The Well Equidistributed Long-period Linear (WELL) is a family of pseudorandom number generators developed in 2006 by François Panneton, Pierre L'Ecuyer, and Makoto Matsumoto(松本 眞). It is a form of linear-feedback shift register optimized for software implementation on a 32-bit machine.

Operational design

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 F 2 {\displaystyle F_{2}}. However, a more complex recurrence produces a denser generator polynomial, producing better statistical properties.

Each step of the generator reads five words of state: the oldest 32 bits (which may straddle a word boundary if the state size is not a multiple of 32), the newest 32 bits, and three other words in between.

Then a series of eight single-word transformations (mostly of the form x := x ⊕ ( x ≫ k ) {\textstyle x:=x\oplus (x\gg k)} and six exclusive-or operations combine those into two words, which become the newest two words of state, one of which will be the output.

Variants

Specific parameters are provided for the following generators:

  • WELL512a
  • WELL521a, WELL521b
  • WELL607a, WELL607b
  • WELL800a, WELL800b
  • WELL1024a, WELL1024b
  • WELL19937a, WELL19937b, WELL19937c
  • WELL21701a
  • WELL23209a, WELL23209b
  • WELL44497a, WELL44497b.

Numbers give the state size in bits; letter suffixes denote variants of the same size.

Implementations

  • (Free for non-commercial use)
  • (Free for non-commercial use)
  • (Public Domain)

External links