aoc2020/9.rkt

#lang curly-fn racket

(require "scripts/aoc.rkt")

;; solution for day 9

;; helper functions here

;; linear time sum checker, assuming sorted subrange
(define (contains-sum? vec start end wanted-sum)
  (define (helper [a start] [b (sub1 end)])
    (define cur-sum (+ (vector-ref vec a) (vector-ref vec b)))
    (cond
      [(>= a b) #f]
      [(= cur-sum wanted-sum) #t]
      [(< cur-sum wanted-sum) (helper (add1 a) b)]
      [(> cur-sum wanted-sum) (helper a (sub1 b))]))
  (vector-sort! vec < start end)
  (helper))

;; continuously sort the part of the vector below the iterator, and call contains-sum?
(define (part1 vec)
  (for*/first ([i (in-range 25 (vector-length vec))] [x (in-value (vector-ref vec i))]
               #:unless (contains-sum? vec 0 i x))
    (cons i x)))

(define (part2 vec pt1-ans)
  (match-define (cons pt1-i pt1-val) pt1-ans)
  (for*/first ([ct (in-range 3 100)] [i (in-range 0 (- pt1-i ct))]
               [sub (in-value (vector-copy vec i (+ i ct)))]
               #:when (= pt1-val (for/fold ([x 0]) ([v (in-vector sub)]) (+ x v))))
    (+ (vector-argmin identity sub) (vector-argmax identity sub))))

(module+ main
  (define input (list->vector (file->list "inputs/9")))
  ;; part 1
  (define pt1 (part1 (vector-copy input)))
  (answer 9 1 (cdr pt1))

  ;; part 2
  (answer 9 2 (part2 (vector-copy input) pt1))

  (displayln "meow"))