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62 lines
2.1 KiB
Racket
62 lines
2.1 KiB
Racket
7 years ago
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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 SIZE) (MOVE ...) ...)
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#`(#%module-begin
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(time (STARS SIZE '(MOVE ...)) ...)))
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(define (spin ps num)
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(define ps-len (vector-length ps))
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(for/vector ([idx (in-range ps-len)])
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(vector-ref ps (modulo (- idx num) ps-len))))
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(define (exchange ps left right)
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(define new-vec (vector-copy ps))
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(vector-set*! new-vec
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left (vector-ref ps right)
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right (vector-ref ps left))
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new-vec)
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(define (partner ps left right)
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(exchange ps (vector-member left ps) (vector-member right ps)))
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(define (starting-vec size)
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(list->vector (take '(a b c d e f g h i j k l m n o p q r s t) size)))
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(define (thunkify move)
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(define move-str (symbol->string move))
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(cond
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[(regexp-match #px"(?<=^s)\\d+(?=$)" move-str)
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=> (λ (m)
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(define num (string->number (car m)))
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(λ (ps) (spin ps num)))]
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[(regexp-match #px"(?<=^x)\\d+/\\d+(?=$)" move-str)
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=> (λ (m)
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(define nums (map string->number (string-split (car m) "/")))
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(λ (ps) (apply exchange ps nums)))]
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[(regexp-match #px"(?<=^p)\\w/\\w(?=$)" move-str)
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=> (λ (m)
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(define syms (map string->symbol (string-split (car m) "/")))
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(λ (ps) (apply partner ps syms)))]
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[else (error move-str)]))
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(define (dance vec moves [rounds 1])
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(for*/fold ([vec vec])
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([round (in-range rounds)]
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[move-thunk (in-list (map thunkify moves))])
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(move-thunk vec)))
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(define (★ size moves [rounds 1])
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(string-append* (map ~a (vector->list (dance (starting-vec size) moves rounds)))))
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(define (★★ size moves)
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(define cycle-length (for/fold ([vecs (list (starting-vec size))]
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#:result (length (cdr vecs)))
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([i (in-naturals)]
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#:break (member (car vecs) (cdr vecs)))
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(cons (dance (car vecs) moves) vecs)))
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(define one-billion (expt 10 9))
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(define iterations-needed (modulo one-billion cycle-length))
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(★ size moves iterations-needed))
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