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55 lines
2.0 KiB
Racket
55 lines
2.0 KiB
Racket
#lang br/quicklang
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(define (read-syntax path port)
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(define turn-strings (string-split (port->string port) ","))
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(define turn-pattern #px"^([LR])(\\d+)$")
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(define turn-datums
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(for*/list ([tstr (in-list turn-strings)])
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(define match-result (regexp-match turn-pattern (string-trim tstr)))
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`(turn ,@(cdr (or match-result empty)))))
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(strip-bindings
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#`(module day01-mod "day01.rkt"
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#,@turn-datums)))
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(module+ reader (provide read-syntax))
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(define-macro (mb . TURNS)
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#'(#%module-begin
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(solve . TURNS)))
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(provide (rename-out [mb #%module-begin]))
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(define (loc-dist loc)
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(+ (abs (imag-part loc)) (abs (real-part loc))))
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(struct $turn (rot dist) #:transparent)
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(define rotate-left +i)
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(define rotate-right -i)
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(define same-dir 1)
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(define (solve . turns)
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(define found-twice-visited-loc #f)
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(define north 0+1i)
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(define starting-loc 0+0i)
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(let loop ([locs (list starting-loc)] [dir north] [turns turns])
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(cond
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[(empty? turns)
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(displayln (format "part 1 (dist of final location): ~a" (loc-dist (car locs))))]
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[(zero? ($turn-dist (car turns))) (loop locs dir (cdr turns))]
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[else
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(define new-dir (* dir ($turn-rot (car turns))))
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(define one-step 1)
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(define new-loc (+ (car locs) (* new-dir one-step)))
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(when (and (not found-twice-visited-loc) (member new-loc locs))
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(set! found-twice-visited-loc new-loc)
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(displayln (format "part 2 (dist of first twice-visited location): ~a"
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(loc-dist new-loc))))
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(define decremented-turn ($turn same-dir (sub1 ($turn-dist (car turns)))))
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(loop (cons new-loc locs) new-dir (cons decremented-turn (cdr turns)))])))
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(define-macro-cases turn
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[(_ DIR DIST)
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(with-pattern ([ENCODED-DIR (syntax-case #'DIR ()
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["L" #'rotate-left]
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["R" #'rotate-right])])
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#'($turn ENCODED-DIR (string->number DIST)))]
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[else #'(void)])
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(provide turn) |