day3.rkt

#lang racket

(module+ test
  (require rackunit))

(define (get-ring n)
  (define m (ceiling (sqrt n)))
  (if (odd? m)
      m
      (add1 m)))

(module+ test
  (test-case "get-ring"
    (check-= (get-ring 1) 1 0)
    (check-= (get-ring 4) 3 0)
    (check-= (get-ring 12) 5 0)
    (check-= (get-ring 23) 5 0)
    (check-= (get-ring 1024) 33 0)))

(define (get-closest-orthagonal n)
  (define ring (get-ring n))
  (if (= 1 ring)
      1
      (argmin (λ (x) (abs (- x n))) (range (+ (quotient ring 2) (sqr (- ring 2)))
                                           (add1 (sqr ring))
                                           (sub1 ring)))))

(define (part1 target)
  (+ (abs (- target (get-closest-orthagonal target)))
     (quotient (get-ring target) 2)))

(module+ test
  (test-case "part1"
    (check-= (part1 1) 0 0)
    (check-= (part1 12) 3 0)
    (check-= (part1 23) 2 0)
    (check-= (part1 1024) 31 0)))