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#lang br/quicklang
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(define (read-syntax path port)
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(strip-bindings
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#`(module mod "day04.rkt"
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#,@(for*/list ([room-str (in-lines port)]
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#:when (not (equal? "" room-str)))
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`(room ,@(cdr (regexp-match #px"^(.*)-(\\d+)\\[(\\w+)\\]$" room-str)))))))
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(module+ reader (provide read-syntax))
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#|
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Each room consists of an encrypted name (lowercase letters separated by dashes)
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followed by a dash, a sector ID, and a checksum in square brackets.
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|#
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(struct $room (name sector checksum) #:transparent)
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(define-macro (room NAME SECTOR CHECKSUM)
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#'($room NAME (string->number SECTOR) CHECKSUM))
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(provide room)
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(define-macro (mb . ROOMS)
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#'(#%module-begin
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(define rooms (list . ROOMS))
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(display "part a: ")
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(displayln (for/sum ([room (in-list rooms)]
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#:when (real-room? room))
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($room-sector room)))
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(display "part b: ")
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(displayln
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(for/first ([room (in-list rooms)]
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#:when (equal? (shift-string ($room-name room) ($room-sector room))
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"northpole object storage"))
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($room-sector room)))))
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(provide (rename-out [mb #%module-begin]))
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#|
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A room is real (not a decoy) if the checksum is
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the five most common letters in the encrypted name, in order,
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with ties broken by alphabetization.
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|#
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(require sugar/list)
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(define (real-room? room)
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(define room-chars (string->list (string-replace ($room-name room) "-" "")))
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(define freqs (hash->list (frequency-hash room-chars)))
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(define sorted-freqs (sort (sort freqs char<? #:key car) > #:key cdr))
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(equal? ($room-checksum room) (list->string (map car (take sorted-freqs 5)))))
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(define (shift-string str shift)
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(list->string
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(for/list ([c (in-string str)])
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(cond
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[(char=? c #\-) #\space]
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[else
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(define a-val (char->integer #\a))
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(integer->char (+ (modulo (+ (char->integer c) (- a-val) shift) 26) a-val))]))))
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