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; 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")

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