checksum at the least significant end.
A message that is m bits long can be viewed as a corner of the m-dimensional hypercube. The effect of a checksum algorithm that yields an n-bit checksum is to map each m-bit message to a corner of a larger hypercube, with dimension m + n. The 2 m + n corners of this hypercube represent all possible received messages.
The term bit length is technical shorthand for this measure. For example, computer processors are often designed to process data grouped into words of a given length of bits (8 bit, 16 bit, 32 bit, 64 bit, etc.). The bit length of each word defines, for one thing, how many memory locations can be independently addressed by the processor.
Below is the relevant part of the GNU sum source code (GPL licensed). It computes a 16-bit checksum by adding up all bytes (8-bit words) of the input data stream. In order to avoid many of the weaknesses of simply adding the data, the checksum accumulator is circular rotated to the right by one bit at each step before the new char is added.
In the early days of computing, it was used for differing numbers of bits based on convention and computer hardware design, but today means 8 bits. A more accurate, but less commonly used name for 8 bits is octet. Commonly, a decimal SI metric prefix (such as kilo-) is used with bit and byte to express larger sizes (kilobit, kilobyte). But ...
This standard defines the format for 32-bit numbers called single precision, as well as 64-bit numbers called double precision and longer numbers called extended precision (used for intermediate results). Floating-point representations can support a much wider range of values than fixed-point, with the ability to represent very small numbers ...
The first pixel in a row is the most significant bit. The extra bits used to make the length equal to a byte are ignored. The following formula can be used to calculate the number of required bytes ⌈width / 8⌉ * height. If we use the example above a ⌈6 / 8⌉ * 10 would be 10 bytes.