day09.scm - advent-of-code - Perserverance, or the lack thereof

day09.scm (1473B)

    1 (use-modules (ice-9 popen))
    2 (use-modules (ice-9 rdelim))
    3 (use-modules (ice-9 format))
    4 (use-modules (ice-9 receive))
    5 (use-modules (srfi srfi-1))
    6 
    7 (define (parse-numbers numbers)
    8   (filter (lambda (x) x) (map string->number (string-split numbers #\ ))))
    9 
   10 (define (parse-input filename)
   11   (let ((file (open-input-file filename))
   12         (report '()))
   13     (while #t
   14            (let ((line (car (%read-line file))))
   15              (if (eof-object? line)
   16                  (break)
   17                  (set! report (cons (parse-numbers line) report)))))
   18     (reverse report)))
   19 
   20 (define (all-zero history)
   21   (if (null? history)
   22       #t
   23       (and (= (car history) 0) (all-zero (cdr history)))))
   24 
   25 (define (calc-diff history)
   26   (if (< (length history) 2)
   27       '()
   28       (cons (- (car (cdr history)) (car history))
   29             (calc-diff (cdr history)))))
   30 
   31 (define (extrapolate history)
   32   (if (all-zero history)
   33       0
   34       (+ (car (last-pair history)) (extrapolate (calc-diff history)))))
   35 
   36 (let* ((report (parse-input "input.txt"))
   37        (extrapolated (map extrapolate report)))
   38   ;; 1647269739
   39   (format #t "Part 1: ~d" (reduce + #nil extrapolated))
   40   (newline))
   41 
   42 (define (backward-extrapolate history)
   43   (if (all-zero history)
   44       0
   45       (- (car history) (backward-extrapolate (calc-diff history)))))
   46 
   47 (let* ((report (parse-input "input.txt"))
   48        (extrapolated (map backward-extrapolate report)))
   49   ;; 864
   50   (format #t "Part 2: ~d" (reduce + #nil extrapolated))
   51   (newline))