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; credit card validation (define (digits n . args) (let ((b (if (null? args) 10 (car args)))) (let loop ((n n) (d '())) (if (zero? n) d (loop (quotient n b) (cons (modulo n b) d)))))) (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-syntax assert (syntax-rules () ((assert expr result) (if (not (equal? expr result)) (for-each display `( #\newline "failed assertion:" #\newline expr #\newline "expected: " ,result #\newline "returned: " ,expr #\newline)))))) (define (luhn n) (define (x2 n) (vector-ref #(0 2 4 6 8 1 3 5 7 9) n)) (let loop ((ds (reverse (digits n))) (s 0) (two? #t)) (cond ((null? ds) (let* ((sum (modulo s 10)) (check (if (zero? sum) 0 (- 10 sum)))) (+ (* n 10) check))) (two? (loop (cdr ds) (+ s (x2 (car ds))) #f)) (else (loop (cdr ds) (+ s (car ds)) #t))))) (define (luhn? n) (define (x2 n) (vector-ref #(0 2 4 6 8 1 3 5 7 9) n)) (let loop ((ds (reverse (digits n))) (s 0) (two? #f)) (cond ((null? ds) (zero? (modulo s 10))) (two? (loop (cdr ds) (+ s (x2 (car ds))) #f)) (else (loop (cdr ds) (+ s (car ds)) #t))))) (define (luhn-test) (assert (luhn 4992739871) 49927398716) (assert (luhn? 49927398716) #t) (assert (luhn? 49927398715) #f) (do ((i 0 (+ i 1))) ((= i 10000)) (assert (luhn? (luhn (randint #e1e8))) #t)) (do ((i 0 (+ i 1))) ((= i 100)) (assert (luhn? (luhn (randint (expt 10 i)))) #t))) (luhn-test) (define luhn #f) (define luhn? #f) (letrec ((x2 (lambda (n) (vector-ref #(0 2 4 6 8 1 3 5 7 9) n))) (luhn-sum (lambda (n two?) (let loop ((ds (reverse (digits n))) (s 0) (two? two?)) (cond ((null? ds) s) (two? (loop (cdr ds) (+ s (x2 (car ds))) #f)) (else (loop (cdr ds) (+ s (car ds)) #t))))))) (set! luhn (lambda (n) (let* ((sum (modulo (luhn-sum n #t) 10)) (check (if (zero? sum) 0 (- 10 sum)))) (+ (* n 10) check)))) (set! luhn? (lambda (n) (zero? (modulo (luhn-sum n #f) 10))))) (luhn-test) (display (luhn 4992739871)) (newline) (display (luhn? 49927398716)) (newline)
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