aoc2021-day7

aoc2021-day7.lisp

;;;; +----------------------------------------------------------------+
;;;; | Advent of Code 2021                                            |
;;;; +----------------------------------------------------------------+

(defpackage #:snippets/aoc2021/day7
  (:use #:cl)
  (:import-from
   #:alexandria)
  (:export
   #:day7))

(in-package #:snippets/aoc2021/day7)

(defun sum-absolute-error (positions choice)
  (loop for position in positions
        sum (abs (- position choice))))

(defun min-sum-absolute-error (positions)
  (sum-absolute-error positions (round (alexandria:median positions))))

(defun sum-triangular-error (positions choice)
  (loop for position in positions
        for n =  (abs (- position choice))
        sum (/ (* n (1+ n)) 2)))

(defun min-sum-triangular-error (positions)
  ;; Can't be arsed to implement weighted median :(
  (loop for choice from 0 to (reduce #'max positions)
        minimize (sum-triangular-error positions choice)))

(defun day7 (input)
  (list (min-sum-absolute-error input)
        (min-sum-triangular-error input)))