bug#63569: 30.0.50; ERC 5.6: Add automatic nickname highlighting to ERC

["J.P." <jp@neverwas.me>]

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v4. Drop erc-buttons patch. Be smarter when looking for faces to skip.


Just a minor update. As noted, I've dropped the first patch

  0001-5.6-Allow-ERC-modules-to-extend-erc-nick-popup-alist.patch

which has been installed on HEAD as

  e560f9af8e8 Allow ERC modules to extend erc-nick-popup-alist


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From 5100a10672355255a80549acde1ca939f61e465d Mon Sep 17 00:00:00 2001
From: "F. Jason Park" <jp@neverwas.me>
Date: Mon, 12 Jun 2023 21:00:28 -0700
Subject: [PATCH 0/1] *** NOT A PATCH ***

*** BLURB HERE ***

David Leatherman (1):
  [5.6] Add module for colorizing nicknames to ERC

 doc/misc/erc.texi                |   4 +
 etc/ERC-NEWS                     |   8 +
 lisp/erc/erc-nicks.el            | 485 +++++++++++++++++++++++++++++++
 lisp/erc/erc.el                  |   1 +
 test/lisp/erc/erc-nicks-tests.el | 340 ++++++++++++++++++++++
 test/lisp/erc/erc-tests.el       |   2 +-
 6 files changed, 839 insertions(+), 1 deletion(-)
 create mode 100644 lisp/erc/erc-nicks.el
 create mode 100644 test/lisp/erc/erc-nicks-tests.el

Interdiff:
diff --git a/lisp/erc/erc-nicks.el b/lisp/erc/erc-nicks.el
index 0e0a481d453..ad4fca523d2 100644
--- a/lisp/erc/erc-nicks.el
+++ b/lisp/erc/erc-nicks.el
@@ -34,7 +34,7 @@
 ;; init.el.  Customize users need only click "Apply and Save", as
 ;; usual.
 
-;; History:
+;;; History:
 
 ;; This module has enjoyed a number of contributors across several
 ;; variants over the years.  To those not mentioned, your efforts are
@@ -69,8 +69,9 @@ erc-nicks
 (defcustom erc-nicks-ignore-chars ",`'_-"
   "Trailing characters in a nick to ignore while highlighting.
 Value should be a string containing characters typically appended
-by IRC clients a la `erc-nick-uniquifier' to secure a nickname
-after a rejection.  A value of nil means don't trim anything."
+by IRC clients to secure a nickname after a rejection (see option
+`erc-nick-uniquifier').  A value of nil means don't trim
+anything."
   :type '(choice (string :tag "Chars to trim")
                  (const :tag "Don't trim" nil)))
 
@@ -95,7 +96,7 @@ erc-nicks-bg-color
   :type 'string)
 
 (defcustom erc-nicks-color-adjustments
-  '(erc-nicks-add-contrast erc-nicks-cap-contrast erc-nicks-ensaturate)
+  '(erc-nicks-invert erc-nicks-cap-contrast erc-nicks-ensaturate)
   "Treatments applied to improve aesthetics or visibility.
 For example, the function `erc-nicks-invert' inverts a nick when
 it's too close to the background, and `erc-nicks-add-contrast'
@@ -110,7 +111,7 @@ erc-nicks-color-adjustments
                   (function-item :tag "Bound saturation" erc-nicks-ensaturate)
                   function)))
 
-(defcustom erc-nicks-contrast-range '(4.0 . 12.5)
+(defcustom erc-nicks-contrast-range '(4.3 . 12.5)
   "Desired range of contrast as a cons of (MIN . MAX).
 When `erc-nicks-add-contrast' and/or `erc-nicks-invert' appear in
 `erc-nicks-color-adjustments', MIN specifies the minimum amount
@@ -118,9 +119,10 @@ erc-nicks-contrast-range
 foreground colors.  Depending on the background, nicks may appear
 tinted in pastels or shaded with muted grays.  MAX works
 similarly for reducing contrast, but only when
-`erc-nicks-cap-contrast' is active.  Values can range from 1.0 to
-21.0(:1) but may produce unsatisfactory results toward either
-extreme."
+`erc-nicks-cap-contrast' is active.  Users with lighter
+backgrounds may want to lower MAX significantly.  Either value
+can range from 1.0 to 21.0(:1) but may produce unsatisfactory
+results toward either extreme."
   :type '(cons float float))
 
 (defcustom erc-nicks-saturation-range '(0.2 . 0.8)
@@ -183,11 +185,14 @@ erc-nicks--bg-mode
 (defvar erc-nicks--grad-steps 9)
 
 ;; https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
-
+;;
+;; TODO see implementation in https://elpa.gnu.org/packages/ement and
+;; maybe copy that instead.
 (defun erc-nicks--adjust-contrast (color target &optional decrease)
   (let* ((lum-bg (or erc-nicks--bg-luminance
                      (setq erc-nicks--bg-luminance
                            (erc-nicks--get-luminance erc-nicks-bg-color))))
+         ;; Shouldn't this use the actual bg color instead of b+w?
          (stop (if (eq (if decrease 'light 'dark) (erc-nicks--bg-mode))
                    '(1.0 1.0 1.0)
                  '(0.0 0.0 0.0)))
@@ -275,6 +280,17 @@ erc-nicks--hash
               n))
     h))
 
+;; From https://elpa.gnu.org/packages/ement.  The resolution has been
+;; scaled up to try and avoid components being exactly 0.0, which our
+;; contrast function doesn't seem to like.  Hopefully, that's OK.
+(defun erc-nicks--gen-color-ement (string)
+  "Generate normalized RGB color from STRING."
+  (let* ((ratio (/ (float (abs (sxhash string))) (float most-positive-fixnum)))
+         (color-num (round (* (* #xffff #xffff #xffff) ratio))))
+    (list (/ (float (logand color-num #xffff)) #xffff)
+          (/ (float (ash (logand color-num #xffff0000) -16)) #xffff)
+          (/ (float (ash (logand color-num #xffff00000000) -32)) #xffff))))
+
 (defvar-local erc-nicks--colors-len nil)
 (defvar-local erc-nicks--custom-keywords '(:group erc-nicks :group erc-faces))
 
@@ -312,12 +328,13 @@ erc-nicks--redirect-face-widget-link
              (cddr args) plist))))
   args)
 
-(defun erc-nicks--reduce (color-string)
-  "Fold contrast strategies over COLOR-STRING."
+(defun erc-nicks--reduce (color)
+  "Fold contrast strategies over COLOR, a string or normalized triple.
+Return a hex string."
   (apply #'color-rgb-to-hex
          (seq-reduce (lambda (color strategy) (funcall strategy color))
                      erc-nicks-color-adjustments
-                     (color-name-to-rgb color-string))))
+                     (if (stringp color) (color-name-to-rgb color) color))))
 
 (defun erc-nicks--get-face (nick key)
   "Retrieve or create a face for NICK, stored locally under KEY.
@@ -332,7 +349,7 @@ erc-nicks--get-face
                         (erc-nicks--revive face face nick (erc-network))))))
         (let ((color (erc-nicks--reduce
                       (pcase erc-nicks-colors
-                        ('all (format "#%012x" (erc-nicks--hash key)))
+                        ('all (erc-nicks--gen-color-ement key))
                         ((or 'defined v)
                          (unless v (setq v (defined-colors (selected-frame))))
                          (unless erc-nicks--colors-len
@@ -345,6 +362,32 @@ erc-nicks--get-face
            new-face (format "Internal face for %s on %s." nick (erc-network)))
           (puthash nick new-face table)))))
 
