day8.rkt

#lang racket
(require threading)

(define test-img '(((0 2)
                    (2 2))
                   ((1 1)
                    (2 2))
                   ((2 2)
                    (1 2))
                   ((0 0)
                    (0 0))))

(define (get-input)
  (car (file->lines "day8.txt")))

(define (split-every num lst)
  (define (step inlist num res)
    (if (empty? inlist)
        (reverse res)
        (step (drop inlist num)
              num
              (cons (take inlist num) res))))
    (step lst num '()))

(define (text->img txt)
  (~>> (string->list (get-input))
       (map char->integer)
       (map (λ (ascii) (- ascii 48)))
       (split-every 25)
       (split-every 6)))

(define (count-layer lst)
  (define (count-chars lst hsh)
    (if (empty? lst)
        hsh
        (count-chars (cdr lst) (hash-update hsh (car lst) (λ (n) (+ n 1)) 0))))
  
  (define (count-list lst hsh)
    (if (empty? lst)
        hsh
        (count-list (cdr lst) (count-chars (car lst) hsh))))

  (count-list lst #hash()))



(define (fewest-zeros a b)
  (if (< (hash-ref a 0 0) (hash-ref b 0 0))
      a b))

(define (part1)
  (let* ([counts (~>> (text->img (get-input)) (map count-layer))]
         [goal (foldl fewest-zeros (car counts) (cdr counts))])
    (* (hash-ref goal 1) (hash-ref goal 2))))

(define (compact-layers layers)
  (define (compact-chars chars res)
    (car (filter-not (λ (n) (= 2 n)) chars)))
  
  (define (compact-rows rows res)
    (if (empty? (car rows))
        (reverse res)
        (compact-rows (map cdr rows) (cons (compact-chars (map car rows) '()) res))))
  
  (define (compact layers res)
    (if (empty? (car layers))
        (reverse res)
        (compact (map cdr layers) (cons (compact-rows (map car layers) '()) res))))
  
  (compact layers '()))

(define (draw img)
  (define (digit->char n)
    (if (= 1 n)
      #\#
      #\ ))
  
  (define (line->str line)
    (list->string(map digit->char line)))

  (string-join (map line->str img) "\n"))

(define (part2)
  (display (draw (compact-layers (text->img (get-input))))))

(part1)
(part2)