Add two optional arguments to 'string-width'

* src/character.c (Fstring_width, lisp_string_width): Accept two
optional arguments FROM and TO, to indicate the substring to be
considered.
(Fstring_width): Add caveats in the doc string about display
features ignored by the function.  (Bug#47712)
* src/character.h (lisp_string_width): Update prototype.
* src/editfns.c (styled_format): Adjust call of lisp_string_width
to its changed signature.

* test/src/character-tests.el (character-test-string-width): New
file with tests for 'string-width'.

* doc/lispref/display.texi (Size of Displayed Text): Document
caveats of using 'string-width'.

* etc/NEWS: Announce the change.

1 files changed, 45 insertions, 0 deletions

diff --git a/test/src/character-tests.el b/test/src/character-tests.el
new file mode 100644
index 00000000000..10fc4dbf353
--- /dev/null
+++ b/test/src/character-tests.el
@@ -0,0 +1,45 @@
+;;; character-tests.el -- tests for character.c  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2021 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/>.
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest character-test-string-width ()
+  "Test `string-width' with and without compositions."
+  (should (= (string-width "1234") 4))
+  (should (= (string-width "12\t34") (+ 4 tab-width)))
+  (should (= (string-width "áëòç") 4))
+  (should (= (string-width "áëòç") 4))
+  (should (= (string-width "הַרְבֵּה אַהֲבָה") 9))
+  (should (= (string-width "1234" 1 3) 2))
+  (should (= (string-width "1234" nil -1) 3))
+  (should (= (string-width "1234" 2) 2))
+  (should-error (string-width "1234" nil 5))
+  (should-error (string-width "1234" -5))
+  (should (= (string-width "12\t34") (+ 4 tab-width)))
+  (should (= (string-width "1234\t56") (+ 6 tab-width)))
+  (should (= (string-width "áëòç") 4))
+  (should (= (string-width "áëòç" nil 3) 3))
+  (should (= (string-width "áëòç" 1 3) 2))
+  (should (= (string-width "áëòç" nil 2) 1))
+  (should (= (string-width "áëòç" nil 3) 2))
+  (should (= (string-width "áëòç" nil 4) 2))
+  (should (= (string-width "הַרְבֵּה אַהֲבָה") 9))
+  (should (= (string-width "הַרְבֵּה אַהֲבָה" nil 8) 4)))