+(define-inline erc-nicks--anon-face-p (face)
+  (inline-quote (and (consp ,face)
+                     (pcase (car ,face)
+                       ((pred keywordp) t)
+                       ('foreground-color t)
+                       ('background-color t)))))
+
+(defvar erc-nicks--max-skip-search 3 ; make this an option?
+  "Max number of faces to visit when testing `erc-nicks-skip-faces'.")
+
+(defun erc-nicks--skip-p (prop option limit)
+  "Return non-nil if a face in PROP appears in OPTION.
+But abandon search after examining LIMIT faces."
+  (setq prop (if (erc-nicks--anon-face-p prop) (list prop) (ensure-list prop)))
+  (catch 'found
+    (while-let (((> limit 0))
+                (elem (pop prop)))
+      (while (and (consp elem) (not (erc-nicks--anon-face-p elem)))
+        (when (cdr elem)
+          (push (cdr elem) prop))
+        (setq elem (car elem)))
+      (when elem
+        (cl-decf limit)
+        (when (if (symbolp elem) (memq elem option) (member elem option))
+          (throw 'found elem))))))
+
 (defvar erc-nicks--phony-face nil
   "Face to pretend is propertizing the nick at point.
 Modules needing to colorize nicks outside of a buttonizing
@@ -366,8 +409,8 @@ erc-nicks--highlight
        (face (or erc-nicks--phony-face
                  (get-text-property (car (erc-button--nick-bounds nick-object))
                                     'font-lock-face)))
-       ((not (seq-some (lambda (f) (memq f erc-nicks-skip-faces))
-                       (erc-list face)))) ; cl-notany
+       ((not (erc-nicks--skip-p face erc-nicks-skip-faces
+                                erc-nicks--max-skip-search)))
        ;; Ensure nicks are colored uniquely (per network) by padding
        ;; from the right, as mentioned above in `erc-nicks--hash'.
        (key (concat (erc-button--nick-downcased nick-object)
diff --git a/test/lisp/erc/erc-nicks-tests.el b/test/lisp/erc/erc-nicks-tests.el
index e84a2fea6ce..0d640ad59c3 100644
--- a/test/lisp/erc/erc-nicks-tests.el
+++ b/test/lisp/erc/erc-nicks-tests.el
@@ -300,4 +300,41 @@ erc-nicks--hash
     (when noninteractive
       (kill-buffer))))
 
+(ert-deftest erc-nicks--skip-p ()
+  ;; Baseline
+  (should-not (erc-nicks--skip-p 'bold nil 10000000))
+  (should-not (erc-nicks--skip-p '(bold) nil 10000000))
+  (should-not (erc-nicks--skip-p nil '(bold) 10000000))
+  (should-not (erc-nicks--skip-p 'bold '(bold) 0))
+  (should-not (erc-nicks--skip-p '(bold) '(bold) 0))
+  (should-not (erc-nicks--skip-p 'bold '(foo bold) 0))
+  (should-not (erc-nicks--skip-p '((:inherit bold)) '(bold) 1))
+  (should (erc-nicks--skip-p 'bold '(bold) 1))
+  (should (erc-nicks--skip-p 'bold '(fake bold) 1))
+  (should (erc-nicks--skip-p 'bold '(foo bar bold) 1))
+  (should (erc-nicks--skip-p '(bold) '(bold) 1))
+  (should (erc-nicks--skip-p '((bold)) '(bold) 1))
+  (should (erc-nicks--skip-p '((((bold)))) '(bold) 1))
+  (should (erc-nicks--skip-p '(bold) '(foo bold) 1))
+  (should (erc-nicks--skip-p '(:inherit bold) '((:inherit bold)) 1))
+  (should (erc-nicks--skip-p '((:inherit bold)) '((:inherit bold)) 1))
+  (should (erc-nicks--skip-p '(((:inherit bold))) '((:inherit bold)) 1))
+
+  ;; Composed
+  (should-not (erc-nicks--skip-p '(italic bold) '(bold) 1))
+  (should-not (erc-nicks--skip-p '((italic) bold) '(bold) 1))
+  (should-not (erc-nicks--skip-p '(italic (bold)) '(bold) 1))
+  (should (erc-nicks--skip-p '(italic bold) '(bold) 2))
+  (should (erc-nicks--skip-p '((italic) bold) '(bold) 2))
+  (should (erc-nicks--skip-p '(italic (bold)) '(bold) 2))
+
+  (should-not (erc-nicks--skip-p '(italic default bold) '(bold) 2))
+  (should-not (erc-nicks--skip-p '((default italic) bold) '(bold) 2))
+  (should-not (erc-nicks--skip-p '(italic (default bold)) '(bold) 2))
+  (should-not (erc-nicks--skip-p '((default italic) (bold shadow)) '(bold) 2))
+  (should (erc-nicks--skip-p '((default italic) bold) '(bold) 3))
+  (should (erc-nicks--skip-p '(italic (default bold)) '(bold) 3))
+  (should (erc-nicks--skip-p '((default italic) (bold shadow)) '(bold) 3))
+  (should (erc-nicks--skip-p '(italic (default (bold shadow))) '(bold) 3)))
+
 ;;; erc-nicks-tests.el ends here
-- 
2.40.1


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From 5100a10672355255a80549acde1ca939f61e465d Mon Sep 17 00:00:00 2001
From: David Leatherman <leathekd@gmail.com>
Date: Sun, 18 Dec 2022 19:01:40 -0800
Subject: [PATCH 1/1] [5.6] Add module for colorizing nicknames to ERC

* doc/misc/erc.texi: Add `nicks' to module lineup.
* etc/ERC-NEWS: Mention new module `nicks'.
* lisp/erc/erc-nicks.el: New file.
* lisp/erc/erc.el: (erc-modules): Add `nicks'.
* test/lisp/erc/erc-nicks-tests.el: New file.
* test/lisp/erc/erc-tests (erc-tests--modules): Add
`nicks'.  (Bug#63569)

Co-authored-by: Andy Stewart <lazycat.manatee@gmail.com>
---
 doc/misc/erc.texi                |   4 +
 etc/ERC-NEWS                     |   8 +
 lisp/erc/erc-nicks.el            | 485 +++++++++++++++++++++++++++++++
 lisp/erc/erc.el                  |   1 +
 test/lisp/erc/erc-nicks-tests.el | 340 ++++++++++++++++++++++
 test/lisp/erc/erc-tests.el       |   2 +-
 6 files changed, 839 insertions(+), 1 deletion(-)
 create mode 100644 lisp/erc/erc-nicks.el
 create mode 100644 test/lisp/erc/erc-nicks-tests.el

diff --git a/doc/misc/erc.texi b/doc/misc/erc.texi
index e848ed21a50..07484122e4b 100644
--- a/doc/misc/erc.texi
+++ b/doc/misc/erc.texi
@@ -459,6 +459,10 @@ Modules
 @item netsplit
 Detect netsplits
 
+@cindex modules, nicks
+@item nicks
+Automatically colorize nicks
+
 @cindex modules, noncommands
 @item noncommands
 Don't display non-IRC commands after evaluation
diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index 68f1083621c..d6383b72557 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -30,6 +30,14 @@ helper called 'erc-fill-wrap-nudge' allows for dynamic "refilling" of
 buffers on the fly.  Set 'erc-fill-function' to 'erc-fill-wrap' to get
 started.
 
