#!/usr/bin/env -S guile -s
!#
(use-modules (ice-9 textual-ports)
             (srfi srfi-1))

(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 (candidates-for type pos)
  (let ((x (car pos))
        (y (cdr pos)))
    (cond ((equal? type #\|) `(,(cons x (1+ y))
                               ,(cons x (1- y))))
          ((equal? type #\-) `(,(cons (1+ x) y)
                               ,(cons (1- x) y)))
          ((equal? type #\L) `(,(cons x (1- y))
                               ,(cons (1+ x) y)))
          ((equal? type #\J) `(,(cons x (1- y))
                               ,(cons (1- x) y)))
          ((equal? type #\7) `(,(cons (1- x) y)
                               ,(cons x (1+ y))))
          ((equal? type #\F) `(,(cons x (1+ y))
                               ,(cons (1+ x) y)))
          ((equal? type #\S) `(,(cons x (1+ y))
                               ,(cons x (1- y))
                               ,(cons (1+ x) y)
                               ,(cons (1- x) y)))
          ((equal? type #\.) '()))))

(let* ((port (open-input-file "10s.txt"))
       (lines (read-lines port))
       (candidates '((0 . 2)))
       (seen '()))
  (close-port port)
  (while (not (null? candidates))
    (set! candidates (append! (cdr candidates) (filter (lambda (x) (member x seen)) (candidates-for (string-ref (list-ref lines (cdar candidates)) (caar candidates)) (car candidates)))))
    (display candidates)
    (newline)))