bug#66948: [PATCH] Add Completion Preview mode

[Eshel Yaron via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Eli Zaretskii <eliz@gnu.org> writes:

>> > Alternatively, we could have a defcustom based on a different design:
>> > show preview only when there are fewer than N completion candidates,
>> > with N being customizable.  That would make much more sense, IMO,
>> > since it replaces a largely "mechanical" limitation with one that is
>> > meaningful for users.
>> 
>> That would indeed be a nice solution, but it has a fatal flaw, sadly.
>> Computing the set of completion candidates is a costly operation,
>> especially with backends such as LSP, so we don't want to do that after
>> each command.
>
> This seems to be an argument in favor of the timer-based design I
> suggested.  But if you aren't inclined to do that, I guess we can
> install the feature as it is designed now.

That'd be great, thanks.

>> >> > The underline face is not universally supported, so this defface
>> >> > should have fallbacks.
>> >>
>> >> The `underline` face in faces.el has `:underline t` in the fallback
>> >> clause too, so I figured that should be alright, no?
>> >
>> > If you are okay with seeing no effect at all when the terminal doesn't
>> > support the underline attribute, then yes.  But I thought we want this
>> > face to stand out no matter what, don't we?
>> 
>> That's okay IMO, the underline just differentiates between when you have
>> a single candidate and when you have multiple candidates.  I don't think
>> that's that crucial, but if you can suggest a universally supported
>> fallback I'd be glad to add it, of course.
>
> How about some color (foreground or background)?

Sure, I've added a fallback that uses `:weight bold` when that's
supported but `:underline` isn't, and another ultimate fallback that
uses a background color instead.  Here's the updated patch (v6):


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From e55c03c2e7010e81682fafbf9b5b291a220f4026 Mon Sep 17 00:00:00 2001
From: Eshel Yaron <me@eshelyaron.com>
Date: Thu, 2 Nov 2023 16:58:31 +0100
Subject: [PATCH v6] Add Completion Preview mode

This adds a new minor mode, 'completion-preview-mode', that displays
in-buffer completion suggestions with an inline "preview" overlay.
(Bug#66948)

* lisp/completion-preview.el: New file.
* doc/emacs/programs.texi (Symbol Completion): Document it.
* etc/NEWS: Announce it.
---
 doc/emacs/programs.texi    |  11 ++
 etc/NEWS                   |   6 +
 lisp/completion-preview.el | 336 +++++++++++++++++++++++++++++++++++++
 3 files changed, 353 insertions(+)
 create mode 100644 lisp/completion-preview.el

diff --git a/doc/emacs/programs.texi b/doc/emacs/programs.texi
index 7746bc8bc23..3f3801abdb4 100644
--- a/doc/emacs/programs.texi
+++ b/doc/emacs/programs.texi
@@ -1701,6 +1701,17 @@ Symbol Completion
   In Text mode and related modes, @kbd{M-@key{TAB}} completes words
 based on the spell-checker's dictionary.  @xref{Spelling}.
 
+@cindex completion preview
+@cindex preview completion
+@cindex suggestion preview
+@cindex Completion Preview mode
+@findex completion-preview-mode
+  Completion Preview mode is a minor mode that shows completion
+suggestions as you type.  When you enable this mode (with @kbd{M-x
+completion-preview-mode}), Emacs automatically displays the
+suggested completion for text around point as an in-line preview
+right after point; type @key{TAB} to accept the suggestion.
+
 @node MixedCase Words
 @section MixedCase Words
 @cindex camel case
diff --git a/etc/NEWS b/etc/NEWS
index 22a08bc97ea..23f4a8b5311 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1078,6 +1078,12 @@ It highlights parens via ‘show-paren-mode’ and ‘blink-matching-paren’ in
 a user-friendly way, avoids reporting alleged paren mismatches and makes
 sexp navigation more intuitive.
 