+** A new module for nickname highlighting has joined ERC.
+Automatic nickname coloring has come to ERC core.  Users familiar with
+'erc-hl-nicks', from which this module directly descends, will already
+be familiar with its suite of handy options.  By default, each
+nickname in an ERC session receives a unique face with a unique (or
+evenly dealt) foreground color.  Add 'nicks' to 'erc-modules' to get
+started.
+
 ** A unified interactive entry point.
 New users are often dismayed to discover that M-x ERC doesn't connect
 to its default network, Libera.Chat, over TLS.  Though perhaps a
diff --git a/lisp/erc/erc-nicks.el b/lisp/erc/erc-nicks.el
new file mode 100644
index 00000000000..ad4fca523d2
--- /dev/null
+++ b/lisp/erc/erc-nicks.el
@@ -0,0 +1,485 @@
+;;; erc-nicks.el -- Nick colors for ERC  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; Author: David Leatherman <leathekd@gmail.com>
+;;         Andy Stewart <lazycat.manatee@gmail.com>
+
+;; 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:
+
+;; This file provides the `nicks' module for automatic nickname
+;; highlighting.  Add `nicks' to `erc-modules' to get started.
+;;
+;; To change the color of a nickname in a target buffer, click on it
+;; and choose "Edit face" from the completion interface, and then
+;; perform your adjustments in the resulting Customize menu.
+;; Non-Customize users can persist their changes permanently by
+;; clicking on the face's "location" hyperlink and copying the
+;; generated code snippet (`defface' or `use-package') to their
+;; init.el.  Customize users need only click "Apply and Save", as
+;; usual.
+
+;;; History:
+
+;; This module has enjoyed a number of contributors across several
+;; variants over the years.  To those not mentioned, your efforts are
+;; no less appreciated.
+
+;; 2023/05 - erc-nicks
+;;           Rewrite using internal API, and rebrand for ERC 5.6
+;; 2020/03 - erc-hl-nicks 1.3.4
+;;           Final release, see [1] for intervening history
+;; 2014/05 - erc-highlight-nicknames.el
+;;           Final release, see [2] for intervening history
+;; 2011/08 - erc-hl-nicks 1.0
+;;           Initial release forked from erc-highlight-nicknames.el
+;; 2008/12 - erc-highlight-nicknames.el
+;;           First release from Andy Stewart
+;; 2007/09 - erc-highlight-nicknames.el
+;;           Initial release by by André Riemann
+
+;; [1] <http://www.github.com/leathekd/erc-nicks>
+;; [2] <https://www.emacswiki.org/emacs/ErcHighlightNicknames>
+
+;;; Code:
+
+(require 'erc-button)
+(require 'color)
+
+(defgroup erc-nicks nil
+  "Colorize nicknames in ERC buffers."
+  :package-version '(ERC . "5.6") ; FIXME sync on release
+  :group 'erc)
+
+(defcustom erc-nicks-ignore-chars ",`'_-"
+  "Trailing characters in a nick to ignore while highlighting.
+Value should be a string containing characters typically appended
+by IRC clients to secure a nickname after a rejection (see option
+`erc-nick-uniquifier').  A value of nil means don't trim
+anything."
+  :type '(choice (string :tag "Chars to trim")
+                 (const :tag "Don't trim" nil)))
+
+(defcustom erc-nicks-skip-nicks nil
+  "Nicks to avoid highlighting."
+  :type '(repeat string))
+
+(defcustom erc-nicks-skip-faces '( erc-notice-face erc-current-nick-face
+                                   erc-my-nick-face erc-pal-face erc-fool-face)
+  "Faces to avoid highlighting atop."
+  :type  '(repeat symbol))
+
+(defcustom erc-nicks-nickname-face erc-button-nickname-face
+  "Face to mix with generated one for emphasizing non-speakers."
+  :type '(choice face (const nil)))
+
+(defcustom erc-nicks-bg-color
+  (frame-parameter (selected-frame) 'background-color)
+  "Background color for calculating contrast.
+Set this explicitly when the background color isn't discoverable,
+which may be the case in terminal Emacs."
+  :type 'string)
+
+(defcustom erc-nicks-color-adjustments
+  '(erc-nicks-invert erc-nicks-cap-contrast erc-nicks-ensaturate)
+  "Treatments applied to improve aesthetics or visibility.
+For example, the function `erc-nicks-invert' inverts a nick when
+it's too close to the background, and `erc-nicks-add-contrast'
+attempts to find a decent contrast ratio by brightening or
+darkening.  Note that ERC still applies adjustments when
+`erc-nicks-colors' is a user-defined list of colors.  Specify a
+value of nil to prevent that."
+  :type '(repeat
+          (choice (function-item :tag "Invert" erc-nicks-invert)
+                  (function-item :tag "Add contrast" erc-nicks-add-contrast)
+                  (function-item :tag "Cap contrast" erc-nicks-cap-contrast)
+                  (function-item :tag "Bound saturation" erc-nicks-ensaturate)
+                  function)))
+
+(defcustom erc-nicks-contrast-range '(4.3 . 12.5)
+  "Desired range of contrast as a cons of (MIN . MAX).
+When `erc-nicks-add-contrast' and/or `erc-nicks-invert' appear in
+`erc-nicks-color-adjustments', MIN specifies the minimum amount
+of contrast allowed between a buffer's background and its
+foreground colors.  Depending on the background, nicks may appear
+tinted in pastels or shaded with muted grays.  MAX works
+similarly for reducing contrast, but only when
+`erc-nicks-cap-contrast' is active.  Users with lighter
+backgrounds may want to lower MAX significantly.  Either value
+can range from 1.0 to 21.0(:1) but may produce unsatisfactory
+results toward either extreme."
+  :type '(cons float float))
+
+(defcustom erc-nicks-saturation-range '(0.2 . 0.8)
+  "Desired range for constraining saturation.
+Expressed as a cons of decimal proportions.  Only matters when
+`erc-nicks-ensaturate' appears in `erc-nicks-color-adjustments'."
+  :type '(cons float float))
+
+;; Should we also accept a list of faces?
+(defcustom erc-nicks-colors 'all
+  "Pool of colors.
+This can be a list of hexes or color names, such as those
+provided by `defined-colors', which can itself be used when the
+value is the symbol `defined'.  With `all', use any 24-bit color."
+  :type '(choice (const all) (const defined) (list string)))
+
+(defvar-local erc-nicks--face-table nil
+  "Hash table containing unique nick faces.")
+
+;; https://stackoverflow.com/questions/596216#answer-56678483
+(defun erc-nicks--get-luminance (color)
+  "Return relative luminance of COLOR.
+COLOR can be a list of normalized values or a name.  This is the
+same as the Y component returned by `color-srgb-to-xyz'."
+  (let ((out 0)
+        (coefficients '(0.2126 0.7152 0.0722))
