Lagged Fibonacci Pseudo-Random Generator

If you need a pseudo-random number generator which works regardless of the coresize, a Lagged Fibonacci generator is one of the better options.

Here's an implementation in 3 instructions with _rndj fixed at 1. _rndk can take the values 3, 4, 6, 7, 15, 22, 60 or 63 depending on the period required. With _rndk = 4 and coresize 8000, the period is 31200.

For more variations, see the Wikipedia article on Lagged Fibonacci generators.

           _rndk  equ 4

random    mov.a  >rdata,        random_out
          mod.ab #_rndk,        rdata
          add.a  *stack,        @rdata

rdata     dat    3711,           0
          dat    1674,           0
          dat    4323,           0
          dat    7835,           0

Sign

Calculates the sign of location's b-field, returning -1, 0 or 1.

sign      jmz.b  done,           location
          slt.b  location,       #1+CORESIZE/2
          mov.ab #-1,            location
          seq.ab #-1,            location
          mov.ab #1,             location
done

Absolute Value

This two-line redcode snippet returns the absolute value of location's b-field.

abs       slt.b  location,       #1+CORESIZE/2
          mul.ab #-1,            location

Euclid's extended

The extended Euclidean algorithm is used to calculate x and y given a and b in the following equation:

ax + by = gcd(a,b)

Implementation in Redcode:

          org    start

          temp   equ r-1

r         dat    55,            89 ; input a b - output gcd(a,b)
s         dat    1,             0
t         dat    0,             1  ; output x y

euclidext mov    r,             temp
          div.ab temp,          temp
          mov.ba temp,          temp
          mul    s,             temp
          sub    temp,          t

          mov    s,             temp
          mov    t,             s
          mov    temp,          t

          mod.ab r,             r
          mov.x  r,             r
start     jmn.a  euclidext,     r