day1.rkt

#lang racket

(require data/maybe point-free)

(provide part1 part2)

(define/compose get-input (curry map string->number) file->lines)

(define input (get-input "input/d1"))

(define (part1) (apply * (from-just! (find-pair input 2020))))

(define (part2) (apply * (from-just! (find-triplet input 2020))))

(define (find-pair entries sum)
  (match entries
    ['() nothing]
    [(list x xs ...) (let ([y (- sum x)]) (if (member y xs)
                                            (just (list x y))
                                            (find-pair xs sum)))]))

(define (find-triplet entries sum)
  (match entries
    ['() nothing]
    [(list x xs ...) (match (find-pair xs (- sum x)) 
                       [(just (list y z)) (just (list x y z))] 
                       [nothing (find-triplet xs sum)])]))