+        (chnls (if (stringp color) (color-name-to-rgb color) color)))
+    (dolist (ch chnls out)
+      (cl-incf out (* (pop coefficients)
+                      (if (<= ch 0.04045)
+                          (/ ch 12.92)
+                        (expt (/ (+ ch 0.055) 1.055) 2.4)))))))
+
+(defvar-local erc-nicks--bg-luminance nil)
+
+(defun erc-nicks--get-contrast (fg &optional bg)
+  "Return a float between 1 and 21 for colors FG and BG.
+If FG or BG are floats, interpret them as luminance values."
+  (let* ((lum-fg (if (numberp fg) fg (erc-nicks--get-luminance fg)))
+         (lum-bg (if bg
+                     (if (numberp bg) bg (erc-nicks--get-luminance bg))
+                   (or erc-nicks--bg-luminance
+                       (setq erc-nicks--bg-luminance
+                             (erc-nicks--get-luminance erc-nicks-bg-color))))))
+    (when (< lum-fg lum-bg) (cl-rotatef lum-fg lum-bg))
+    (/ (+ 0.05 lum-fg) (+ 0.05 lum-bg))))
+
+(defvar-local erc-nicks--bg-mode-value nil)
+
+(defmacro erc-nicks--bg-mode ()
+  `(or erc-nicks--bg-mode-value
+       (setq erc-nicks--bg-mode-value
+             ,(cond ((fboundp 'frame--current-background-mode)
+                     '(frame--current-background-mode (selected-frame)))
+                    ((fboundp 'frame--current-backround-mode)
+                     '(frame--current-backround-mode (selected-frame)))
+                    (t
+                     '(frame-parameter (selected-frame) 'background-mode))))))
+
+(defvar erc-nicks--grad-steps 9)
+
+;; https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
+;;
+;; TODO see implementation in https://elpa.gnu.org/packages/ement and
+;; maybe copy that instead.
+(defun erc-nicks--adjust-contrast (color target &optional decrease)
+  (let* ((lum-bg (or erc-nicks--bg-luminance
+                     (setq erc-nicks--bg-luminance
+                           (erc-nicks--get-luminance erc-nicks-bg-color))))
+         ;; Shouldn't this use the actual bg color instead of b+w?
+         (stop (if (eq (if decrease 'light 'dark) (erc-nicks--bg-mode))
+                   '(1.0 1.0 1.0)
+                 '(0.0 0.0 0.0)))
+         ;; From `color-gradient' in color.el
+         (r (nth 0 color))
+         (g (nth 1 color))
+         (b (nth 2 color))
+         (interval (float (1+ (expt 2 erc-nicks--grad-steps))))
+         (r-step (/ (- (nth 0 stop) r) interval))
+         (g-step (/ (- (nth 1 stop) g) interval))
+         (b-step (/ (- (nth 2 stop) b) interval))
+         (maxtries erc-nicks--grad-steps)
+         started)
+    ;; FIXME stop when sufficiently close instead of exhausting.
+    (while (let* ((lum-fg (erc-nicks--get-luminance (list r g b)))
+                  (darker (if (< lum-bg lum-fg) lum-bg lum-fg))
+                  (lighter (if (= darker lum-bg) lum-fg lum-bg))
+                  (cur (/ (+ 0.05 lighter) (+ 0.05 darker)))
+                  (scale (expt 2 maxtries)))
+             (cond ((if decrease (> cur target) (< cur target))
+                    (setq r (+ r (* r-step scale))
+                          g (+ g (* g-step scale))
+                          b (+ b (* b-step scale))))
+                   (started
+                    (setq r (- r (* r-step scale))
+                          g (- g (* g-step scale))
+                          b (- b (* b-step scale))))
+                   (t (setq maxtries 1)))
+             (unless started
+               (setq started t))
+             (setq r (min 1.0 (max 0 r))
+                   g (min 1.0 (max 0 g))
+                   b (min 1.0 (max 0 b)))
+             (not (zerop (cl-decf maxtries)))))
+    (list r g b)))
+
+(defun erc-nicks-add-contrast (color)
+  "Increase COLOR's contrast by blending it with white or black.
+Unless sufficient contrast exists between COLOR and the
+background, raise it to somewhere around the lower bound of
+`erc-nicks-contrast-range'."
+  (erc-nicks--adjust-contrast color (car erc-nicks-contrast-range)))
+
+(defun erc-nicks-cap-contrast (color)
+  "Reduce COLOR's contrast by blending it with white or black.
+If excessive contrast exists between COLOR and the background,
+lower it to the upper bound of `erc-nicks-contrast-range'."
+  (erc-nicks--adjust-contrast color (cdr erc-nicks-contrast-range) 'remove))
+
+(defun erc-nicks-invert (color)
+  "Invert COLOR based on the CAR of `erc-nicks-contrast-range'.
+Don't bother if the inverted color has less contrast than the
+input."
+  (if-let ((con-input (erc-nicks--get-contrast color))
+           ((< con-input (car erc-nicks-contrast-range)))
+           (flipped (mapcar (lambda (c) (- 1.0 c)) color))
+           ((> (erc-nicks--get-contrast flipped) con-input)))
+      flipped
+    color))
+
+(defun erc-nicks-ensaturate (color)
+  "Ensure COLOR falls within `erc-nicks-saturation-range'."
+  (pcase-let ((`(,min . ,max) erc-nicks-saturation-range)
+              (`(,h ,s ,l) (apply #'color-rgb-to-hsl color)))
+    (cond ((> s max) (setq color (color-hsl-to-rgb h max l)))
+          ((< s min) (setq color (color-hsl-to-rgb h min l)))))
+  color)
+
+;; http://www.cse.yorku.ca/~oz/hash.html
+;; See also gui_nick_hash_djb2_64 in weechat/src/gui/gui-nick.c,
+;; which is originally from https://savannah.nongnu.org/patch/?8062.
+;;
+;; Short strings of the same length and those differing only in their
+;; low order bits tend to land in neighboring buckets, which are often
+;; similar in color.  Padding on the right with at least nine added
+;; chars seems to scramble things sufficiently enough for our needs.
+
+(defun erc-nicks--hash (s &optional nchoices)
+  (let ((h 5381) ; seed and multiplier (33) hardcoded for now
+        (p (or nchoices 281474976710656)) ; 48-bits (expt 2 48)
+        (i 0)
+        (n (length s)))
+    (while (< (setq h (% (+ (* h 33) (aref s i)) p)
+                    i (1+ i))
+              n))
+    h))
+
+;; From https://elpa.gnu.org/packages/ement.  The resolution has been
+;; scaled up to try and avoid components being exactly 0.0, which our
+;; contrast function doesn't seem to like.  Hopefully, that's OK.
+(defun erc-nicks--gen-color-ement (string)
+  "Generate normalized RGB color from STRING."
+  (let* ((ratio (/ (float (abs (sxhash string))) (float most-positive-fixnum)))
+         (color-num (round (* (* #xffff #xffff #xffff) ratio))))
+    (list (/ (float (logand color-num #xffff)) #xffff)
+          (/ (float (ash (logand color-num #xffff0000) -16)) #xffff)
