10.rkt

#lang racket
(require threading racket/trace)

(define FILE "/tmp/input")

(define lines
  (with-input-from-file
      FILE
    (λ ()
      (port->lines (current-input-port)))))

(define (delim->kind c)
  (match c
    [#\{ 'CURLY]
    [#\} 'CURLY]
    [#\( 'PAREN]
    [#\) 'PAREN]
    [#\[ 'BLOCK]
    [#\] 'BLOCK]
    [#\< 'ANGLE]
    [#\> 'ANGLE]))

(define (delim->dir c)
  (match c 
    [#\{ 'OPEN]
    [#\} 'CLOSE]
    [#\( 'OPEN]
    [#\) 'CLOSE]
    [#\[ 'OPEN]
    [#\] 'CLOSE]
    [#\< 'OPEN]
    [#\> 'CLOSE]))

(define (fresh-state)
  (box '()))

(define (push! state kind)
  (set-box! state (cons kind (unbox state))))

(define (pop! state kind)
  (define top (car (unbox state)))
  (set-box! state (cdr (unbox state)))
  (when (not (equal? top kind))
    (raise kind)))
             


(define (update-state! state kind dir)
  (match dir
    ['OPEN (push! state kind)]
    ['CLOSE (pop! state kind)]))


       
(define (process-line line)
  (define score 0 )
  (define state (fresh-state))
  (for [(c (in-list (string->list line)))]
    (update-state! state (delim->kind c) (delim->dir c)))
  (define scores (map kind->score (unbox state)))
  (for [(indv-score (in-list scores))]
    (set! score (+ (* score 5) indv-score)))
  score)




(define (kind->score e)
  (match  e
    ['PAREN 1]
    ['BLOCK 2]
    ['CURLY 3]
    ['ANGLE 4]))

(define scores
  (filter
   identity
   (for/list [(line (in-list lines))]
     (with-handlers
         [(symbol? (const #f))]
       (process-line line)))))

(set! scores (sort scores <))

(define odd/c (and/c number? odd?))

(define/contract (halfway-idx len)
  (-> odd/c number?)
  (/ (sub1 len) 2))

(list-ref scores (halfway-idx (length scores)))