Advent of Code 2025 Discussions

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For me today was a big lesson in keeping it simple. I did a lot of thinking about k-d trees and grids before staring at the ceiling for a few minutes, going "wait a second... this isn't that much" and just brute-forcing it (with some pretty ugly code).

Part 2 full solution

I'm so sorry about this code :see_no_evil_monkey:

#lang racket

(define (dist x y)
  (let ([delta (for/list ([nth `(,first ,second ,third)])
                 (- (nth y) (nth x)))])
    (apply + (map (curryr expt 2) delta))))

(define
  coords
  (map (λ (v) (map string->number v))
   (map (curryr string-split ",")
    (file->lines "test"))))

(define distances
  (sort #:key first
        (for*/list ([i (combinations coords 2)])
          (list (dist (first i) (second i)) (first i) (second i))) <))

(define circuits (for/list ([coord coords]) (set coord)))

(for ([distance distances])
  (define new-set (set))

  (set! circuits
        (filter identity
                (for/list ([circuit circuits])
                  (if (or (set-member? circuit (second distance)) (set-member? circuit (third distance)))
                      (begin
                        (set! new-set (set-union new-set circuit))
                        (when (= (length coords) (length (set->list new-set)))
                          (display (* (first (second distance)) (first (third distance))))
                          (exit))
                        #f)
                      circuit))))

  (set! circuits (cons new-set circuits)))
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