diff: Fix inline word diff to treat non-alpha multibyte chars as non-word

Previously inline word diff simply used Vim's definition of keyword to
determine what is a word, which leads to multi-byte character classes
such as emojis and CJK (Chinese/Japanese/Korean) characters all
classifying as word characters, leading to entire sentences being
grouped as a single word which does not provide meaningful information
in a diff highlight.

Fix this by treating all non-alphanumeric characters (with class number
above 2) as non-word characters, as there is usually no benefit in using
word diff on them. These include CJK characters, emojis, and also
subscript/superscript numbers. Meanwhile, multi-byte characters like
Cyrillic and Greek letters will still continue to considered as words.

Note that this is slightly inconsistent with how words are defined
elsewhere, as Vim usually considers any character with class >=2 to be
a "word".

Related: vim#16881 (diff inline highlight)

Author: ychin

runtime/doc/options.txt

@@ -2989,7 +2989,10 @@ A jump table for the options with a short description can be found at |Q_op|.
 					difference.
 				word    Use internal diff to perform a
 					|word|-wise diff and highlight the
-					difference.
+					difference.  Non-alphanumeric
+					multi-byte characters such as emoji
+					and CJK characters are considered
+					individual words.
 
 		internal	Use the internal diff library.  This is
 				ignored when 'diffexpr' is set.  *E960*

src/diff.c

@@ -3309,10 +3309,17 @@ diff_find_change_inline_diff(
 	    char_u *s;
 	    for (s = curline; *s != NUL;)
 	    {
-		// Always use the first buffer's 'iskeyword' to have a consistent diff
 		int new_in_keyword = FALSE;
 		if (diff_flags & DIFF_INLINE_WORD)
-		    new_in_keyword = vim_iswordp_buf(s, curtab->tp_diffbuf[file1_idx]);
+		{
+		    // Always use the first buffer's 'iskeyword' to have a
+		    // consistent diff.
+		    // For multibyte chars, only treat alphanumeric chars
+		    // (class 2) as "word", as other classes such as emojis and
+		    // CJK ideographs do not usually benefit from word diff as
+		    // Vim doesn't have a good way to segment them.
+		    new_in_keyword = (mb_get_class_buf(s, curtab->tp_diffbuf[file1_idx]) == 2);
+		}
 		if (in_keyword && !new_in_keyword)
 		{
 		    ga_append(curstr, NL);

src/mbyte.c

@@ -828,8 +828,8 @@ remove_bom(char_u *s)
  * Get class of pointer:
  * 0 for blank or NUL
  * 1 for punctuation
- * 2 for an (ASCII) word character
- * >2 for other word characters
+ * 2 for an alphanumeric word character
+ * >2 for other word characters, including CJK and emoji
  */
     int
 mb_get_class(char_u *p)

src/testdir/dumps/Test_diff_inline_word_03.dump

@@ -0,0 +1,20 @@
+| +0#0000e05#a8a8a8255@1|🚀*0#0000000#ffd7ff255|⛵️*2&#ff404010|一*0&#ffd7ff255|二|三*2&#ff404010|ひ*0&#ffd7ff255|ら|が*0࿈ff13|な*0&#ffd7ff255|Δ+2&#ff404010|έ|λ|τ|α| +0&#ffd7ff255|Δ+2&#ff404010|e|l|t|a| +0&#ffd7ff255|f|o@1|b|a||+1&#ffffff0| +0#0000e05#a8a8a8255@1|🚀*0#0000000#ffd7ff255|🛸*2&#ff404010|一*0&#ffd7ff255|二|四*2&#ff404010|ひ*0&#ffd7ff255|ら|な|δ+2&#ff404010|έ|λ|τ|α| +0&#ffd7ff255|δ+2&#ff404010|e|l|t|a| +0&#ffd7ff255|f|o@1|b|a|r| 
+|~+0#4040ff13#ffffff0| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|~| @35||+1#0000000&|~+0#4040ff13&| @35
+|X+3#0000000&|d|i|f|i|l|e|1| @10|1|,|1| @11|A|l@1| |X+1&&|d|i|f|i|l|e|2| @10|1|,|1| @11|A|l@1
+|:+0&&> @73

src/testdir/test_diffmode.vim

@@ -2444,6 +2444,11 @@ func Test_diff_inline()
 
   call term_sendkeys(buf, ":windo set iskeyword&\:1wincmd w\")
 
+  " word diff: test handling of multi-byte characters. Only alphanumeric chars
+  " (e.g. Greek alphabet, but not CJK/emoji) count as words.
+  call WriteDiffFiles(buf, ["🚀⛵️一二三ひらがなΔέλτα Δelta foobar"], ["🚀🛸一二四ひらなδέλτα δelta foobar"])
+  call VerifyInternal(buf, "Test_diff_inline_word_03", " diffopt+=inline:word")
+
   " char diff: should slide highlight to whitespace boundary if possible for
   " better readability (by using forced indent-heuristics). A wrong result
   " would be if the highlight is "Bar, prefix". It should be "prefixBar, "