(eval-when-compile (require 'cl))
(defun format-spec (format specification)
"Return a string based on FORMAT and SPECIFICATION.
FORMAT is a string containing `format'-like specs like \"bash %u %k\",
while SPECIFICATION is an alist mapping from format spec characters
to values. Any text properties on a %-spec itself are propagated to
the text that it generates."
(with-temp-buffer
(insert format)
(goto-char (point-min))
(while (search-forward "%" nil t)
(cond
((eq (char-after) ?%)
(delete-char 1))
((looking-at "\\([-0-9.]*\\)\\([a-zA-Z]\\)")
(let* ((num (match-string 1))
(spec (string-to-char (match-string 2)))
(val (cdr (assq spec specification))))
(unless val
(error "Invalid format character: %s" spec))
(let ((text (format (concat "%" num "s") val)))
(insert-and-inherit text)
(delete-region (+ (match-beginning 0) (length text))
(+ (match-end 0) (length text)))
(delete-region (1- (match-beginning 0)) (match-beginning 0)))))
(t
(error "Invalid format string"))))
(buffer-string)))
(defun format-spec-make (&rest pairs)
"Return an alist suitable for use in `format-spec' based on PAIRS.
PAIRS is a list where every other element is a character and a value,
starting with a character."
(let (alist)
(while pairs
(unless (cdr pairs)
(error "Invalid list of pairs"))
(push (cons (car pairs) (cadr pairs)) alist)
(setq pairs (cddr pairs)))
(nreverse alist)))
(provide 'format-spec)