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#lang racket
(require "../../jj-aoc.rkt"
threading)
(define tickets
(for/list ([l (in-lines (open-day 5 2020))])
(~>> l (regexp-match #px"(.{7})(.{3})") rest)))
(define (find-place str min-p max-p l r)
(if (string=? "" str)
min-p
(let ([p-range (/ (add1 (- max-p min-p)) 2)] [c (substring str 0 1)])
(cond
[(string=? c l) (find-place (substring str 1) min-p (- max-p p-range) l r)]
[(string=? c r) (find-place (substring str 1) (+ min-p p-range) max-p l r)]))))
(define (find-row str)
(find-place str 0 127 "F" "B"))
(define (find-col str)
(find-place str 0 7 "L" "R"))
(define (ticket-id t)
(let ([row (find-row (first t))] [col (find-col (second t))]) (+ col (* 8 row))))
;; part 1
(define occupied-seats
(~>> (for/list ([t (in-list tickets)])
(ticket-id t))))
(apply max occupied-seats)
;; part 2
(set-first (set-subtract
(list->set (inclusive-range (apply min occupied-seats) (apply max occupied-seats)))
(list->set occupied-seats)))
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