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#lang racket
(define/contract (replace-digits s)
(-> string? string?)
(define/contract (shift-letter c x)
(-> char? char? char?)
(integer->char (+ (string->number (string x)) (char->integer c))))
(define letters (string->list (string-replace s #rx"[0-9]" "")))
(define digits (string->list (string-replace s #rx"[a-z]" "")))
(foldl (λ (c x acc)
(if (equal? x #\X)
(string-append acc (string c))
(string-append acc (string c) (string (shift-letter c x)))))
""
letters
(if (= (length digits) (length letters))
digits
(append digits '(#\X)))
))
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