; dawkins weasel
(define (range . args)
(case (length args)
((1) (range 0 (car args) (if (negative? (car args)) -1 1)))
((2) (range (car args) (cadr args) (if (< (car args) (cadr args)) 1 -1)))
((3) (let ((le? (if (negative? (caddr args)) >= <=)))
(let loop ((x(car args)) (xs '()))
(if (le? (cadr args) x)
(reverse xs)
(loop (+ x (caddr args)) (cons x xs))))))
(else (error 'range "unrecognized arguments"))))
(define rand #f)
(define randint #f)
(let ((two31 #x80000000) (a (make-vector 56 -1)) (fptr #f))
(define (mod-diff x y) (modulo (- x y) two31)) ; generic version
; (define (mod-diff x y) (logand (- x y) #x7FFFFFFF)) ; fast version
(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)))))
(init-rand 19380110) ; happy birthday donald e knuth
(set! rand (lambda seed
(cond ((null? seed) (/ (next-rand) two31))
((eq? (car seed) 'get) (cons fptr (vector->list a)))
((eq? (car seed) 'set) (set! fptr (caadr seed))
(set! a (list->vector (cdadr seed))))
(else (/ (init-rand (modulo (numerator
(inexact->exact (car seed))) two31)) two31)))))
(set! randint (lambda args
(cond ((null? (cdr args))
(if (< (car args) two31) (unif-rand (car args))
(floor (* (next-rand) (car args)))))
((< (car args) (cadr args))
(let ((span (- (cadr args) (car args))))
(+ (car args)
(if (< span two31) (unif-rand span)
(floor (* (next-rand) span))))))
(else (let ((span (- (car args) (cadr args))))
(- (car args)
(if (< span two31) (unif-rand span)
(floor (* (next-rand) span))))))))))
(define (rept n . cs)
(make-string n (if (pair? cs) (car cs) #\space)))
(define (align str wid aline)
(let ((len (string-length str)))
(cond ((< wid len) (rept wid #\#))
((eq? aline'left) (string-append str (rept (- wid len))))
((eq? aline 'center)
(let* ((left (quotient (- wid len) 2)) (right (- wid len left)))
(string-append (rept left) str (rept right))))
((eq? aline 'right) (string-append (rept (- wid len)) str))
(else (error 'align "invalid alignment specifier")))))
(define (rand-letter)
(let ((r (randint 27)))
(if (zero? r) #\space
(integer->char (+ r 64)))))
(define (mutate guess)
(map (lambda (g) (if (zero? (randint 20)) (rand-letter) g)) guess))
(define (score guess target)
(apply + (map (lambda (g t) (if (char=? g t) 1 0)) guess target)))
(define (get-best guess target)
(let loop ((k 100) (best 0) (mutation guess))
(if (zero? k) (values best mutation)
(let* ((m (mutate guess)) (s (score m target)))
(if (< best s)
(loop (- k 1) s m)
(loop (- k 1) best mutation))))))
(define (display-weasel guess target)
(for-each (lambda (t g)
(if (char=? t g)
(display g)
(if (char=? #\space g)
(display #\_)
(display (char-downcase g)))))
target guess))
(define (dawkins target)
(let* ((target (string->list (string-upcase target)))
(len (length target))
(guess (map (lambda (x) (rand-letter)) (range len)))
(current (score guess target)))
(let loop ((i 0) (current current) (guess guess))
(display (align (number->string i) 4 'left))
(display (align (number->string current) 4 'left))
(display-weasel guess target) (newline)
(when (< current len)
(call-with-values
(lambda () (get-best guess target))
(lambda (best mutation)
(loop (+ i 1) best mutation)))))))
(dawkins "METHINKS IT IS LIKE A WEASEL")
programmingpraxis
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Scheme,
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on Nov 13: