>From a108605cad5c054a68c0ddbe2f576094d6eaa526 Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <jp@neverwas.me>
Date: Fri, 13 Jan 2023 00:00:56 -0800
Subject: [PATCH 4/4] [5.6] Add erc-fill style based on visual-line-mode

* lisp/erc/erc-common.el (erc--features-to-modules): Add mapping for
local module `fill-wrap'.
* lisp/erc/erc-fill.el (erc-fill-function): Add new value,
`erc-fill-wrap'.
(erc-fill-static-center): Extend meaning of option to also affect
`erc-wrap-mode'.
(erc-fill-wrap-mode, erc-fill--wrap-prefix, erc-fill--wrap-value): New
minor mode and variables to support it.
(erc-fill-wrap): New function implementing
`erc-fill-function' (behavioral) interface.
(erc-fill-wrap-nudge, erc-fill--wrap-nudge): New command and helper
for growing and shrinking visual fill prefix.
---
 lisp/erc/erc-common.el |   1 +
 lisp/erc/erc-fill.el   | 159 ++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 158 insertions(+), 2 deletions(-)

diff --git a/lisp/erc/erc-common.el b/lisp/erc/erc-common.el
index 9eb4f1a9000..456d2bc204d 100644
--- a/lisp/erc/erc-common.el
+++ b/lisp/erc/erc-common.el
@@ -96,6 +96,7 @@ erc--features-to-modules
     (erc-page page ctcp-page)
     (erc-sound sound ctcp-sound)
     (erc-stamp stamp timestamp)
+    (erc-fill fill-wrap)
     (erc-services services nickserv))
   "Migration alist mapping a library feature to module names.
 Keys need not be unique: a library may define more than one
diff --git a/lisp/erc/erc-fill.el b/lisp/erc/erc-fill.el
index caf401bf222..95b388cbf84 100644
--- a/lisp/erc/erc-fill.el
+++ b/lisp/erc/erc-fill.el
@@ -79,16 +79,27 @@ erc-fill-function
 These two styles are implemented using `erc-fill-variable' and
 `erc-fill-static'.  You can, of course, define your own filling
 function.  Narrowing to the region in question is in effect while your
-function is called."
+function is called.
+
+A third style resembles static filling but \"wraps\" instead of
+fills, courtesy of `visual-line-mode' mode, which ERC
+automatically enables when this option is `erc-fill-wrap' or
+`erc-fill-wrap-mode' is active.  Set `erc-fill-static-center' to
+your preferred initial \"prefix\" width.  For adjusting the width
+during a session, see the command `erc-fill-wrap-nudge'."
   :type '(choice (const :tag "Variable Filling" erc-fill-variable)
                  (const :tag "Static Filling" erc-fill-static)
+                 (const :tag "Dynamic word-wrap" erc-fill-wrap)
                  function))
 
 (defcustom erc-fill-static-center 27
   "Column around which all statically filled messages will be centered.
 This column denotes the point where the ` ' character between
 <nickname> and the entered text will be put, thus aligning nick
-names right and text left."
+names right and text left.
+
+Also used by the `erc-fill-function' variant `erc-fill-wrap' for
+its initial leading \"prefix\" width."
   :type 'integer)
 
 (defcustom erc-fill-variable-maximum-indentation 17
@@ -155,6 +166,150 @@ erc-fill-variable
           (erc-fill-regarding-timestamp))))
     (erc-restore-text-properties)))
 
