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#lang reader "../aoc-lang.rkt"
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(provide (rename-out [#%mb #%module-begin]))
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(define-macro (#%mb (STARS) (BANK ...))
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#`(#%module-begin
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(time (count-redists (list->vector '(BANK ...)) 'STARS))))
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(define (redist starting-vec)
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(define vec (vector-copy starting-vec))
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(define max-blocks (vector-argmax values vec))
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(define start-at (vector-member max-blocks vec))
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(vector-set! vec start-at 0)
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(for ([block (in-range max-blocks)]
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[idx (in-cycle (shift-left-cycle (range (vector-length vec)) (add1 start-at)))])
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(vector-set! vec idx (+ (vector-ref vec idx) 1)))
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vec)
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(define (count-redists bankvec stars)
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(let/ec exit
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(for/fold ([bankvecs-seen (list bankvec)])
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([i (in-naturals)])
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(cond
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[(member (car bankvecs-seen) (cdr bankvecs-seen))
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=> (λ (tail) (exit (if (eq? stars '★)
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i
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(- (length bankvecs-seen) (length tail)))))])
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(cons (redist (car bankvecs-seen)) bankvecs-seen))))
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