7.rkt

#lang racket
(require threading)

(define sample-pos
  (~>
   "16,1,2,0,4,2,7,1,2,14"
   #;
   (with-input-from-file
    "/tmp/input"
    (λ () (port->string (current-input-port))))
   string-trim
   (string-split _ ",")
   (map string->number _)))

(define real-pos
  (~>
   (with-input-from-file
    "/tmp/input"
    (λ () (port->string (current-input-port))))
   string-trim
   (string-split _ ",")
   (map string->number _)))

(define (average lst)
  (define sum (apply + lst))
  (floor (/ sum (length lst))))

(define memo (make-hash '()))

(module+ test
  (require rackunit))

(define (indv-cost target at)
  (define dist (abs (- target at)))
  (define (f dist acc)
    (if (zero? dist)
        acc
        (f (sub1 dist) (+ dist acc))))
  (f dist 0))

(module+ test
  (check-equal? (indv-cost 16 5) 66)
  (check-equal? (indv-cost 1 5) 10))


(define (cost target lst)
  (if (hash-has-key? memo target)
      (hash-ref memo target)
      (let* [
             (result (apply + (map (λ (i) (indv-cost i target)) lst)))]
        (begin
          (hash-set! memo target result)
          result))))
(module+ test
  (check-equal? (cost 5 sample-pos) 168)
  (check-equal? (cost 2 sample-pos) 206))

(define (list-range lst)
  (define mx (foldl max (first lst) lst))
  (define mn (foldl min (first lst) lst))
  (in-range mn mx))

(define start (average sample-pos))

(define (go-left? target lst)
  (< (cost (sub1 target) lst)
     (cost target lst)))

(define (go-right? target lst)
  (< (cost (add1 target) lst)
     (cost target lst)))

(define (done? target lst)
  (not (or (go-left? target lst) (go-right? target lst))))


(define (step target lst)
  (if (go-right? target lst)
      (add1 target)
      (sub1 target)))


(define (loop target lst)
  (if (done? target lst)
      (cost target lst)
      (loop (step target lst) lst)))