Matthew Butterick 8 years ago
parent 4f738c22e8
commit cfd27965fb

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#lang reader "lang.rkt"
hhhxzeay

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#lang br/quicklang ;; http://adventofcode.com/2016/day/17
(require openssl/md5 sugar/cache)
(provide read-syntax
(rename-out [mb #%module-begin]))
(define (read-syntax path port)
(strip-bindings
#`(module mod "lang.rkt"
#,(string-trim (port->string port)))))
(define-macro (mb STR)
#'(#%module-begin
(displayln (solve-shortest STR))
(displayln (solve-longest STR))))
(define (path->dirs path)
(regexp-match* #rx"[UDLR]" path))
(define (on-grid? pos)
(and (<= 0 (real-part pos) 3)
(<= 0 (imag-part pos) 3)))
(define/caching (follow-path path)
(define result (regexp-match #rx"^(.*)([UDLR])$" path))
(define end
(cond
[result
(match-define (list _ prefix suffix) result)
(+ (follow-path prefix) (case suffix
[("D") +i]
[("U") -i]
[("L") -1]
[("R") 1]))]
[else 0]))
(and (on-grid? end) end))
(define/caching (get-hash str)
(md5 (open-input-string str)))
(define (take-step path)
(define hash (get-hash path))
(define prefix (car (regexp-match #rx"^...." hash)))
(define possible-dirs
(for/list ([dir (in-list (list "U" "D" "L" "R"))]
[c (in-string prefix)]
#:when (member c '(#\b #\c #\d #\e #\f)))
dir))
(for*/list ([dir (in-list possible-dirs)]
[path+dir (in-value (string-append path dir))]
#:when (follow-path path+dir))
path+dir))
(define vault 3+3i)
(define (solve-shortest str)
(let loop ([paths (list str)])
(define stepped-paths (append-map take-step paths))
(if (empty? stepped-paths)
'no-solution
(or (for/first ([sp (in-list stepped-paths)]
#:when (= vault (follow-path sp)))
(apply string-append (path->dirs sp)))
(loop stepped-paths)))))
(define (solve-longest str)
(length
(path->dirs
(argmax string-length
(let loop ([paths (list str)][vault-paths empty][i 0])
(cond
[(empty? paths) vault-paths]
[else
(define stepped-paths (append-map take-step paths))
(define-values (new-vault-paths other-paths)
(partition (λ(sp) (= vault (follow-path sp))) stepped-paths))
(loop other-paths (if (pair? new-vault-paths) new-vault-paths vault-paths) (add1 i))]))))))

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#lang reader "lang.rkt"
hijkl