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#lang racket
(define/contract (full-justify words max-width)
(-> (listof string?) exact-integer? (listof string?))
(define/contract (justify-line line [last-line #f])
(->* ((listof string?)) (boolean?) string?)
(define gaps (sub1 (length line)))
(cond [last-line
(~a (string-join line " ") #:min-width max-width)] ; Right-pad the last line
[(= 1 (length line))
(~a (first line) #:min-width max-width)] ; Right-pad single-word lines
[else
(let* ([words-length (apply + (map string-length line))]
[spacing-length (- max-width words-length)] ; How many spaces do we need?
[spacing-list (make-list gaps 1)] ; Every gap needs to be at least
[distribute (- spacing-length gaps)] ; 1 space long, so we need to
[distributed-spaces ; distribute the excess
(for/list ([space (in-list spacing-list)]
[i (in-naturals 1)])
(+ space
(quotient distribute gaps) ; Add an equal number of spaces
(if (<= i ; to each gap, then add the
(modulo distribute gaps)) 1 0)))]) ; remainder at the front
(apply string-append
(append (map (λ (w s)
(string-append ; Knit together the first (n-1)
w (make-string s #\space))) ; words and gaps, then append
(drop-right line 1) ; the final word at the end
distributed-spaces)
(take-right line 1))))]))
(for/fold ([lines '()] ; List of justified lines
[line-acc '()] ; Words to fit into the next line
#:result (append lines ; Only return the list of lines
(list (justify-line line-acc #t)))) ; and append the final line
([word (in-list words)])
(let* ([candidate-acc (append line-acc (list word))]
[candidate-length
(string-length (string-join candidate-acc " "))])
(if (candidate-length . <= . max-width) ; If the word fits into the line,
(values lines ; keep the current line list
candidate-acc) ; and add it to the accumulator
(values (append lines (list (justify-line line-acc))) ; Otherwise, wrap up this line
(list word)))))) ; and start a new accumulator
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