## GB_FLIP

### May 25, 2010

Here is our version:

`(define two31 #x80000000)`

`(define a (make-vector 56 -1))`

`(define fptr #f)`

`(define (mod-diff x y)`

(modulo (- x y) two31))

`; use this if you have logand`

; (define (mod-diff x y)

; (logand (- x y) #x7FFFFFFF))

`(define (flip-cycle)`

(do ((ii 1 (+ ii 1)) (jj 32 (+ jj 1))) ((< 55 jj))

(vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))

(do ((ii 25 (+ ii 1)) (jj 1 (+ jj 1))) ((< 55 ii))

(vector-set! a ii (mod-diff (vector-ref a ii) (vector-ref a jj))))

(set! fptr 54)

(vector-ref a 55))

`(define (init-rand seed)`

(let* ((seed (mod-diff seed 0)) (prev seed) (next 1))

(vector-set! a 55 prev)

(do ((i 21 (modulo (+ i 21) 55))) ((zero? i))

(vector-set! a i next)

(set! next (mod-diff prev next))

(set! seed (+ (quotient seed 2) (if (odd? seed) #x40000000 0)))

(set! next (mod-diff next seed))

(set! prev (vector-ref a i)))

(flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle) (flip-cycle)))

`(define (next-rand)`

(if (negative? (vector-ref a fptr)) (flip-cycle)

(let ((next (vector-ref a fptr)))

(set! fptr (- fptr 1)) next)))

`(define (unif-rand m)`

(let ((t (- two31 (modulo two31 m))))

(let loop ((r (next-rand)))

(if (<= t r) (loop (next-rand)) (modulo r m)))))

Standard Scheme doesn’t provide a `logand`

function, so we gave an alternative to Knuth’s `mod-diff`

macro. You should use the `logand`

version if possible, since bit operations are faster than division.

Knuth provides a test function:

`(define (test-flip)`

(init-rand -314159)

(when (not (= (next-rand) 119318998))

(error 'test-flip "Failure on the first try!"))

(do ((j 1 (+ j 1))) ((< 133 j)) (next-rand))

(when (not (= (unif-rand #x55555555) 748103812))

(error 'test-flip "Failure on the second try!"))

(display "OK, the gb-flip routines seem to work!") (newline))

You can run the program at http://programmingpraxis.codepad.org/nBm7d0wF. We’ll install GB_FLIP in our Standard Prelude, replacing the current random number generator, but using our own interface.

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I’ve been trying to come up with a Python version, but the global variables are giving me the run around.

Here’s my Python version. It’s a fairly direct translation of Knuth’s C-code to Python.

[…] random number generators in past exercises, including Donald Knuth’s lagged-fibonacci generator that is used in the Standard Prelude. We also looked at cellular automata in a previous exercise. […]

Java implementation.