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#lang racket
(require json)
(define lists (file->list "day13" read-json))
(define data
(let loop ([lists lists]
[pairs '()])
(if (null? lists)
(reverse pairs)
(let-values ([(pair rest) (split-at lists 2)])
(loop rest (cons pair pairs))))))
(define (compare left right)
(cond [(and (number? left) (number? right)) (- left right)]
[(and (list? left) (list? right))
(let loop ([leaning (compare (length left) (length right))]
[left left]
[right right])
(cond [(null? left) leaning]
[(null? right) 1]
[(compare (car left) (car right)) =>
(lambda (c)
(if (zero? c)
(loop leaning (cdr left) (cdr right))
c))]))]
[(and (number? left) (list? right))
(compare (list left) right)]
[(and (list? left) (number? right))
(compare left (list right))]
[else (error "can't happen")]))
(for/sum ([(pair i) (in-indexed data)])
(if (negative? (apply compare pair))
(+ i 1)
0))
; 6420
(define part2
(sort (list* '((2)) '((6)) lists)
(compose negative? compare)))
(* (+ 1 (index-of part2 '((2))))
(+ 1 (index-of part2 '((6)))))
;; 22000
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