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#lang racket
(define/contract (add-strings num1 num2)
(-> string? string? string?)
(define (char->integer c)
((compose string->number string) c))
(define pad-length
(add1 (apply max (map string-length (list num1 num2)))))
(define (pad-with-zeroes n)
(~a n
#:align 'right
#:min-width pad-length
#:pad-string "0"))
(define (string-reverse s)
((compose list->string reverse string->list) s))
(define raw-sum
(for/fold ([sum-string ""]
[carry 0]
#:result sum-string)
([n1 (string-reverse (pad-with-zeroes num1))]
[n2 (string-reverse (pad-with-zeroes num2))])
(let* ([digit-sum (+ carry (char->integer n1) (char->integer n2))]
[sum-place (number->string (modulo digit-sum 10))]
[sum-carry (quotient digit-sum 10)])
(values (string-append sum-place sum-string)
sum-carry))))
(cond [(equal? raw-sum "00") "0"]
[else (string-trim raw-sum "0" #:repeat? #t #:right? #f)]))
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