#lang racket/base

(provide read-line-limited)

;; Port Natural -> (or String EofObject)
;; Uses Internet (CRLF) convention. Limit does not cover the CRLF
;; bytes.
(define (read-line-limited port limit)
  (let collect-chars ((acc '())
		      (remaining limit))
    (let ((ch (read-char port)))
      (cond
       ((eof-object? ch) (if (null? acc)
			     ch
			     (list->string (reverse acc))))
       ((eqv? ch #\return) (let ((ch (read-char port)))
			     (if (eqv? ch #\linefeed)
				 (list->string (reverse acc))
				 (error 'read-line-limited
					"Invalid character ~v after #\\return"
					ch))))
       ((eqv? ch #\newline)
	;; Is this a good idea?
	(error 'read-line-limited "Bare #\\linefeed encountered"))
       ((positive? remaining) (collect-chars (cons ch acc) (- remaining 1)))
       (else (error 'read-line-limited
		    "Line too long (more than ~v bytes before CRLF)"
		    limit))))))