++++
+*** New minor mode 'completion-preview-mode'.
+This minor mode shows you symbol completion suggestions as you type,
+using an inline preview.  New user options in the 'completion-preview'
+customization group control exactly when Emacs displays this preview.
+
 ---
 ** The highly accessible Modus themes collection has eight items.
 The 'modus-operandi' and 'modus-vivendi' are the main themes that have
diff --git a/lisp/completion-preview.el b/lisp/completion-preview.el
new file mode 100644
index 00000000000..a6e6e10c034
--- /dev/null
+++ b/lisp/completion-preview.el
@@ -0,0 +1,336 @@
+;;; completion-preview.el --- Preview completion with inline overlay  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; Author: Eshel Yaron <me@eshelyaron.com>
+;; Maintainer: Eshel Yaron <me@eshelyaron.com>
+;; Keywords: abbrev convenience
+
+;; This program 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.
+
+;; This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This library provides the Completion Preview mode.  This minor mode
+;; displays the top completion candidate for the symbol at point in an
+;; overlay after point.  Check out the customization group
+;; `completion-preview' for user options that you may want to tweak.
+;;
+;; To accept the completion suggestion, press TAB.  If you want to
+;; ignore a completion suggestion, just go on editing or moving around
+;; the buffer.  Completion Preview mode continues to update the
+;; suggestion as you type according to the text around point.
+;;
+;; The commands `completion-preview-next-candidate' and
+;; `completion-preview-prev-candidate' allow you to cycle the
+;; completion candidate that the preview suggests.  These commands
+;; don't have a default keybinding, but you can bind them, for
+;; example, to M-n and M-p in `completion-preview-active-mode-map' to
+;; have them handy whenever the preview is visible.
+;;
+;; If you set the user option `completion-preview-exact-match-only' to
+;; non-nil, Completion Preview mode only suggests a completion
+;; candidate when its the only possible completion for the (partial)
+;; symbol at point.  The user option `completion-preview-commands'
+;; says which commands should trigger the completion preview.  The
+;; user option `completion-preview-minimum-symbol-length' specifies a
+;; minimum number of consecutive characters with word or symbol syntax
+;; that should appear around point for Emacs to suggest a completion.
+;; By default, this option is set to 3, so Emacs suggests a completion
+;; if you type "foo", but typing just "fo" doesn't show the preview.
+;;
+;; The user option `completion-preview-insert-on-completion' controls
+;; what happens when you invoke `completion-at-point' while the
+;; completion preview is visible.  By default this option is nil,
+;; which tells `completion-at-point' to ignore the completion preview
+;; and show the list of completion candidates as usual.  If you set
+;; `completion-preview-insert-on-completion' to non-nil, then
+;; `completion-at-point' inserts the preview directly without looking
+;; for more candidates.
+
+;;; Code:
+
+(defgroup completion-preview nil
+  "In-buffer completion preview."
+  :group 'completion)
+
+(defcustom completion-preview-exact-match-only nil
+  "Whether to show completion preview only when there is an exact match.
+
+If this option is non-nil, Completion Preview mode only shows the
+preview when there is exactly one completion candidate that
+matches the symbol at point.  Otherwise, if this option is nil,
+when there are multiple matching candidates the preview shows the
+first candidate, and you can cycle between the candidates with
+\\[completion-preview-next-candidate] and
+\\[completion-preview-prev-candidate]."
+  :type 'boolean
+  :version "30.1")
+
+(defcustom completion-preview-commands '(self-insert-command
+                                         insert-char
+                                         delete-backward-char
+                                         backward-delete-char-untabify)
+  "List of commands that should trigger completion preview."
+  :type '(repeat (function :tag "Command" :value self-insert-command))
+  :version "30.1")
+
+(defcustom completion-preview-minimum-symbol-length 3
+  "Minimum length of the symbol at point for showing completion preview."
+  :type 'natnum
+  :version "30.1")
+
+(defcustom completion-preview-insert-on-completion nil
+  "Whether \\[completion-at-point] inserts the previewed suggestion."
+  :type 'boolean
+  :version "30.1")
+
+(defvar completion-preview-sort-function #'minibuffer--sort-by-length-alpha
+  "Sort function to use for choosing a completion candidate to preview.")
+
+(defface completion-preview
+  '((t :inherit shadow))
+  "Face for completion preview overlay."
+  :version "30.1")
+
+(defface completion-preview-exact
+  '((((supports :underline t))
+     :underline t :inherit completion-preview)
+    (((supports :weight bold))
+     :weight bold :inherit completion-preview)
+    (t :background "gray"))
+  "Face for exact completion preview overlay."
+  :version "30.1")
+
+(defvar-keymap completion-preview-active-mode-map
+  :doc "Keymap for Completion Preview Active mode."
+  "C-i" #'completion-preview-insert
+  ;; "M-n" #'completion-preview-next-candidate