+          (/ (float (ash (logand color-num #xffff00000000) -32)) #xffff))))
+
+(defvar-local erc-nicks--colors-len nil)
+(defvar-local erc-nicks--custom-keywords '(:group erc-nicks :group erc-faces))
+
+(defun erc-nicks--revive (new-face old-face nick net)
+  (put new-face 'erc-nicks--custom-nick (cons nick net))
+  (apply #'custom-declare-face new-face (face-user-default-spec old-face)
+         (format "Persistent `erc-nicks' color for %s on %s." nick net)
+         erc-nicks--custom-keywords))
+
+(defun erc-nicks--create-defface-template (face)
+  (pop-to-buffer (get-buffer-create (format "*New face %s*" face)))
+  (erase-buffer)
+  (lisp-interaction-mode)
+  (insert ";; If you *don't* use Customize, put something like this in your\n"
+          (substitute-command-keys
+           ";; init.el and use \\[eval-last-sexp] to apply any edits.\n\n")
+          (format "(defface %s\n  '%S\n  %S"
+                  face (face-user-default-spec face) (face-documentation face))
+          (cl-loop for (k v) on erc-nicks--custom-keywords by #'cddr
+                   concat (format "\n  %s %S" k (list 'quote v)))
+          ")\n\n;; Or, if you use use-package\n(use-package erc-nicks\n"
+          "  :custom-face\n"
+          (format "  (%s %S)" face (face-user-default-spec face))
+          ")\n"))
+
+(defun erc-nicks--redirect-face-widget-link (args)
+  (pcase args
+    (`(,widget face-link . ,plist)
+     (when-let* ((face (widget-value widget))
+                 ((get face 'erc-nicks--custom-nick)))
+       (unless (symbol-file face)
+         (setf (plist-get plist :action)
+               (lambda (&rest _) (erc-nicks--create-defface-template face))))
+       (setf (plist-get plist :help-echo) "Create or edit `defface'."
+             (cddr args) plist))))
+  args)
+
+(defun erc-nicks--reduce (color)
+  "Fold contrast strategies over COLOR, a string or normalized triple.
+Return a hex string."
+  (apply #'color-rgb-to-hex
+         (seq-reduce (lambda (color strategy) (funcall strategy color))
+                     erc-nicks-color-adjustments
+                     (if (stringp color) (color-name-to-rgb color) color))))
+
+(defun erc-nicks--get-face (nick key)
+  "Retrieve or create a face for NICK, stored locally under KEY.
+But favor a custom erc-nicks-NICK@NETWORK-face, when defined."
+  (setq nick (erc-downcase nick))
+  (let ((table (buffer-local-value 'erc-nicks--face-table
+                                   (erc-server-buffer))))
+    (or (gethash nick table)
+        (and-let* ((face (intern-soft (concat "erc-nicks-" nick "@"
+                                              (erc-network-name) "-face")))
+                   ((or (and (facep face) face)
+                        (erc-nicks--revive face face nick (erc-network))))))
+        (let ((color (erc-nicks--reduce
+                      (pcase erc-nicks-colors
+                        ('all (erc-nicks--gen-color-ement key))
+                        ((or 'defined v)
+                         (unless v (setq v (defined-colors (selected-frame))))
+                         (unless erc-nicks--colors-len
+                           (setq erc-nicks--colors-len (length v)))
+                         (nth (erc-nicks--hash key erc-nicks--colors-len)
+                              v)))))
+              (new-face (make-symbol (concat "erc-nicks-" nick "-face"))))
+          (face-spec-set new-face `((t :foreground ,color)) 'face-defface-spec)
+          (set-face-documentation
+           new-face (format "Internal face for %s on %s." nick (erc-network)))
+          (puthash nick new-face table)))))
+
+(define-inline erc-nicks--anon-face-p (face)
+  (inline-quote (and (consp ,face)
+                     (pcase (car ,face)
+                       ((pred keywordp) t)
+                       ('foreground-color t)
+                       ('background-color t)))))
+
+(defvar erc-nicks--max-skip-search 3 ; make this an option?
+  "Max number of faces to visit when testing `erc-nicks-skip-faces'.")
+
+(defun erc-nicks--skip-p (prop option limit)
+  "Return non-nil if a face in PROP appears in OPTION.
+But abandon search after examining LIMIT faces."
+  (setq prop (if (erc-nicks--anon-face-p prop) (list prop) (ensure-list prop)))
+  (catch 'found
+    (while-let (((> limit 0))
+                (elem (pop prop)))
+      (while (and (consp elem) (not (erc-nicks--anon-face-p elem)))
+        (when (cdr elem)
+          (push (cdr elem) prop))
+        (setq elem (car elem)))
+      (when elem
+        (cl-decf limit)
+        (when (if (symbolp elem) (memq elem option) (member elem option))
+          (throw 'found elem))))))
+
+(defvar erc-nicks--phony-face nil
+  "Face to pretend is propertizing the nick at point.
+Modules needing to colorize nicks outside of a buttonizing
+context can use this instead of setting fictitious bounds on the
+`erc-button--nick' object passed to `erc-nicks--highlight'.")
+
+(defun erc-nicks--highlight (nick-object)
+  "Possibly highlight a single nick."
+  (when-let*
+      ((nick-object)
+       (server-user (erc-button--nick-user nick-object))
+       (trimmed (if erc-nicks-ignore-chars
+                    (string-trim-right (erc-server-user-nickname server-user)
+                                       (rx-to-string
+                                        `(: (+ (any ,erc-nicks-ignore-chars))
+                                            eot)))
+                  (erc-server-user-nickname server-user)))
+       ((not (member trimmed erc-nicks-skip-nicks)))
+       (face (or erc-nicks--phony-face
+                 (get-text-property (car (erc-button--nick-bounds nick-object))
+                                    'font-lock-face)))
+       ((not (erc-nicks--skip-p face erc-nicks-skip-faces
+                                erc-nicks--max-skip-search)))
+       ;; Ensure nicks are colored uniquely (per network) by padding
+       ;; from the right, as mentioned above in `erc-nicks--hash'.
+       (key (concat (erc-button--nick-downcased nick-object)
+                    (and-let* ((net (erc-network))) (format "%9s" net))))
+       (out (erc-nicks--get-face trimmed key)))
+    ;; `font-lock-prepend-text-property' could also work if preserving
+    ;; history isn't needed (in which case this var should be nil).
+    (setf (erc-button--nick-erc-button-nickname-face nick-object)
+          (if (or (not erc-nicks-nickname-face)
+                  (eq face erc-nicks-nickname-face))
+              out
