emacs/lisp/format-spec.el

;;; format-spec.el --- functions for formatting arbitrary formatting strings

;; Copyright (C) 1999-2020 Free Software Foundation, Inc.

;; Author: Lars Magne Ingebrigtsen <larsi@gnus.org>
;; Keywords: tools

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;;; Code:

(eval-when-compile
  (require 'subr-x))

(defun format-spec (format specification &optional only-present)
  "Return a string based on FORMAT and SPECIFICATION.
FORMAT is a string containing `format'-like specs like \"su - %u %k\",
while SPECIFICATION is an alist mapping from format spec characters
to values.

For instance:

  (format-spec \"su - %u %l\"
               `((?u . ,(user-login-name))
                 (?l . \"ls\")))

Each format spec can have modifiers, where \"%<010b\" means \"if
the expansion is shorter than ten characters, zero-pad it, and if
it's longer, chop off characters from the left size\".

The following modifiers are allowed:

* 0: Use zero-padding.
* -: Pad to the right.
* ^: Upper-case the expansion.
* _: Lower-case the expansion.
* <: Limit the length by removing chars from the left.
* >: Limit the length by removing chars from the right.

Any text properties on a %-spec itself are propagated to the text
that it generates.

If ONLY-PRESENT, format spec characters not present in
SPECIFICATION are ignored, and the \"%\" characters are left
where they are, including \"%%\" strings."
  (with-temp-buffer
    (insert format)
    (goto-char (point-min))
    (while (search-forward "%" nil t)
      (cond
        ;; Quoted percent sign.
        ((eq (char-after) ?%)
         (unless only-present
	   (delete-char 1)))
        ;; Valid format spec.
        ((looking-at "\\([-0 _^<>]*\\)\\([0-9.]*\\)\\([a-zA-Z]\\)")
	 (let* ((modifiers (match-string 1))
                (num (match-string 2))
	        (spec (string-to-char (match-string 3)))
	        (val (assq spec specification)))
	   (if (not val)
               (unless only-present
	         (error "Invalid format character: `%%%c'" spec))
	     (setq val (cdr val)
                   modifiers (format-spec--parse-modifiers modifiers))
	     ;; Pad result to desired length.
	     (let ((text (format "%s" val)))
               (when num
                 (setq num (string-to-number num))
                 (setq text (format-spec--pad text num modifiers))
                 (when (> (length text) num)
                   (cond
                    ((memq :chop-left modifiers)
                     (setq text (substring text (- (length text) num))))
                    ((memq :chop-right modifiers)
                     (setq text (substring text 0 num))))))
               (when (memq :uppercase modifiers)
                 (setq text (upcase text)))
               (when (memq :lowercase modifiers)
                 (setq text (downcase text)))
	       ;; Insert first, to preserve text properties.
	       (insert-and-inherit text)
	       ;; Delete the specifier body.
               (delete-region (+ (match-beginning 0) (length text))
                              (+ (match-end 0) (length text)))
               ;; Delete the percent sign.
               (delete-region (1- (match-beginning 0)) (match-beginning 0))))))
        ;; Signal an error on bogus format strings.
        (t
          (unless only-present
	    (error "Invalid format string")))))
    (buffer-string)))

(defun format-spec--pad (text total-length modifiers)
  (if (> (length text) total-length)
      ;; The text is longer than the specified length; do nothing.
      text
    (let ((padding (make-string (- total-length (length text))
                                (if (memq :zero-pad modifiers)
                                    ?0
                                  ?\s))))
      (if (memq :right-pad modifiers)
          (concat text padding)
        (concat padding text)))))

(defun format-spec--parse-modifiers (modifiers)
  (mapcan (lambda (char)
            (when-let ((modifier
                        (pcase char
                          (?0 :zero-pad)
                          (?\s :space-pad)
                          (?^ :uppercase)
                          (?_ :lowercase)
                          (?- :right-pad)
                          (?< :chop-left)
                          (?> :chop-right))))
              (list modifier)))
          modifiers))

(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)

;;; format-spec.el ends here