Make color-distance symmetric and more accurate

* src/xfaces.c (color_distance): Don't throw away the low 8 bits of
the colours, and make the function symmetric (bug41544)
(Fcolor_distance): Add caution about this not being a true metric.
* test/src/xfaces-tests.el: New file.

1 files changed, 27 insertions, 0 deletions

diff --git a/test/src/xfaces-tests.el b/test/src/xfaces-tests.el
new file mode 100644
index 00000000000..f08a87a518a
--- /dev/null
+++ b/test/src/xfaces-tests.el
@@ -0,0 +1,27 @@
+;;; xfaces-tests.el --- tests for xfaces.c           -*- lexical-binding: t -*-
+
+;; Copyright (C) 2020 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/>.
+
+(require 'ert)
+
+(ert-deftest xfaces-color-distance ()
+  ;; Check symmetry (bug#51455).
+  (should (equal (color-distance "#222222" "#ffffff")
+                 (color-distance "#ffffff" "#222222"))))
+
+(provide 'xfaces-tests)