aoc2023-day9.scm

(use-modules (ice-9 textual-ports) (srfi srfi-1) (srfi srfi-26) (ice-9 format))

(define (read-input)
  (let ([line (get-line (current-input-port))])
    (if (eof-object? line)
	'()
	(cons (map string->number (string-split line #\space))
	      (read-input)))))

(define (list-of-differences lst)
  (map (cut apply - <>) (zip (cdr lst) lst)))

(define (%part-common op getter lst)
  (if (eqv? (length (delete-duplicates lst)) 1)
      (car lst)
      (op (getter lst)
	  (%part-common op getter (list-of-differences lst)))))

(define (part-common op getter input)
  (apply + (map (cut %part-common op getter <>) input)))

(let ([input (read-input)])
  (format #t "Part 1: ~a~%" (part-common + last input))
  (format #t "Part 2: ~a~%" (part-common - first input)))