+  ;; "M-p" #'completion-preview-prev-candidate
+  )
+
+(defvar-local completion-preview--overlay nil)
+
+(defvar completion-preview--internal-commands
+  '(completion-preview-next-candidate completion-preview-prev-candidate)
+  "List of commands that manipulate the completion preview.")
+
+(defsubst completion-preview--internal-command-p ()
+  "Return non-nil if `this-command' manipulates the completion preview."
+  (memq this-command completion-preview--internal-commands))
+
+(defsubst completion-preview-require-certain-commands ()
+  "Check if `this-command' is one of `completion-preview-commands'."
+  (or (completion-preview--internal-command-p)
+      (memq this-command completion-preview-commands)))
+
+(defun completion-preview-require-minimum-symbol-length ()
+  "Check if the length of symbol at point is at least above a certain threshold.
+`completion-preview-minimum-symbol-length' determines that threshold."
+  (let ((bounds (bounds-of-thing-at-point 'symbol)))
+    (and bounds (<= completion-preview-minimum-symbol-length
+                    (- (cdr bounds) (car bounds))))))
+
+(defun completion-preview-hide ()
+  "Hide the completion preview."
+  (when completion-preview--overlay
+    (delete-overlay completion-preview--overlay)
+    (setq completion-preview--overlay nil)))
+
+(defun completion-preview--make-overlay (pos string)
+  "Make a new completion preview overlay at POS showing STRING."
+  (if completion-preview--overlay
+      (move-overlay completion-preview--overlay pos pos)
+    (setq completion-preview--overlay (make-overlay pos pos))
+    (overlay-put completion-preview--overlay 'window (selected-window)))
+  (let ((previous (overlay-get completion-preview--overlay 'after-string)))
+    (unless (and previous (string= previous string))
+      (add-text-properties 0 1 '(cursor 1) string)
+      (overlay-put completion-preview--overlay 'after-string string))
+    completion-preview--overlay))
+
+(defun completion-preview--get (prop)
+  "Return property PROP of the completion preview overlay."
+  (overlay-get completion-preview--overlay prop))
+
+(define-minor-mode completion-preview-active-mode
+  "Mode for when the completion preview is shown."
+  :interactive nil
+  (if completion-preview-active-mode
+      (add-hook 'completion-at-point-functions #'completion-preview--insert -1 t)
+    (remove-hook 'completion-at-point-functions #'completion-preview--insert t)
+    (completion-preview-hide)))
+
+(defun completion-preview--exit-function (func)
+  "Return an exit function that hides the completion preview and calls FUNC."
+  (lambda (&rest args)
+    (completion-preview-active-mode -1)
+    (when (functionp func) (apply func args))))
+
+(defun completion-preview--update ()
+  "Update completion preview."
+  (seq-let (beg end table &rest plist)
+      (let ((completion-preview-insert-on-completion nil))
+        (run-hook-with-args-until-success 'completion-at-point-functions))
+    (when (and beg end table)
+      (let* ((pred (plist-get plist :predicate))
+             (exit-fn (completion-preview--exit-function
+                       (plist-get plist :exit-function)))
+             (string (buffer-substring beg end))
+             (md (completion-metadata string table pred))
+             (sort-fn (or (completion-metadata-get md 'cycle-sort-function)
+                          (completion-metadata-get md 'display-sort-function)
+                          completion-preview-sort-function))
+             (all (let ((completion-lazy-hilit t))
+                    (completion-all-completions string table pred
+                                                (- (point) beg) md)))
+             (last (last all))
+             (base (or (cdr last) 0))
+             (bbeg (+ beg base))
+             (prefix (substring string base)))
+        (when last
+          (setcdr last nil)
+          (let* ((filtered (remove prefix (all-completions prefix all)))
+                 (sorted (funcall sort-fn filtered))
+                 (multi (cadr sorted))  ; multiple candidates
+                 (cand (car sorted)))
+            (when (and cand
+                       (not (and multi
+                                 completion-preview-exact-match-only)))
+              (let* ((face (if multi
+                               'completion-preview
+                             'completion-preview-exact))
+                     (after (propertize (substring cand (length prefix))
+                                        'face face))
+                     (ov (completion-preview--make-overlay end after)))
+                (overlay-put ov 'completion-preview-beg bbeg)
+                (overlay-put ov 'completion-preview-end end)
+                (overlay-put ov 'completion-preview-index 0)
+                (overlay-put ov 'completion-preview-cands sorted)
+                (overlay-put ov 'completion-preview-exit-fn exit-fn)
+                (completion-preview-active-mode)))))))))
+
+(defun completion-preview--show ()
+  "Show a new completion preview.
+
+Call `completion-at-point-functions' in order to obtain and
+display a completion candidate for the text around point.
+
+If the preview is already shown, first check whether the
