2021/day07.scm

(import (chicken string))
(import (chicken format))
(import (chicken io))
(import (chicken sort))
(import srfi-141)

(define numbers (map string->number (string-split (read-line) ",")))

; PART 1
(let* ((sorted (sort numbers <))
       (median (list-ref sorted (floor/ (length numbers) 2)))
       (part1
        (foldl (lambda (x y) (+ x (abs (- y median))))
               0
               numbers)))
  (format #t "part 1: ~A~%" part1))

; PART 2
(define (triangular n) (/ (* n (+ n 1)) 2))

(define (part2- avg)
  (foldl (lambda (x y) (+ x (triangular (abs (- y avg)))))
         0
         numbers))

(let* ((len (length numbers))
       (sum (foldl + 0 numbers))
       (avg-ceil (ceiling/ sum len))
       (avg-floor (floor/ sum len))
       (part2 (min (part2- avg-ceil) (part2- avg-floor))))
  (format #t "part 2: ~A~%" part2))