+            (cons out (erc-list erc-nicks-nickname-face)))))
+  nick-object)
+
+(define-erc-module nicks nil
+  "Uniquely colorize nicknames in target buffers."
+  ((if erc--target
+       (progn
+         (add-function :filter-return (local 'erc-button--modify-nick-function)
+                       #'erc-nicks--highlight '((depth . 80)))
+         (erc-button--phantom-users-mode +1))
+     (unless erc-button-mode
+       (unless (memq 'button erc-modules)
+         (erc--warn-once-before-connect 'erc-nicks-mode
+           "Enabling default global module `button' needed by local"
+           " module `nicks'. This will impact \C-]all\C-] ERC"
+           " sessions. Add `nicks' to `erc-modules' to avoid this"
+           " warning. See Info:\"(erc) Modules\" for more."))
+       (erc-button-mode +1))
+     (when (equal erc-nicks-bg-color "unspecified-bg")
+       (let ((temp (if (eq (erc-nicks--bg-mode) 'light) "white" "black")))
+         (erc-button--display-error-notice-with-keys
+          "Module `nicks' unable to determine background color.  Setting to \""
+          temp "\" globally.  Please see `erc-nicks-bg-color'.")
+         (custom-set-variables (list 'erc-nicks-bg-color temp))))
+     (setq erc-nicks--face-table (make-hash-table :test #'equal)))
+   (setf (alist-get "Edit face" erc-button--nick-popup-alist nil nil #'equal)
+         #'erc-nicks-customize-face)
+   (advice-add 'widget-create-child-and-convert :filter-args
+               #'erc-nicks--redirect-face-widget-link))
+  ((kill-local-variable 'erc-nicks--face-table)
+   (kill-local-variable 'erc-nicks--bg-mode-value)
+   (kill-local-variable 'erc-nicks--bg-luminance)
+   (kill-local-variable 'erc-nicks--colors-len)
+   (when (fboundp 'erc-button--phantom-users-mode)
+     (erc-button--phantom-users-mode -1))
+   (remove-function (local 'erc-button--modify-nick-function)
+                    #'erc-nicks--highlight)
+   (setf (alist-get "Edit face"
+                    erc-button--nick-popup-alist nil 'remove #'equal)
+         nil))
+  'local)
+
+(defun erc-nicks-customize-face (nick)
+  "Customize or create persistent face for NICK."
+  (interactive (list (or (car (get-text-property (point) 'erc-data))
+                         (completing-read "nick: " (or erc-channel-users
+                                                       erc-server-users)))))
+  (setq nick (erc-downcase (substring-no-properties nick)))
+  (let* ((net (erc-network))
+         (key (concat nick (and net (format "%9s" net))))
+         (old-face (erc-nicks--get-face nick key))
+         (new-face (intern (format "erc-nicks-%s@%s-face" nick net))))
+    (unless (eq new-face old-face)
+      (erc-nicks--revive new-face old-face nick net)
+      (set-face-attribute old-face nil :foreground 'unspecified)
+      (set-face-attribute old-face nil :inherit new-face))
+    (customize-face new-face)))
+
+(provide 'erc-nicks)
+
+;;; erc-nicks.el ends here
diff --git a/lisp/erc/erc.el b/lisp/erc/erc.el
index a1538962602..7b54b5db276 100644
--- a/lisp/erc/erc.el
+++ b/lisp/erc/erc.el
@@ -2017,6 +2017,7 @@ erc-modules
            move-to-prompt)
     (const :tag "netsplit: Detect netsplits" netsplit)
     (const :tag "networks: Provide data about IRC networks" networks)
+    (const :tag "nicks: Uniquely colorize nicknames in target buffers" nicks)
     (const :tag "noncommands: Don't display non-IRC commands after evaluation"
            noncommands)
     (const :tag "notifications: Desktop alerts on PRIVMSG or mentions"
diff --git a/test/lisp/erc/erc-nicks-tests.el b/test/lisp/erc/erc-nicks-tests.el
new file mode 100644
index 00000000000..0d640ad59c3
--- /dev/null
+++ b/test/lisp/erc/erc-nicks-tests.el
@@ -0,0 +1,340 @@
+;;; erc-nicks-tests.el --- Tests for erc-nicks  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+
+;; 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:
+
+;; Unlike most of ERC's tests, the ones in this file can be run
+;; interactively in the same session.
+
+;; TODO:
+;;
+;; * Add mock session (or scenario) with buffer snapshots, like those
+;;   in erc-fill-tests.el.  (Should probably move helpers to a common
+;;   library under ./resources.)
+
+;;; Code:
+
+(require 'ert)
+(require 'erc-nicks)
+
+;; This function replicates the behavior of older "invert" strategy
+;; implementations from EmacsWiki, etc.  The values for the lower and
+;; upper bounds (0.33 and 0.66) are likewise inherited.  See
+;; `erc-nicks--invert-classic--dark' below for one reason its results
+;; may not be plainly obvious.
+(defun erc-nicks-tests--invert-classic (color)
+  (if (pcase (erc-nicks--bg-mode)
+        ('dark (< (erc-nicks--get-luminance color) (/ 1 3.0)))
+        ('light (> (erc-nicks--get-luminance color) (/ 2 3.0))))
+      (list (- 1.0 (nth 0 color)) (- 1.0 (nth 1 color)) (- 1.0 (nth 2 color)))
+    color))
+
+
+(ert-deftest erc-nicks--get-luminance ()
+  (should (eql 0.0 (erc-nicks--get-luminance "black")))
+  (should (eql 1.0 (erc-nicks--get-luminance "white")))
+  (should (eql 21.0 (/ (+ 0.05 1.0) (+ 0.05 0.0))))
+
+  ;; RGB floats from a `display-graphic-p' session.
+  (let ((a (erc-nicks--get-luminance ; #9439ad
+            '(0.5803921568627451 0.2235294117647059 0.6784313725490196)))
+        (b (erc-nicks--get-luminance ; #ae54c7
+            '(0.6823529411764706 0.32941176470588235 0.7803921568627451)))
+        (c (erc-nicks--get-luminance ; #d19ddf
+            '(0.8196078431372549 0.615686274509804 0.8745098039215686)))
+        (d (erc-nicks--get-luminance ; #f5e8f8
+            '(0.9607843137254902 0.9098039215686274 0.9725490196078431))))
+    ;; Low, med, high contrast comparisons against known values from
+    ;; an external source.
+    (should (eql 1.42 (/ (round (* 100 (/ (+ 0.05 b) (+ 0.05 a)))) 100.0)))
+    (should (eql 2.78 (/ (round (* 100 (/ (+ 0.05 c) (+ 0.05 a)))) 100.0)))
+    (should (eql 5.16 (/ (round (* 100 (/ (+ 0.05 d) (+ 0.05 a)))) 100.0)))))
+
+(ert-deftest erc-nicks-invert--classic ()
+  (let ((convert (lambda (n) (apply #'color-rgb-to-hex
+                                    (erc-nicks-tests--invert-classic
+                                     (color-name-to-rgb n))))))
+    (let ((erc-nicks--bg-mode-value 'dark))
+      (should (equal (funcall convert "white") "#ffffffffffff"))
+      (should (equal (funcall convert "black") "#ffffffffffff"))
+      (should (equal (funcall convert "green") "#0000ffff0000")))
+    (let ((erc-nicks--bg-mode-value 'light))
