Created
February 11, 2010 14:43
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The Seasoned Schemer, scramble
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;; scramble | |
; (1 1 1 3 4 2 1 1 9 2) -> (1 1 1 1 1 4 1 1 1 9) | |
; (1 2 3 4 5 6 7 8 9) -> (1 1 1 1 1 1 1 1 1) | |
; (1 2 3 1 2 3 4 1 8 2 10) -> (1 1 1 1 1 1 1 1 2 8 2) | |
(define one? | |
(lambda (n) | |
(= n 1))) | |
(define sub1 | |
(lambda (n) | |
(- n 1))) | |
(define pick | |
(lambda (n lat) | |
(if (one? n) | |
(car lat) | |
(pick (sub1 n)(cdr lat))))) | |
(define pick | |
(lambda (n lat) | |
(list-ref lat (sub1 n)))) | |
(pick 3 '(a b c d e f g)) | |
; -> c | |
(define scramble-b | |
(lambda (tup rev-pre) | |
(if (null? tup) | |
'() | |
(cons (pick (car tup) | |
(cons (car tup) rev-pre)) | |
(scramble-b (cdr tup) | |
(cons (car tup) rev-pre)))))) | |
(define scramble | |
(lambda (tup) | |
(scramble-b tup '()))) | |
(define scramble-b | |
(lambda (tup rev-pre) | |
(if (null? tup) | |
'() | |
(let* ((n (car tup)) | |
(rev (cons n rev-pre))) | |
(display (format "n = ~a, ret = ~a, tup = ~a, rev-pre = ~a\n" | |
n (pick n rev) tup rev-pre)) | |
(cons (pick n rev) | |
(scramble-b (cdr tup) rev)))))) | |
; (1 1 1 3 4 2 1 1 9 2) -> (1 1 1 1 1 4 1 1 1 9) | |
(scramble '(1 1 1 3 4 2 1 1 9 2)) | |
;; n = 1, ret = 1, tup = (1 1 1 3 4 2 1 1 9 2), rev-pre = () | |
;; n = 1, ret = 1, tup = (1 1 3 4 2 1 1 9 2), rev-pre = (1) | |
;; n = 1, ret = 1, tup = (1 3 4 2 1 1 9 2), rev-pre = (1 1) | |
;; n = 3, ret = 1, tup = (3 4 2 1 1 9 2), rev-pre = (1 1 1) | |
;; n = 4, ret = 1, tup = (4 2 1 1 9 2), rev-pre = (3 1 1 1) | |
;; n = 2, ret = 4, tup = (2 1 1 9 2), rev-pre = (4 3 1 1 1) | |
;; n = 1, ret = 1, tup = (1 1 9 2), rev-pre = (2 4 3 1 1 1) | |
;; n = 1, ret = 1, tup = (1 9 2), rev-pre = (1 2 4 3 1 1 1) | |
;; n = 9, ret = 1, tup = (9 2), rev-pre = (1 1 2 4 3 1 1 1) | |
;; n = 2, ret = 9, tup = (2), rev-pre = (9 1 1 2 4 3 1 1 1) | |
;; (1 1 1 1 1 4 1 1 1 9) | |
; (1 2 3 4 5 6 7 8 9) -> (1 1 1 1 1 1 1 1 1) | |
(scramble '(1 2 3 4 5 6 7 8 9)) | |
;; n = 1, ret = 1, tup = (1 2 3 4 5 6 7 8 9), rev-pre = () | |
;; n = 2, ret = 1, tup = (2 3 4 5 6 7 8 9), rev-pre = (1) | |
;; n = 3, ret = 1, tup = (3 4 5 6 7 8 9), rev-pre = (2 1) | |
;; n = 4, ret = 1, tup = (4 5 6 7 8 9), rev-pre = (3 2 1) | |
;; n = 5, ret = 1, tup = (5 6 7 8 9), rev-pre = (4 3 2 1) | |
;; n = 6, ret = 1, tup = (6 7 8 9), rev-pre = (5 4 3 2 1) | |
;; n = 7, ret = 1, tup = (7 8 9), rev-pre = (6 5 4 3 2 1) | |
;; n = 8, ret = 1, tup = (8 9), rev-pre = (7 6 5 4 3 2 1) | |
;; n = 9, ret = 1, tup = (9), rev-pre = (8 7 6 5 4 3 2 1) | |
;; (1 1 1 1 1 1 1 1 1) | |
; (1 2 3 1 2 3 4 1 8 2 10) -> (1 1 1 1 1 1 1 1 2 8 2) | |
(scramble '(1 2 3 1 2 3 4 1 8 2 10)) | |
;; n = 1, ret = 1, tup = (1 2 3 1 2 3 4 1 8 2 10), rev-pre = () | |
;; n = 2, ret = 1, tup = (2 3 1 2 3 4 1 8 2 10), rev-pre = (1) | |
;; n = 3, ret = 1, tup = (3 1 2 3 4 1 8 2 10), rev-pre = (2 1) | |
;; n = 1, ret = 1, tup = (1 2 3 4 1 8 2 10), rev-pre = (3 2 1) | |
;; n = 2, ret = 1, tup = (2 3 4 1 8 2 10), rev-pre = (1 3 2 1) | |
;; n = 3, ret = 1, tup = (3 4 1 8 2 10), rev-pre = (2 1 3 2 1) | |
;; n = 4, ret = 1, tup = (4 1 8 2 10), rev-pre = (3 2 1 3 2 1) | |
;; n = 1, ret = 1, tup = (1 8 2 10), rev-pre = (4 3 2 1 3 2 1) | |
;; n = 8, ret = 2, tup = (8 2 10), rev-pre = (1 4 3 2 1 3 2 1) | |
;; n = 2, ret = 8, tup = (2 10), rev-pre = (8 1 4 3 2 1 3 2 1) | |
;; n = 10, ret = 2, tup = (10), rev-pre = (2 8 1 4 3 2 1 3 2 1) | |
;; (1 1 1 1 1 1 1 1 2 8 2) | |
(define scramble | |
(lambda (tup) | |
(letrec ((iter (lambda (t rev-pre) | |
(if (null? t) | |
'() | |
(let* ((n (car t)) | |
(rev (cons n rev-pre))) | |
(cons (pick n rev) | |
(iter (cdr t) rev))))))) | |
(iter tup '())))) |
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