+suggested candidate remains a valid completion for the text at
+point.  If so, update the preview according the new text at
+point, otherwise hide it."
+  (when completion-preview-active-mode
+    ;; We were already showing a preview before this command, so we
+    ;; check if the text before point is still a prefix of the
+    ;; candidate that the preview suggested, and if so we first update
+    ;; existing preview according to the changes made by this command,
+    ;; and only then try to get a new candidate.  This ensures that we
+    ;; never display a stale preview and that the preview doesn't
+    ;; flicker, even with slow completion backends.
+    (let* ((beg (completion-preview--get 'completion-preview-beg))
+           (cands (completion-preview--get 'completion-preview-cands))
+           (index (completion-preview--get 'completion-preview-index))
+           (cand (nth index cands))
+           (len (length cand))
+           (end (+ beg len))
+           (cur (point))
+           (face (get-text-property 0 'face (completion-preview--get 'after-string))))
+      (if (and (< beg cur end) (string-prefix-p (buffer-substring beg cur) cand))
+          ;; The previous preview is still applicable, update it.
+          (overlay-put (completion-preview--make-overlay
+                        cur (propertize (substring cand (- cur beg))
+                                        'face face))
+                       'completion-preview-end cur)
+        ;; The previous preview is no longer applicable, hide it.
+        (completion-preview-active-mode -1))))
+  ;; Run `completion-at-point-functions' to get a new candidate.
+  (while-no-input (completion-preview--update)))
+
+(defun completion-preview--post-command ()
+  "Create, update or delete completion preview post last command."
+  (if (and (completion-preview-require-certain-commands)
+           (completion-preview-require-minimum-symbol-length))
+      ;; We should show the preview.
+      (or
+       ;; If we're called after a command that itself updates the
+       ;; preview, don't do anything.
+       (completion-preview--internal-command-p)
+       ;; Otherwise, show the preview.
+       (completion-preview--show))
+    (completion-preview-active-mode -1)))
+
+(defun completion-preview--insert ()
+  "Completion at point function for inserting the current preview.
+
+When `completion-preview-insert-on-completion' is nil, this
+function returns nil.  Completion Preview mode adds this function
+to `completion-at-point-functions' when the preview is shown,
+such that `completion-at-point' inserts the preview candidate if
+and only if `completion-preview-insert-on-completion' is non-nil."
+  (when (and completion-preview-active-mode
+             completion-preview-insert-on-completion)
+    (list (completion-preview--get 'completion-preview-beg)
+          (completion-preview--get 'completion-preview-end)
+          (list (nth (completion-preview--get 'completion-preview-index)
+                     (completion-preview--get 'completion-preview-cands)))
+          :exit-function (completion-preview--get 'completion-preview-exit-fn))))
+
+(defun completion-preview-insert ()
+  "Insert the completion candidate that the preview shows."
+  (interactive)
+  (let ((completion-preview-insert-on-completion t))
+    (completion-at-point)))
+
+(defun completion-preview-prev-candidate ()
+  "Cycle the candidate that the preview shows to the previous suggestion."
+  (interactive)
+  (completion-preview-next-candidate -1))
+
+(defun completion-preview-next-candidate (direction)
+  "Cycle the candidate that the preview shows in direction DIRECTION.
+
+DIRECTION should be either 1 which means cycle forward, or -1
+which means cycle backward.  Interactively, DIRECTION is the
+prefix argument and defaults to 1."
+  (interactive "p")
+  (when completion-preview-active-mode
+    (let* ((beg (completion-preview--get 'completion-preview-beg))
+           (all (completion-preview--get 'completion-preview-cands))
+           (cur (completion-preview--get 'completion-preview-index))
+           (len (length all))
+           (new (mod (+ cur direction) len))
+           (str (nth new all))
+           (pos (point)))
+      (while (or (<= (+ beg (length str)) pos)
+                 (not (string-prefix-p (buffer-substring beg pos) str)))
+        (setq new (mod (+ new direction) len) str (nth new all)))
+      (let ((aft (propertize (substring str (- pos beg))
+                             'face (if (< 1 len)
+                                       'completion-preview
+                                     'completion-preview-exact))))
+        (add-text-properties 0 1 '(cursor 1) aft)
+        (overlay-put completion-preview--overlay 'completion-preview-index new)
+        (overlay-put completion-preview--overlay 'after-string aft)))))
+
+;;;###autoload
+(define-minor-mode completion-preview-mode
+  "Show in-buffer completion preview as you type."
+  :lighter " CP"
+  (if completion-preview-mode
+      (add-hook 'post-command-hook #'completion-preview--post-command nil t)
+    (remove-hook 'post-command-hook #'completion-preview--post-command t)
+    (completion-preview-active-mode -1)))
+
+(provide 'completion-preview)
+;;; completion-preview.el ends here
-- 
2.42.0