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#lang racket
;; https://github.com/progheal/adventofcode/blob/master/2022/22.cpp
(define-values (field path)
(let-values ([(field path) (splitf-at (file->lines "day22") (lambda (line) (not (equal? line ""))))])
(let ([max-line (+ 1 (apply max (map string-length field)))])
(values (for/vector ([line (append field (list ""))])
(for/vector ([char (~a line #:min-width max-line)])
char))
(for/list ([step (regexp-match* #rx"[0-9]+|." (second path))])
(or (string->number step) step))))))
;; field
;; path
(define (vector-index-of v e)
(let loop ([i 0])
(if (>= i (vector-length v))
#f
(if (equal? (vector-ref v i) e)
i
(loop (+ i 1))))))
(define dir #(0 1 0 -1 0))
(define (do-step x y d)
(values (modulo (+ x (vector-ref dir d))
(vector-length field))
(modulo (+ y (vector-ref dir (+ d 1)))
(vector-length (vector-ref field 0)))
d))
(define (turn-left d)
(modulo (- d 1) 4))
(define (turn-right d)
(modulo (+ d 1) 4))
(define (walk field path)
(define d 0)
(define x 0)
(define y (vector-index-of (vector-ref field x) #\.))
(for ([step path])
(match step
["L" (set! d (turn-left d))]
["R" (set! d (turn-right d))]
[n (let loop ([n n])
(when (> n 0)
(let ([px x]
[py y]
[pd d])
(let loop2 ()
(set!-values (x y d) (do-step x y d))
(when (equal? (vector-ref (vector-ref field x) y) #\space)
(loop2)))
(if (equal? (vector-ref (vector-ref field x) y) #\#)
(set!-values (x y d) (values px py pd))
(loop (- n 1))))))]))
; (list x y d)
(+ (* 1000 (+ x 1)) (* 4 (+ y 1)) d)
)
(walk field path)
;; 13566
(define cube-edge (make-hash))
(define d 0)
(define x 0)
(define y (vector-index-of (vector-ref field x) #\.))
(define cube-size (gcd (- (vector-length field) 1)
(- (vector-length (vector-ref field 0)) 1)))
(displayln (list "cube size:" cube-size))
(let ([reverse false]
[unmatched-edge '()]
[unmatched-turn (list 1)]
[ex x]
[ey y]
[ed d]
[ignore 42]
)
(let loop ()
(if reverse
(for ([i (in-range 0 cube-size)])
(let-values ([(rx ry rd) (apply values (car unmatched-edge))])
(set! unmatched-edge (cdr unmatched-edge))
(hash-set! cube-edge
(vector rx ry (turn-left rd))
(vector ex ey (turn-right ed)))
(hash-set! cube-edge
(vector ex ey (turn-left ed))
(vector rx ry (turn-right rd)))
(set!-values (ex ey ed) (do-step ex ey ed))))
(for ([i (in-range 0 cube-size)])
(set! unmatched-edge (cons (list ex ey ed) unmatched-edge))
(set!-values (ex ey ed) (do-step ex ey ed))))
(let*-values ([(now) (vector-ref (vector-ref field ex) ey)]
[(lx ly ld) (do-step ex ey (turn-left ed))]
[(myleft) (vector-ref (vector-ref field lx) ly)]
[(edge-turn) -1])
(cond [(and (equal? myleft #\space) (equal? now #\space))
(set! edge-turn 1)]
[(and (equal? myleft #\space) (not (equal? now #\space)))
(set! edge-turn 0)])
(if reverse
(let ([last-unmatched-turn (car unmatched-turn)])
(if (= 1 (+ last-unmatched-turn edge-turn))
(begin
(set! unmatched-turn (cdr unmatched-turn))
(when (null? unmatched-turn)
(set! unmatched-turn (cons edge-turn unmatched-turn))
(set! reverse #f)))
(begin
(set! unmatched-turn (cons 0 (cdr unmatched-turn)))
(set! reverse #f))))
(if (= edge-turn -1)
(set! reverse #t)
(set! unmatched-turn (cons edge-turn unmatched-turn))))
(cond [(= edge-turn 1)
(set! ed (turn-right ed))
(set!-values (ex ey ignore) (do-step ex ey (turn-right ed)))]
[(= edge-turn -1)
(set! ed (turn-left ed))
(set!-values (ex ey ignore) (do-step ex ey ed))]))
(unless (and (equal? ex x) (equal? ey y) (equal? ed d))
(loop)))
)
;; cube-edge
(define (do-cube-step x y d)
(if (hash-has-key? cube-edge (vector x y d))
(vector->values (hash-ref cube-edge (vector x y d)))
(do-step x y d)))
(define (walk2 field path)
(define d 0)
(define x 0)
(define y (vector-index-of (vector-ref field x) #\.))
(for ([step path])
(match step
["L" (set! d (modulo (- d 1) 4))]
["R" (set! d (modulo (+ d 1) 4))]
[n (let loop ([n n])
(when (> n 0)
(let ([px x]
[py y]
[pd d])
(set!-values (x y d) (do-cube-step x y d))
(if (equal? (vector-ref (vector-ref field x) y) #\#)
(set!-values (x y d) (values px py pd))
(loop (- n 1))))))]))
; (list x y d)
(+ (* 1000 (+ x 1)) (* 4 (+ y 1)) d)
)
(walk2 field path)
;; 11451
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