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@@ -0,0 +1,60 @@
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+#lang racket
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+
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+(require "../../lib/utils.rkt")
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+
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+(define lines
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+ (call-with-input-file "input"
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+ (lambda (fp)
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+ (get-lines fp))))
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+
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+(define instructions (string->list (car lines)))
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+
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+(define (make-instruction-generator)
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+ (define cur instructions)
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+ (lambda ()
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+ (when (null? cur)
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+ (set! cur instructions))
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+ (define ret (car cur))
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+ (set! cur (cdr cur))
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+ ret))
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+
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+(define next-instruction (make-instruction-generator))
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+
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+(set! lines (cddr lines))
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+
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+(define (parse-map str)
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+ (define p1 (string-split str "="))
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+ (define key (string-trim (car p1)))
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+ (define val-str (string-trim (cadr p1)))
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+ (define value (list (substring val-str 1 4) (substring val-str 6 9)))
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+ (cons key value))
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+
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+(define the-map (make-hash (map parse-map lines)))
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+
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+(define (is-start? str)
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+ (char=? #\A (string-ref str 2)))
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+
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+(define (is-end? str)
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+ (char=? #\Z (string-ref str 2)))
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+
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+(define (simulate start)
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+ (let loop ((i 0) (cur start))
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+ (if (is-end? cur)
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+ i
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+ (let ()
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+ (define instruction (next-instruction))
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+ (define (next-point point)
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+ (define mapping (hash-ref the-map point))
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+ (if (char=? instruction #\L)
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+ (car mapping)
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+ (cadr mapping)))
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+ (loop (+ 1 i) (next-point cur))))))
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+
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+
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+(define starts
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+ (let ()
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+ (define lst (hash->list the-map))
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+ (define keys (map car lst))
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+ (filter is-start? keys)))
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+
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+(apply lcm (map simulate starts))
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