+      (should (equal (funcall convert "white") "#000000000000"))
+      (should (equal (funcall convert "black") "#000000000000"))
+      (should (equal (funcall convert "green") "#ffff0000ffff")))))
+
+(ert-deftest erc-nicks--get-contrast ()
+  (should (= 21.0 (erc-nicks--get-contrast "white" "black")))
+  (should (= 21.0 (erc-nicks--get-contrast "black" "white")))
+  (should (= 1.0 (erc-nicks--get-contrast "black" "black")))
+  (should (= 1.0 (erc-nicks--get-contrast "white" "white"))))
+
+(defun erc-nicks-tests--print-contrast (fn color)
+  (let* ((erc-nicks-color-adjustments (list fn))
+         (result (erc-nicks--reduce color))
+         (start (point)))
+    (insert (format "%16s%-16s%16s%-16s\n"
+                    (concat color "-")
+                    (concat ">" result)
+                    (concat color " ")
+                    (concat " " result)))
+    (put-text-property (+ start 32) (+ start 48) 'face
+                       (list :background color :foreground result))
+    (put-text-property (+ start 48) (+ start 64) 'face
+                       (list :background result :foreground color))
+    result))
+
+(ert-deftest erc-nicks--invert-classic--light ()
+  (let ((erc-nicks--bg-luminance 1.0)
+        (erc-nicks--bg-mode-value 'light)
+        (show (lambda (c) (erc-nicks-tests--print-contrast
+                           #'erc-nicks-tests--invert-classic c))))
+
+    (with-current-buffer (get-buffer-create
+                          "*erc-nicks--invert-classic--light*")
+      (should (equal "#000000000000" (funcall show "white")))
+      (should (equal "#000000000000" (funcall show "black")))
+      (should (equal "#ffff00000000" (funcall show "red")))
+      (should (equal "#ffff0000ffff" (funcall show "green"))) ; magenta
+      (should (equal "#00000000ffff" (funcall show "blue")))
+
+      (unless noninteractive
+        (should (equal "#bbbbbbbbbbbb" (funcall show "#bbbbbbbbbbbb")))
+        (should (equal "#cccccccccccc" (funcall show "#cccccccccccc")))
+        (should (equal "#222122212221" (funcall show "#dddddddddddd")))
+        (should (equal "#111011101110" (funcall show "#eeeeeeeeeeee"))))
+
+      (when noninteractive
+        (kill-buffer)))))
+
+;; This shows that the output can be darker (have less contrast) than
+;; the input.
+(ert-deftest erc-nicks--invert-classic--dark ()
+  (let ((erc-nicks--bg-luminance 0.0)
+        (erc-nicks--bg-mode-value 'dark)
+        (show (lambda (c) (erc-nicks-tests--print-contrast
+                           #'erc-nicks-tests--invert-classic c))))
+
+    (with-current-buffer (get-buffer-create
+                          "*erc-nicks--invert-classic--dark*")
+      (should (equal "#ffffffffffff" (funcall show "white")))
+      (should (equal "#ffffffffffff" (funcall show "black")))
+      (should (equal "#0000ffffffff" (funcall show "red"))) ; cyan
+      (should (equal "#0000ffff0000" (funcall show "green")))
+      (should (equal "#ffffffff0000" (funcall show "blue"))) ; yellow
+
+      (unless noninteractive
+        (should (equal "#aaaaaaaaaaaa" (funcall show "#555555555555")))
+        (should (equal "#999999999999" (funcall show "#666666666666")))
+        (should (equal "#888888888888" (funcall show "#777777777777")))
+        (should (equal "#777777777777" (funcall show "#888888888888")))
+        (should (equal "#666666666666" (funcall show "#999999999999")))
+        (should (equal "#aaaaaaaaaaaa" (funcall show "#aaaaaaaaaaaa"))))
+
+      (when noninteractive
+        (kill-buffer)))))
+
+;; These are the same as the legacy version but work in terms of
+;; contrast ratios.  Converting the original bounds to contrast ratios
+;; (assuming pure white and black backgrounds) gives:
+;;
+;;   min-lum of 0.33 ~~> 1.465
+;;   max-lum of 0.66 ~~> 7.666
+;;
+(ert-deftest erc-nicks-invert--light ()
+  (let ((erc-nicks--bg-luminance 1.0)
+        (erc-nicks--bg-mode-value 'light)
+        (erc-nicks-contrast-range '(1.465))
+        (show (lambda (c) (erc-nicks-tests--print-contrast
+                           #'erc-nicks-invert c))))
+
+    (with-current-buffer (get-buffer-create
+                          "*erc-nicks--invert-classic--light*")
+      (should (equal "#000000000000" (funcall show "white")))
+      (should (equal "#000000000000" (funcall show "black")))
+      (should (equal "#ffff00000000" (funcall show "red")))
+      (should (equal "#ffff0000ffff" (funcall show "green"))) ; magenta
+      (should (equal "#00000000ffff" (funcall show "blue")))
+
+      (unless noninteractive
+        (should (equal "#bbbbbbbbbbbb" (funcall show "#bbbbbbbbbbbb")))
+        (should (equal "#cccccccccccc" (funcall show "#cccccccccccc")))
+        (should (equal "#222122212221" (funcall show "#dddddddddddd")))
+        (should (equal "#111011101110" (funcall show "#eeeeeeeeeeee"))))
+
+      (when noninteractive
+        (kill-buffer)))))
+
+(ert-deftest erc-nicks-invert--dark ()
+  (let ((erc-nicks--bg-luminance 0.0)
+        (erc-nicks--bg-mode-value 'dark)
+        (erc-nicks-contrast-range '(7.666))
+        (show (lambda (c) (erc-nicks-tests--print-contrast
+                           #'erc-nicks-invert c))))
+
+    (with-current-buffer (get-buffer-create "*erc-nicks-invert--dark*")
+      (should (equal "#ffffffffffff" (funcall show "white")))
+      (should (equal "#ffffffffffff" (funcall show "black")))
+      (should (equal "#0000ffffffff" (funcall show "red"))) ; cyan
+      (should (equal "#0000ffff0000" (funcall show "green")))
+      (should (equal "#ffffffff0000" (funcall show "blue"))) ; yellow
+
+      (unless noninteractive
+        (should (equal "#aaaaaaaaaaaa" (funcall show "#555555555555")))
+        (should (equal "#999999999999" (funcall show "#666666666666")))
+        (should (equal "#888888888888" (funcall show "#777777777777")))
+        (should (equal "#888888888888" (funcall show "#888888888888")))
+        (should (equal "#999999999999" (funcall show "#999999999999"))))
+
+      (when noninteractive
+        (kill-buffer)))))
+
+(ert-deftest erc-nicks-add-contrast ()
+  (let ((erc-nicks--bg-luminance 1.0)
+        (erc-nicks--bg-mode-value 'light)
+        (erc-nicks-contrast-range '(3.5))
+        (show (lambda (c) (erc-nicks-tests--print-contrast
+                           #'erc-nicks-add-contrast c))))
+
+    (with-current-buffer (get-buffer-create "*erc-nicks-add-contrast*")
+      (should (equal "#893a893a893a" (funcall show "white")))
