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Xorshift random number generators, also called shift-register generators, are a class of pseudorandom number generators that were invented by George Marsaglia. [1] They are a subset of linear-feedback shift registers (LFSRs) which allow a particularly efficient implementation in software without the excessive use of sparse polynomials. [2]
A structure similar to LCGs, but not equivalent, is the multiple-recursive generator: X n = (a 1 X n−1 + a 2 X n−2 + ··· + a k X n−k) mod m for k ≥ 2. With a prime modulus, this can generate periods up to m k −1, so is a useful extension of the LCG structure to larger periods.
The structure is similar to the Mersenne Twister, a large state made up of previous output words (32 bits each), from which a new output word is generated using linear recurrences modulo 2 over a finite binary field. However, a more complex recurrence produces a denser generator polynomial, producing better statistical properties.
A newer version from 1999 is based on a linear congruential generator, a 3-shift linear feedback shift-register and two multiply-with-carry generators. It is 10–20% slower than the 1993 version but has a larger period 2 123 and passes all tests in TestU01.
If addition or subtraction is used, the maximum period is (2 k − 1) × 2 M−1. If multiplication is used, the maximum period is (2 k − 1) × 2 M−3, or 1/4 of period of the additive case. If bitwise xor is used, the maximum period is 2 k − 1. For the generator to achieve this maximum period, the polynomial: y = x k + x j + 1
Blum Blum Shub takes the form + =, where M = pq is the product of two large primes p and q.At each step of the algorithm, some output is derived from x n+1; the output is commonly either the bit parity of x n+1 or one or more of the least significant bits of x n+1.
It was covered under the now-expired U.S. patent 5,732,138, titled "Method for seeding a pseudo-random number generator with a cryptographic hash of a digitization of a chaotic system." by Landon Curt Noll, Robert G. Mende, and Sanjeev Sisodiya. From 1997 to 2001, [2] there was a website at lavarand.sgi.com demonstrating the technique.