;;; minibuf-eldef.el --- Only show defaults in prompts when applicable  -*- lexical-binding: t -*-
;;
;; Copyright (C) 2000-2019 Free Software Foundation, Inc.
;;
;; Author: Miles Bader <miles@gnu.org>
;; Keywords: convenience

;; 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:
;;
;; Defines the mode `minibuffer-electric-default-mode'.
;;
;; When active, minibuffer prompts that show a default value only show
;; the default when it's applicable -- that is, when hitting RET would
;; yield the default value.  If the user modifies the input such that
;; hitting RET would enter a non-default value, the prompt is modified
;; to remove the default indication (which is restored if the input is
;; ever restore to the match the initial input).

;;; Code:

(defvar minibuffer-eldef-shorten-default)

(defun minibuffer-default--in-prompt-regexps ()
  `(("\\( (default\\(?: is\\)? \\(.*\\))\\):? \\'"
     1 ,(if minibuffer-eldef-shorten-default " [\\2]"))
    ("([^(]+?\\(, default\\(?: is\\)? \\(.*\\)\\)):? \\'" 1)
    ("\\( \\[.*\\]\\):? *\\'" 1)))

(defcustom minibuffer-eldef-shorten-default nil
  "If non-nil, shorten \"(default ...)\" to \"[...]\" in minibuffer prompts."
  :set (lambda (symbol value)
         (set-default symbol value)
	 (setq-default minibuffer-default-in-prompt-regexps
		       (minibuffer-default--in-prompt-regexps)))
  :type 'boolean
  :group 'minibuffer
  :version "24.3")

(defvar minibuffer-default-in-prompt-regexps
  (minibuffer-default--in-prompt-regexps)
  "A list of regexps matching the parts of minibuffer prompts showing defaults.
When `minibuffer-electric-default-mode' is active, these regexps are
used to identify the portions of prompts to elide.

Each entry is of the form (REGEXP MATCH-NUM &optional REWRITE),
where REGEXP should match the default part of the prompt,
MATCH-NUM is the subgroup that matched the actual default indicator,
and REWRITE, if present, is a string to pass to `replace-match' that
should be displayed in its place.")


;;; Internal variables

;; A list of minibuffers to which we've added a post-command-hook.
(defvar minibuf-eldef-frobbed-minibufs nil)

;;; The following are all local variables in the minibuffer

;; Input pre-inserted into the minibuffer before the user can edit it.
(defvar minibuf-eldef-initial-input)
(make-variable-buffer-local 'minibuf-eldef-initial-input)
;; and the length of the buffer with it inserted.
(defvar minibuf-eldef-initial-buffer-length)
(make-variable-buffer-local 'minibuf-eldef-initial-buffer-length)

;; True if the current minibuffer prompt contains the default spec.
(defvar minibuf-eldef-showing-default-in-prompt)
(make-variable-buffer-local 'minibuf-eldef-showing-default-in-prompt)

;; An overlay covering the default portion of the prompt
(defvar minibuf-eldef-overlay)
(make-variable-buffer-local 'minibuf-eldef-overlay)


;;; Hook functions

;; This function goes on minibuffer-setup-hook
(defun minibuf-eldef-setup-minibuffer ()
  "Set up a minibuffer for `minibuffer-electric-default-mode'.
The prompt and initial input should already have been inserted."
  (let ((regexps minibuffer-default-in-prompt-regexps)
	(match nil)
	(inhibit-point-motion-hooks t))
    (save-excursion
      (save-restriction
	;; Narrow to only the prompt.
	(goto-char (point-min))
	(narrow-to-region (point) (minibuffer-prompt-end))
	;; See if the prompt contains a default input indicator.
	(while regexps
	  (setq match (pop regexps))
	  (cond
           ((not (re-search-forward (if (stringp match) match (car match))
                                    nil t))
            ;; No match yet, try the next rule.
	    (setq match nil))
           ((and (consp (cdr-safe match)) (nth 2 match))
            ;; Matched a replacement rule.
            (let* ((inhibit-read-only t)
                   (buffer-undo-list t)
                   (submatch (nth 1 match))
                   (replacement (nth 2 match))
                   (props (text-properties-at (match-beginning submatch))))
              (replace-match replacement nil nil nil submatch)
              (set-text-properties (match-beginning submatch)
                                   (match-end submatch)
                                   props)
              ;; Replacement done, now keep trying with subsequent rules.
              (setq match nil)
              (goto-char (point-min))))
           ;; Matched a non-replacement (i.e. electric hide) rule, no need to
           ;; keep trying.
           (t (setq regexps nil))))))
    (if (not match)
	;; No match for electric hiding, so just make sure our
	;; post-command-hook isn't left around.
	(remove-hook 'post-command-hook #'minibuf-eldef-update-minibuffer t)
      ;; Yup; set things up so we can frob the prompt as the state of
      ;; the input string changes.
      (setq match (if (consp match) (cdr match) 0))
      (setq match (if (consp match) (car match) match))
      (setq minibuf-eldef-overlay
	    (make-overlay (match-beginning match) (match-end match)))
      (setq minibuf-eldef-showing-default-in-prompt t)
      (setq minibuf-eldef-initial-input
	    (minibuffer-contents-no-properties))
      (setq minibuf-eldef-initial-buffer-length (point-max))
      (add-to-list 'minibuf-eldef-frobbed-minibufs (current-buffer))
      (add-hook 'post-command-hook #'minibuf-eldef-update-minibuffer nil t))))

;; post-command-hook to swap prompts when necessary
(defun minibuf-eldef-update-minibuffer ()
  "Update a minibuffer's prompt to include a default only when applicable.
This is intended to be used as a minibuffer post-command-hook for
`minibuffer-electric-default-mode'; the minibuffer should have already
been set up by `minibuf-eldef-setup-minibuffer'."
  (unless (eq minibuf-eldef-showing-default-in-prompt
	      (and (= (point-max) minibuf-eldef-initial-buffer-length)
		   (string-equal (minibuffer-contents-no-properties)
				 minibuf-eldef-initial-input)))
    ;; Swap state.
    (setq minibuf-eldef-showing-default-in-prompt
	  (not minibuf-eldef-showing-default-in-prompt))
    (overlay-put minibuf-eldef-overlay 'invisible
                 (not minibuf-eldef-showing-default-in-prompt))))


;;;###autoload
(define-minor-mode minibuffer-electric-default-mode
  "Toggle Minibuffer Electric Default mode.
With a prefix argument ARG, enable Minibuffer Electric Default
mode if ARG is positive, and disable it otherwise.  If called
from Lisp, enable the mode if ARG is omitted or nil.

Minibuffer Electric Default mode is a global minor mode.  When
enabled, minibuffer prompts that show a default value only show
the default when it's applicable -- that is, when hitting RET
would yield the default value.  If the user modifies the input
such that hitting RET would enter a non-default value, the prompt
is modified to remove the default indication."
  :global t
  :group 'minibuffer
  (if minibuffer-electric-default-mode
      ;; Enable the mode
      (add-hook 'minibuffer-setup-hook 'minibuf-eldef-setup-minibuffer)
    ;; Disable the mode
    (remove-hook 'minibuffer-setup-hook 'minibuf-eldef-setup-minibuffer)
    ;; Remove our entry from any post-command-hook variable's it's still in
    (dolist (minibuf minibuf-eldef-frobbed-minibufs)
      (with-current-buffer minibuf
	(remove-hook 'post-command-hook #'minibuf-eldef-update-minibuffer t)))
    (setq minibuf-eldef-frobbed-minibufs nil)))


(provide 'minibuf-eldef)

;;; minibuf-eldef.el ends here
