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#lang racket
(define-values (stacks raw-steps)
(let ((lines (file->lines "day05")))
(split-at lines (index-of lines ""))))
(define initial-state
(for/vector ([x (in-range 1 35 4)])
(dropf (for/list ([stack (drop-right stacks 1)])
(string-ref stack x))
(curry equal? #\space))))
(define steps
(for/list ([line (cdr raw-steps)])
(filter number? (map string->number (string-split line " ")))))
(define (solve f)
(define state (vector-copy initial-state))
(for ([step steps])
(let* ([n (first step)]
[from (sub1 (second step))]
[to (sub1 (third step))]
[taken (take (vector-ref state from) n)]
[left (drop (vector-ref state from) n)]
[placed (append (f taken) (vector-ref state to))])
(vector-set! state to placed)
(vector-set! state from left)))
(list->string (for/list ([s state]) (first s))))
(solve reverse)
(solve identity)
; MQTPGLLDN
; LVZPSTTCZ
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