Emacs Lisp for flowed text email

Here’s some Emacs Lisp for encoding flowed text a la RFC 2646. This is useful for composing email in Emacs. For example, if you use a separate email client and emacsclient as your editor, add this and (add-hook 'server-switch-hook 'flow-encode-buffer) to your .emacs to encode your emails as flowed text automatically.

(defgroup flow-fill nil
  "Decoding and encoding flowed text a la RFC2646."
  :group 'fill)

(defcustom flow-fill-min-line-length
  65
  "The minimum length, in characters, for a line to be considered for encoding
   as flowed text."
  :type '(integer)
  :group 'flow-fill)

(defun starts-with (string prefix)
  (and (>= (length string) (length prefix))
       (equal prefix (substring string 0 (length prefix)))))

(defun starts-with-word-char ()
  "Return t if this line starts with a word-constituent character, nil if not."
  ; use point-at-{b,e}ol for xemacs compatibility
  (and (char-after (point-at-bol))
       (eq (char-syntax (char-after (point-at-bol))) ?w)))

(defun flow-encode-buffer ()
  "Encode the buffer as flowed text, a la RFC 2646.

Flowed text, defined by RFC2646, is a lightweight way to specify that a line
is part of the same paragraph as the next line, by appending a trailing space
character to the end of the line. It's often used in RFC2822 messages.

This function encodes the current buffer as RFC2646 flowed text. It uses a
heuristic to determine which lines should be flowed. If the first character on
a line is word-constituent, and the last character is non-whitespace, and the
same applies to the next line, and the first line is at least
`flow-fill-min-line-length' characters long, then flow the first line.

Due to the heuristic, if this function is run on a buffer that's already
encoded as flowed text, it will leave the buffer unchanged."
 (goto-char (point-min))
  (while (and (not (eobp))
              (not (equal "---------- Forwarded message ----------\n"
                          (thing-at-point 'line))))
    (let ((long-enough
           (> (- (point-at-eol) (point-at-bol)) flow-fill-min-line-length))
          (ends-in-whitespace
           (and (not (eq (point-at-bol) (point-at-eol)))
                (eq (char-syntax (char-before (point-at-eol))) ? )))
          (line (thing-at-point 'line)))
      (when (save-excursion (and (not (starts-with line "> "))
                                 (not (starts-with line "http://"))
                                 long-enough
                                 (not ends-in-whitespace)
                                 (forward-line)
                                 (starts-with-word-char)))
        (end-of-line)
        (insert ? )))
    (forward-line)))