day3.rkt

(struct pos (x y) #:transparent)

(define (pos+ p1 p2)
  (match-let ([(pos x1 y1) p1]
              [(pos x2 y2) p2])
    (pos (+ x1 x2)
         (+ y1 y2))))

(define (flip f)
  (lambda (x y)
    (f y x)))

(define (gen-points start-pos dir dis)
  (let loop ([cur start-pos]
             [dis dis]
             [points null])
    (if (zero? dis)
        points
        (let ([new (pos+ cur dir)])
          (loop new (sub1 dis) (cons new points))))))

(define (calculate-segment-points line)
  (let loop ([line line]
             [cur-pos (pos 0 0)]
             [segment (set)])
    (if (empty? line)
        segment
        (let* ([dir (match (caar line)
                      ['R (pos 1 0)]
                      ['U (pos 0 1)]
                      ['L (pos -1 0)]
                      ['D (pos 0 -1)])]
               [points (gen-points cur-pos dir (cadar line))])
          (loop (cdr line)
                (car points)
                (set-union segment
                           (list->set points)))))))

(define (parse-line line)
  (map (lambda (x) (list (string->symbol (substring x 0 1))
                         (string->number (substring x 1))))
       (string-split line ",")))

(define (find-distances ps)
  (sort (map (lambda (p) (+ (abs (pos-x p))
                            (abs (pos-y p))))
             (set->list ps))
        <))

(define (part1 f)
  (find-distances (set-intersect (calculate-segment-points (parse-line (read-line f)))
                                 (calculate-segment-points (parse-line (read-line f))))))