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#!/usr/bin/env -S guile -s
!#
(use-modules (ice-9 regex)
(ice-9 textual-ports)
(srfi srfi-1))
(define line-length 140) ;; nasty :(
(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-line line)
(letrec ((loop (lambda (l ls)
(if (not l) ls
(loop (string-match "[0-9]+" line (match:end l)) (cons l ls))))))
(loop (string-match "[0-9]+" line) '())))
(define (find-candidates lines)
(let ((i -1))
(concatenate (map-in-order (lambda (line) (let ((cands (candidates-for-line line)))
(set! i (+ i 1))
(map (lambda (cand)
`(,i ,(match:start cand) ,(- (match:end cand) (match:start cand)) ,(string->number (match:substring cand)))) cands))) lines))))
(define (saturating-add lhs rhs max)
(let ((res (+ lhs rhs)))
(if (> res max) max res)))
(define (saturating-sub lhs rhs min)
(let ((res (- lhs rhs)))
(if (< res min) min res)))
(define (contains-part-symbol? str)
(string-match "[^0-9|\\.]" str))
(define (part-number? candidate lines)
(let ((above (substring (list-ref lines (saturating-sub (car candidate) 1 0))
(saturating-sub (cadr candidate) 1 0)
(saturating-add (+ (cadr candidate) (caddr candidate)) 1 line-length)))
(below (substring (list-ref lines (saturating-add (car candidate) 1 (- (length lines) 1)))
(saturating-sub (cadr candidate) 1 0)
(saturating-add (+ (cadr candidate) (caddr candidate)) 1 line-length)))
(left (substring (list-ref lines (car candidate))
(saturating-sub (cadr candidate) 1 0)
(cadr candidate)))
(right (substring (list-ref lines (car candidate))
(+ (cadr candidate) (caddr candidate))
(saturating-add (+ (cadr candidate) (caddr candidate)) 1 line-length))))
(or (contains-part-symbol? above)
(contains-part-symbol? below)
(contains-part-symbol? left)
(contains-part-symbol? right))))
(let* ((port (open-input-file "3.txt"))
(lines (read-lines port)))
(close-port port)
(display (fold + 0 (map cadddr (filter (lambda (cand) (part-number? cand lines)) (find-candidates lines))))))
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