Advent of Code 2021 https://adventofcode.com/
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#lang racket
(define file "day2.txt")
(define lines (file->lines file))
(define steps (map string-split lines))
; part 1
(define (modify-position step position)
; (writeln position)
(let ([instruction (car step)]
[magnitude (string->number (second step))]
[forwardness (car position)]
[depth (cdr position)])
(cond [(equal? instruction "forward") (cons (+ forwardness magnitude) depth)]
[(equal? instruction "up") (cons forwardness (- depth magnitude))]
[(equal? instruction "down") (cons forwardness (+ depth magnitude))])))
(define position (foldl modify-position (cons 0 0) steps)) ; (forwardness, depth)
(define part1 (* (car position) (cdr position)))
; part 2
(define (modify-position2 step position)
; (writeln position)
(let ([instruction (first step)]
[magnitude (string->number (second step))]
[forwardness (first position)]
[depth (second position)]
[aim (third position)])
(cond [(equal? instruction "forward") (list (+ forwardness magnitude) (+ depth (* aim magnitude)) aim)]
[(equal? instruction "up") (list forwardness depth (- aim magnitude))]
[(equal? instruction "down") (list forwardness depth (+ aim magnitude))])))
(define position2 (foldl modify-position2 (list 0 0 0) steps)) ; (forwardness, depth, aim)
(define part2 (* (first position2) (second position2)))