patch 9.1.0694: matchparen is slow on a long line

Problem:  The matchparen plugin is slow on a long line.
Solution: Don't use a regexp to get char at and before cursor.
          (zeertzjq)

Example:

```vim
  call setline(1, repeat(' foobar', 100000))
  runtime plugin/matchparen.vim
  normal! $hhhhhhhh
```

closes: #15568

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/plugin/matchparen.vim

@@ -60,12 +60,8 @@ func s:Highlight_Matching_Pair()
   let before = 0
 
   let text = getline(c_lnum)
-  let matches = matchlist(text, '\(.\)\=\%'.c_col.'c\(.\=\)')
+  let c_before = text->strpart(0, c_col - 1)->slice(-1)
-  if empty(matches)
+  let c = text->strpart(c_col - 1)->slice(0, 1)
-    let [c_before, c] = ['', '']
-  else
-    let [c_before, c] = matches[1:2]
-  endif
   let plist = split(&matchpairs, '.\zs[:,]')
   let i = index(plist, c)
   if i < 0

src/testdir/dumps/Test_matchparen_mbyte_1.dump

@@ -0,0 +1,10 @@
+>a+0&#ffffff0@7|（*&| +&@64
+|b@3|）*&|c+&@1| @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|1|,|1| @10|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_2.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7>（*0&#40ffff15| +0&#ffffff0@64
+|b@3|）*0&#40ffff15|c+0&#ffffff0@1| @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|1|,|9| @10|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_3.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7|（*&| +&@64
+|b@3|）*&|c+&>c| @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|2|,|9|-|8| @8|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_4.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7|（*0&#40ffff15| +0&#ffffff0@64
+|b@3>）*0&#40ffff15|c+0&#ffffff0@1| @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|2|,|5| @10|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_5.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7|（*&| +&@64
+>b@3|）*&|c+&@1| @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@56|2|,|1| @10|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_6.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7|（*0&#40ffff15> +0&#ffffff0@64
+|b@3|）*0&#40ffff15|c+0&#ffffff0@1| @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|1|,|1|2|-|1@1| @6|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_7.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7|（*&| +&@64
+|b@3|）*&|c+&@1> @66
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|2|,|1|0|-|9| @7|A|l@1| 

src/testdir/dumps/Test_matchparen_mbyte_8.dump

@@ -0,0 +1,10 @@
+|a+0&#ffffff0@7|（*0&#40ffff15| +0&#ffffff0@64
+|b@3|）*0&#40ffff15> +0&#ffffff0@68
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+|-+2#0000000&@1| |I|N|S|E|R|T| |-@1| +0&&@44|2|,|8|-|7| @8|A|l@1| 

src/testdir/test_matchparen.vim

@@ -108,5 +108,35 @@ func Test_matchparen_pum_clear()
   call StopVimInTerminal(buf)
 endfunc
 
+" Test that matchparen works with multibyte chars in 'matchpairs'
+func Test_matchparen_mbyte()
+  CheckScreendump
+
+  let lines =<< trim END
+    source $VIMRUNTIME/plugin/matchparen.vim
+    call setline(1, ['aaaaaaaa（', 'bbbb）cc'])
+    set matchpairs+=（:）
+  END
+
+  call writefile(lines, 'XmatchparenMbyte', 'D')
+  let buf = RunVimInTerminal('-S XmatchparenMbyte', #{rows: 10})
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_1', {})
+  call term_sendkeys(buf, "$")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_2', {})
+  call term_sendkeys(buf, "j")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_3', {})
+  call term_sendkeys(buf, "2h")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_4', {})
+  call term_sendkeys(buf, "0")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_5', {})
+  call term_sendkeys(buf, "kA")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_6', {})
+  call term_sendkeys(buf, "\<Down>")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_7', {})
+  call term_sendkeys(buf, "\<C-W>")
+  call VerifyScreenDump(buf, 'Test_matchparen_mbyte_8', {})
+
+  call StopVimInTerminal(buf)
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    694,
 /**/
     693,
 /**/