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#!/usr/bin/env -S guile -s
!#
(use-modules (ice-9 regex)
(ice-9 textual-ports)
(srfi srfi-1)
(srfi srfi-69))
(define (read-lines port)
(letrec ((loop (lambda (l ls)
(if (eof-object? l)
ls
(loop (get-line port) (cons l ls))))))
(reverse (loop (get-line port) '()))))
(define (parse-line line)
(let ((m (string-match "([A-Z]{3}) = \\(([A-Z]{3}), ([A-Z]{3})\\)" line)))
`(,(match:substring m 1) (,(match:substring m 2) ,(match:substring m 3)))))
(define (find-node tag tree)
(car (filter (lambda (x) (equal? tag (car x))) tree)))
(define (solve-path start tree instructions)
(let ((current-instruction 0)
(current-node start)
(count 0))
(while (not (equal? (string-ref current-node 2) #\Z))
(if (equal? (string-ref instructions current-instruction) #\L)
(set! current-node (caadr (find-node current-node tree)))
(set! current-node (cadadr (find-node current-node tree))))
(set! current-instruction (modulo (1+ current-instruction) (string-length instructions)))
(set! count (1+ count)))
count))
(let* ((port (open-input-file "8.txt"))
(lines (read-lines port))
(instructions (car lines))
(tree (map parse-line (cddr lines)))
(start-nodes (filter (lambda (x) (equal? (string-ref x 2) #\A)) (map car tree))))
(close-port port)
(display (apply lcm (map (lambda (x) (solve-path x tree instructions)) start-nodes))))
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