#!/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 (split-up nums)
  (map string->number (filter (lambda (x) (not (string-null? x))) (string-split nums char-set:whitespace))))

(define (seq-diff seq)
  (map (lambda (x y) (- y x)) (drop-right seq 1) (cdr seq)))

(define (all-seq-diffs seq)
  (letrec ((loop (lambda (l ls)
                   (if (every zero? l) ls
                       (loop (seq-diff l) (cons l ls))))))
    (loop seq '())))

(define (predict seq-diffs)
  (fold + 0 (map (lambda (x) (car (take-right x 1))) seq-diffs)))

(let* ((port (open-input-file "9.txt"))
       (lines (map split-up (read-lines port))))
  (close-port port)
  (display (fold + 0 (map (lambda (x) (predict (all-seq-diffs x))) lines))))