Range coding

Range coding (or range encoding) is an entropy coding method defined by G. Nigel N. Martin in a 1979 paper,^[1] which effectively rediscovered the FIFO arithmetic code first introduced by Richard Clark Pasco in 1976.^[2] Given a stream of symbols and their probabilities, a range coder produces a space-efficient stream of bits to represent these symbols and, given the stream and the probabilities, a range decoder reverses the process.

Range coding is very similar to arithmetic coding, except that coding is done with digits in any base, instead of with bits, and so it is faster when using larger bases (e.g. a byte) at small cost in compression efficiency.^[3] After the expiration of the first (1978) arithmetic coding patent,^[4] range coding appeared to clearly be free of patent encumbrances. This particularly drove interest in the technique in the open source community. Since that time, patents on various well-known arithmetic coding techniques have also expired.