+(defvar-local erc-fill--wrap-prefix nil)
+(defvar-local erc-fill--wrap-value nil)
+
+(define-erc-module fill-wrap nil
+  "Fill style leveraging `visual-line-mode'.
+This local module depends on the global `fill' module.  To use
+it, either include `fill-wrap' in `erc-modules' or set
+`erc-fill-function' to `erc-fill-wrap'.  You can also manually
+invoke one of the minor-mode toggles."
+  ((let (msg)
+     (unless erc-fill-mode
+       (unless (memq 'fill erc-modules)
+         (setq msg
+               (concat "WARNING: enabling default global module `fill' needed "
+                       " by local module `fill-wrap'.  This will impact all"
+                       " ERC sessions.  Add `fill' to `erc-modules' to avoid "
+                       " this warning. See Info:\"(erc) Modules\" for more.")))
+       (erc-fill-mode +1))
+     (unless (eq erc-fill-function #'erc-fill-wrap)
+       (setq-local erc-fill-function #'erc-fill-wrap))
+     (when-let* ((vars (or erc--server-reconnecting erc--target-priors))
+                 ((alist-get 'erc-fill-wrap-mode vars)))
+       (setq erc-fill--wrap-value (alist-get 'erc-fill--wrap-value vars)
+             erc-fill--wrap-prefix (alist-get 'erc-fill--wrap-prefix vars)))
+     (when (eq erc-timestamp-use-align-to 'margin)
+       (erc-timestamp--display-margin-mode +1))
+     (setq erc-fill--wrap-value
+           (or erc-fill--wrap-value erc-fill-static-center)
+           ;;
+           erc-fill--wrap-prefix
+           (or erc-fill--wrap-prefix
+               (list 'space :width erc-fill--wrap-value)))
+     (visual-line-mode +1)
+     (when msg
+       (erc-display-error-notice nil msg))))
+  ((when erc-timestamp--display-margin-mode
+     (erc-timestamp--display-margin-mode -1))
+   (kill-local-variable 'erc-button--add-nickname-face-function)
+   (kill-local-variable 'erc-fill--wrap-prefix)
+   (kill-local-variable 'erc-fill--wrap-value)
+   (kill-local-variable 'erc-fill-function)
+   (visual-line-mode -1))
+  'local)
+
+(defvar-local erc-fill--wrap-length-function nil
+  "Function to determine length of perceived nickname.
+It should return an integer representing the length of the
+nickname, including any enclosing brackets, or nil, to fall back
+to the default behavior of taking the length from the first word.")
+
+(defun erc-fill-wrap ()
+  "Use text props to mimic the effect of `erc-fill-static'.
+See `erc-fill-wrap-mode' for details."
+  (unless erc-fill-wrap-mode
+    (erc-fill-wrap-mode +1))
+  (save-excursion
+    (goto-char (point-min))
+    (let ((len (or (and erc-fill--wrap-length-function
+                        (funcall erc-fill--wrap-length-function))
+                   (progn (skip-syntax-forward "^-")
+                          (- (point) (point-min))))))
+      (erc-put-text-properties (point-min) (point-max)
+                               '(line-prefix wrap-prefix) nil
+                               `((space :width ,(- erc-fill--wrap-value 1 len))
+                                 ,erc-fill--wrap-prefix)))))
+
+;; This is an experimental helper for third-party modules.  You could,
+;; for example, use this to automatically resize the prefix to a
+;; fraction of the window's width on some event change.
+
+(defun erc-fill--wrap-fix (&optional value)
+  "Re-wrap from `point-min' to `point-max'.
+Reset prefix to VALUE, when given."
+  (save-excursion
+    (when value
+      (setq erc-fill--wrap-value value
+            erc-fill--wrap-prefix (list 'space :width value)))
+    (let ((inhibit-field-text-motion t)
+          (inhibit-read-only t))
+      (goto-char (point-min))
+      (while (and (zerop (forward-line))
+                  (< (point) (min (point-max) erc-insert-marker)))
+        (save-restriction
+          (narrow-to-region (pos-bol) (pos-eol))
+          (erc-fill-wrap))))))
+
+(defun erc-fill--wrap-nudge (arg)
+  (save-excursion
+    (save-restriction
+      (widen)
+      (let ((inhibit-field-text-motion t)
+            (inhibit-read-only t) ; necessary?
+            (p (goto-char (point-min))))
+        (when (zerop arg)
+          (setq arg (- erc-fill-static-center erc-fill--wrap-value)))
+        (cl-incf (caddr erc-fill--wrap-prefix) arg)
+        (cl-incf erc-fill--wrap-value arg)
+        (while (setq p (next-single-property-change p 'line-prefix))
+          (when-let ((v (get-text-property p 'line-prefix)))
+            (cl-incf (caddr v) arg)
+            (when-let
+                ((e (text-property-not-all p (point-max) 'line-prefix v)))
+              (goto-char e)))))))
+  arg)
+
+(defun erc-fill-wrap-nudge (arg)
+  "Adjust `erc-fill-wrap' by ARG columns.
+Offer to repeat command in a manner similar to
+`text-scale-adjust'.  Note that misalignment may occur when
+messages contain decorations applied by third-party modules.
+See `erc-fill--wrap-fix' for a workaround."
+  (interactive "p")
+  (unless erc-fill--wrap-value
+    (cl-assert (not erc-fill-wrap-mode))
+    (user-error "Minor mode `erc-fill-wrap-mode' disabled"))
+  (let ((total (erc-fill--wrap-nudge arg))
+        (start (window-start))
+        (marker (set-marker (make-marker) (point))))
+    (when (zerop arg)
+      (setq arg 1))
+    (set-transient-map
+     (let ((map (make-sparse-keymap)))
+       (dolist (key '(?+ ?= ?- ?0))
+         (let ((a (pcase key
+                    (?0 0)
+                    (?- (- (abs arg)))
+                    (_ (abs arg)))))
+           (define-key map (vector (list key))
+                       (lambda ()
+                         (interactive)
+                         (cl-incf total (erc-fill--wrap-nudge a))
+                         (set-window-start (selected-window) start)
+                         (goto-char marker)))))
+       map)
+     t
+     (lambda ()
+       (set-marker marker nil)
+       (message "Fill prefix: %d (%+d col%s)"
+                erc-fill--wrap-value total (if (> (abs total) 1) "s" "")))
+     "Use %k for further adjustment"
+     1)
+    (goto-char marker)
+    (set-window-start (selected-window) start)))
+
 (defun erc-fill-regarding-timestamp ()
   "Fills a text such that messages start at column `erc-fill-static-center'."
   (fill-region (point-min) (point-max) t t)
-- 
2.38.1