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25 lines
845 B
Racket
25 lines
845 B
Racket
#lang reader "../aoc-lang.rkt"
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(provide (rename-out [#%mb #%module-begin]) ★ ★★)
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(define-macro (#%mb (STARS) (STEP-SIZE))
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#`(#%module-begin
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(time (STARS STEP-SIZE))))
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(define (★ step-size)
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(define iterations 2017)
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(for/fold ([buf '(0)]
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[current-pos 0]
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#:result (second (member iterations buf)))
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([i (in-range iterations)])
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(define pos (add1 (modulo (+ current-pos step-size) (add1 i))))
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(define-values (head tail) (split-at buf pos))
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(values (append head (list (add1 i)) tail) pos)))
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(define (★★ step-size)
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(for/fold ([in-first-pos #f]
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[current-pos 0]
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#:result in-first-pos)
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([i (in-range (* 50 1e6))])
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(define pos (add1 (modulo (+ current-pos step-size) (add1 i))))
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(values (if (= 1 pos) (add1 i) in-first-pos) pos)))
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