day07.rkt

#lang racket/base

(require racket/string)

(define positions
  (call-with-input-file "day07.txt"
    (lambda (in)
      (map string->number (string-split (read-line in) ",")))))

(define (compute-costs poss cost-proc)
  (for*/fold ([costs (hasheqv)])
             ([target-pos (in-inclusive-range 0 (apply max poss))]
              [source-pos (in-list poss)])
    (hash-update costs target-pos (λ (v) (+ v (cost-proc (abs (- target-pos source-pos))))) 0)))

(define (solution poss [cost-proc values])
  (for/fold ([res #f] [fuel +inf.0] #:result fuel)
            ([(pos cost) (in-hash (compute-costs poss cost-proc))])
    (if (< cost fuel)
        (values pos cost)
        (values res fuel))))

(define part1 (time (solution positions)))
(define part2 (time (solution positions (λ (d) (* d (add1 d) 1/2)))))

(module+ test
  (require rackunit)
  (check-= part1 348664 0)
  (check-= part2 100220525 0))