; formatted output

(define (sprintf fmt . args)
  (define (escape cs)
    (define (octal? c) (char<=? #\0 c #\7))
    (define (c->o c) (- (char->integer c) 48))
    (cond ((null? cs) (error 'escape "incomplete"))
          ((not (char=? (car cs) #\\)) (values (car cs) (cdr cs)))
          ((null? (cdr cs)) (error 'escape "malformed"))
          ((char=? (cadr cs) #\b) (values #\backspace (cddr cs)))
          ((char=? (cadr cs) #\f) (values #\page (cddr cs)))
          ((char=? (cadr cs) #\n) (values #\newline (cddr cs)))
          ((char=? (cadr cs) #\r) (values #\return (cddr cs)))
          ((char=? (cadr cs) #\t) (values #\tab (cddr cs)))
          ((octal? (cadr cs))
            (let loop ((k 3) (cs (cdr cs)) (oct 0))
              (if (and (positive? k) (pair? cs) (octal? (car cs)))
                  (loop (- k 1) (cdr cs) (+ (* oct 8) (c->o (car cs))))
                  (values (integer->char oct) cs))))
          (else (values (cadr cs) (cddr cs)))))
  (define (specifier cs arg)
    (define (c->d c) (- (char->integer c) 48))
    (define (justify str left? pad? width)
      (let ((len (string-length str)))
        (cond ((<= width len) str)
              (left? (string-append str (make-string (- width len) #\space)))
              ((and pad? (not left?)) (string-append (make-string (- width len) #\0) str))
              (else (string-append (make-string (- width len) #\space) str)))))
    (define (rnd num prec)
        (if prec (/ (round (* num (expt 10 prec))) (expt 10 prec)) num))
    (define (trunc num) (inexact->exact (truncate num)))
    (let ((cs (cdr cs)) (left? #f) (pad? #f) (width 0) (prec #f))
      (when (and (pair? cs) (char=? (car cs) #\-))
        (set! left? #t) (set! cs (cdr cs)))
      (when (and (pair? cs) (char=? (car cs) #\0))
        (set! pad? #t) (set! cs (cdr cs)))
      (do () ((or (null? cs) (not (char-numeric? (car cs)))))
        (set! width (+ (* width 10) (c->d (car cs)))) (set! cs (cdr cs)))
      (when (and (pair? cs) (char=? (car cs) #\.))
        (set! cs (cdr cs)) (set! prec 0)
        (do () ((or (null? cs) (not (char-numeric? (car cs)))))
          (set! prec (+ (* prec 10) (c->d (car cs)))) (set! cs (cdr cs))))
      (if (null? cs) (error 'specifier "incomplete")
        (case (car cs)
        ((#\c) (values (justify (string (integer->char arg)) left? #f width) (cdr cs)))
        ((#\d) (values (justify (number->string (trunc arg)) left? pad? width) (cdr cs)))
        ((#\f) (values (justify (number->string (rnd arg prec)) left? pad? width) (cdr cs)))
        ((#\o) (values (justify (number->string (trunc arg) 8) left? pad? width) (cdr cs)))
        ((#\s) (values (justify (if prec (substring arg 0 prec) arg) left? #f width) (cdr cs)))
        ((#\x) (values (justify (number->string (trunc arg) 16) left? pad? width) (cdr cs)))
        ((#\%) (values "%" cs))
        (else (error 'specifier "unsupported"))))))
  (let loop ((cs (string->list fmt)) (args args) (out (list)))
    (cond ((null? cs)
            (if (pair? args) (error 'printf "too many arguments")
              (list->string (reverse out))))
          ((char=? (car cs) #\\)
            (call-with-values (lambda () (escape cs))
              (lambda (c rest) (loop rest args (cons c out)))))
          ((char=? (car cs) #\%)
            (if (null? (cdr cs)) (error 'sprintf "incomplete specifier")
              (if (char=? (cadr cs) #\%) (loop (cddr cs) args (cons #\% out))
                (if (null? args) (error 'printf "not enough arguments")
                  (call-with-values (lambda () (specifier cs (car args)))
                    (lambda (str rest)
                      (loop rest (cdr args)
                            (append (reverse (string->list str)) out))))))))
          (else (loop (cdr cs) args (cons (car cs) out))))))

(define (printf fmt . args) (display (apply sprintf fmt args)))
(define (fprintf port fmt . args) (display (apply sprintf fmt args) port))

(printf "%c%%hello\\n" 97)
(printf "|%s|%10s|%-10s|%.3s|%10.3s|%-10.3s|\\n" "January" "January" "January" "January" "January" "January")
(printf "|%d|%5d|%-5d|%o|%06o|\\\\x%x|\\n" 97 97 97 97 97 97)
(printf "|%f|%.2f|%9.2f|%-9.2f|%09.2f|\\n" 123.4567 123.4567 123.4567 123.4567 123.4567)

a%hello
|January|   January|January   |Jan|       Jan|Jan       |
|97|   97|97   |141|000141|\x61|
|123.4567|123.46|   123.46|123.46   |000123.46|