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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) (JMP) ...)
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#`(#%module-begin
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(escape (list->vector '(JMP ...)) 'STARS)))
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(define (escape vec stars)
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(let/ec exit
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(for/fold ([pos 0])
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([i (in-naturals)])
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(unless (<= 0 pos (sub1 (vector-length vec)))
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(exit i))
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(define jmp (vector-ref vec pos))
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(vector-set! vec pos (if (and (eq? stars '★★) (>= jmp 3))
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(sub1 jmp)
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(add1 jmp)))
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(+ pos jmp))))
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