+      (should (equal "#893a893a893a" (funcall show "#893a893a893a")))
+      (should (equal "#000000000000" (funcall show "black")))
+      (should (equal "#ffff00000000" (funcall show "red")))
+      (should (equal "#0000a12e0000" (funcall show "green")))
+      (should (equal "#00000000ffff" (funcall show "blue")))
+
+      ;; When the input is already near the desired ratio, the result
+      ;; may not be in bounds, only close.  But the difference is
+      ;; usually imperceptible.
+      (unless noninteractive
+        ;; Well inside (light slate gray)
+        (should (equal "#777788889999" (funcall show "#777788889999")))
+        ;; Slightly outside -> just outside
+        (should (equal "#7c498bd39b5c" (funcall show "#88889999aaaa")))
+        ;; Just outside -> just inside
+        (should (equal "#7bcc8b479ac0" (funcall show "#7c498bd39b5c")))
+        ;; Just inside
+        (should (equal "#7bcc8b479ac0" (funcall show "#7bcc8b479ac0"))))
+
+      (when noninteractive
+        (kill-buffer)))))
+
+(ert-deftest erc-nicks-cap-contrast ()
+  (should (= 12.5 (cdr erc-nicks-contrast-range)))
+  (let ((erc-nicks--bg-luminance 1.0)
+        (erc-nicks--bg-mode-value 'light)
+        (show (lambda (c) (erc-nicks-tests--print-contrast
+                           #'erc-nicks-cap-contrast c))))
+
+    (with-current-buffer (get-buffer-create "*erc-nicks-remove-contrast*")
+      (should (equal (funcall show "black") "#34e534e534e5" )) ; 21.0 -> 12.14
+      (should ; 12.32 -> 12.32 (same)
+       (equal (funcall show "#34e534e534e5") "#34e534e534e5"))
+      (should (equal (funcall show "white") "#ffffffffffff"))
+
+      (unless noninteractive
+        (should (equal (funcall show "DarkRed") "#8b8b00000000"))
+        (should (equal (funcall show "DarkGreen") "#000064640000"))
+        ;; 15.29 -> 12.38
+        (should (equal (funcall show "DarkBlue") "#1cf11cf198b5"))
+
+        ;; 12.50 -> 12.22
+        (should (equal (funcall show "#33e033e033e0") "#34ab34ab34ab"))
+        ;; 12.57 -> 12.28
+        (should (equal (funcall show "#338033803380") "#344c344c344c"))
+        ;; 12.67 -> 12.37
+        (should (equal (funcall show "#330033003300") "#33cc33cc33cc")))
+
+      (when noninteractive
+        (kill-buffer)))))
+
+;; Here is an example of how filters can steer us wrong (don't always
+;; DTRT).  Two keys with similar names hash to very different values:
+;;
+;;   1) "awbLibera.Chat" -> #x1e3b5ca4edbc ; deep blue
+;;   2) "twbLibera.Chat" -> #xdeb4c26934af ; yellow/orange
+;;
+;; But on a dark bg, (1) falls below `erc-nicks-invert's min threshold
+;; and thus gets treated, becoming #xe1c4a35b1243, which is quite
+;; close to and thus easily confused with (2).
+
+(ert-deftest erc-nicks--hash ()
+  (with-current-buffer (get-buffer-create "*erc-nicks--hash*")
+    ;; Here, we're just using `erc-nicks-tests--show-contrast' for show.
+    (let ((show (lambda (c) (erc-nicks-tests--print-contrast #'identity c))))
+
+      ;; Similar nicks yielding similar colors is likely undesirable.
+      (should (= (erc-nicks--hash "00000000") #xe4deaa6df385))
+      (should (= (erc-nicks--hash "00000001") #xe4deaa6df386))
+      (funcall show "#e4deaa6df385")
+      (funcall show "#e4deaa6df386")
+
+      ;; So we currently pad from the right to avoid this.
+      (should (= (erc-nicks--hash "0Libera.Chat") #x32fdc0d63a92))
+      (should (= (erc-nicks--hash "1Libera.Chat") #xc2c4f1c997f3))
+      (funcall show "#32fdc0d63a92")
+      (funcall show "#c2c4f1c997f3")
+
+      (should (= (erc-nicks--hash "0       OFTC") #x6805b7521261))
+      (should (= (erc-nicks--hash "1       OFTC") #xf7cce8456fc2))
+      (funcall show "#6805b7521261")
+      (funcall show "#f7cce8456fc2"))
+
+    (when noninteractive
+      (kill-buffer))))
+
+(ert-deftest erc-nicks--skip-p ()
+  ;; Baseline
+  (should-not (erc-nicks--skip-p 'bold nil 10000000))
+  (should-not (erc-nicks--skip-p '(bold) nil 10000000))
+  (should-not (erc-nicks--skip-p nil '(bold) 10000000))
+  (should-not (erc-nicks--skip-p 'bold '(bold) 0))
+  (should-not (erc-nicks--skip-p '(bold) '(bold) 0))
+  (should-not (erc-nicks--skip-p 'bold '(foo bold) 0))
+  (should-not (erc-nicks--skip-p '((:inherit bold)) '(bold) 1))
+  (should (erc-nicks--skip-p 'bold '(bold) 1))
+  (should (erc-nicks--skip-p 'bold '(fake bold) 1))
+  (should (erc-nicks--skip-p 'bold '(foo bar bold) 1))
+  (should (erc-nicks--skip-p '(bold) '(bold) 1))
+  (should (erc-nicks--skip-p '((bold)) '(bold) 1))
+  (should (erc-nicks--skip-p '((((bold)))) '(bold) 1))
+  (should (erc-nicks--skip-p '(bold) '(foo bold) 1))
+  (should (erc-nicks--skip-p '(:inherit bold) '((:inherit bold)) 1))
+  (should (erc-nicks--skip-p '((:inherit bold)) '((:inherit bold)) 1))
+  (should (erc-nicks--skip-p '(((:inherit bold))) '((:inherit bold)) 1))
+
+  ;; Composed
+  (should-not (erc-nicks--skip-p '(italic bold) '(bold) 1))
+  (should-not (erc-nicks--skip-p '((italic) bold) '(bold) 1))
+  (should-not (erc-nicks--skip-p '(italic (bold)) '(bold) 1))
+  (should (erc-nicks--skip-p '(italic bold) '(bold) 2))
+  (should (erc-nicks--skip-p '((italic) bold) '(bold) 2))
+  (should (erc-nicks--skip-p '(italic (bold)) '(bold) 2))
+
+  (should-not (erc-nicks--skip-p '(italic default bold) '(bold) 2))
+  (should-not (erc-nicks--skip-p '((default italic) bold) '(bold) 2))
+  (should-not (erc-nicks--skip-p '(italic (default bold)) '(bold) 2))
+  (should-not (erc-nicks--skip-p '((default italic) (bold shadow)) '(bold) 2))
+  (should (erc-nicks--skip-p '((default italic) bold) '(bold) 3))
+  (should (erc-nicks--skip-p '(italic (default bold)) '(bold) 3))
+  (should (erc-nicks--skip-p '((default italic) (bold shadow)) '(bold) 3))
+  (should (erc-nicks--skip-p '(italic (default (bold shadow))) '(bold) 3)))
+
+;;; erc-nicks-tests.el ends here
diff --git a/test/lisp/erc/erc-tests.el b/test/lisp/erc/erc-tests.el
index f3489a16386..ebe49bcece2 100644
--- a/test/lisp/erc/erc-tests.el
+++ b/test/lisp/erc/erc-tests.el
@@ -2057,7 +2057,7 @@ erc-handle-irc-url
 (defconst erc-tests--modules
   '( autoaway autojoin button capab-identify completion dcc fill identd
      imenu irccontrols keep-place list log match menu move-to-prompt netsplit
-     networks noncommands notifications notify page readonly
+     networks nicks noncommands notifications notify page readonly
      replace ring sasl scrolltobottom services smiley sound
      spelling stamp track truncate unmorse xdcc))
 
-- 
2.40.1