;;;;
;;;; Standard strings in Lisp are immutable.  There seem to be two
;;;; relatively easy ways around this problem:
;;;;
;;;;     1) Use what C++ would call an ostringstream:
;;;;
;;;;            (defvar s (make-string-output-stream))
;;;;            (format s "~a~%" "Hello, World!")
;;;;            (format s "~a~%" "Goodbye, Cruel World!")
;;;;            (get-output-stream-string s)
;;;;
;;;;        The only problem with this is that the call to
;;;;        (get-output-stream-string) resets s making it hard to
;;;;        append to s later.
;;;;
;;;;     2) Strings in lisp are vectors of characters.  It doesn't
;;;;        matter if the vector is adjustable or not.  This is the
;;;;        approach taken in the code below.  Note that the code uses
;;;;        macros to avoid unnecessary function calls.
;;;;
;;;; Also see the dstring2.lisp file for an equivalent class that is
;;;; implemented using lists of characters instead.
;;;;

(defmacro dstring ()
  `(make-array 0 :fill-pointer 0 :adjustable t :element-type 'character))

(defmacro dstring->string (s)
  s)

(defmacro dstring->list (s)
  `(coerce ,s 'list))

(defmacro dstring-append (s x)
  `(format ,s "~a" ,x))

(defmacro dstring-append-char (s c)
  `(format ,s "~a" ,c))

(defmacro dstring-append-line (s &optional (x ""))
  `(format ,s "~a~%" ,x))

(defmacro dstring-clear (s)
  `(setf (fill-pointer ,s) 0))