(define-library (csc format)
  (export
    fprintf
    printf
    sprintf)
  (import (scheme base)
          (only (scheme write)
            display)
          (only (csc loop)
            loop)
          (only (csc strings)
            index
            has-prefix?))
  (begin


    (define (fprintf port format-string . args)
      (loop with s = format-string
            until (string=? "" s)
            if (has-prefix? s "{{")
              do (write-string "{" port)
                 (set! s (string-copy s 2))
            else if (has-prefix? s "}}")
              do (write-string "}" port)
                 (set! s (string-copy s 2))
            else if (has-prefix? s "{}")
              do (display (car args) port)
                 (set! args (cdr args))
                 (set! s (string-copy s 2))
            else if (or (has-prefix? s "{")
                        (has-prefix? s "}"))
              do (error "invalid format string" format-string)
            else
              do (let* ((open-brace-pos (index s "{"))
                        (close-brace-pos (index s "}"))
                        (format-pos (min open-brace-pos close-brace-pos)))
                   (when (negative? format-pos)
                     (set! format-pos (string-length s)))
                   (write-string s port 0 format-pos)
                   (set! s (string-copy s format-pos)))))


    (define (printf format-string . format-args)
      (apply fprintf (current-output-port) format-string format-args))


    (define (sprintf format-string . format-args)
      (let ((string-builder (open-output-string)))
        (apply fprintf string-builder format-string format-args)
        (get-output-string string-builder)))))