vim-patch:9.1.0694: matchparen is slow on a long line (#30134)

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: vim/vim#15568

https://github.com/vim/vim/commit/81e7513c86459c40676bd983f73c2722096d67a9

3 files changed, 107 insertions, 10 deletions

diff --git a/runtime/plugin/matchparen.vim b/runtime/plugin/matchparen.vim
index 6c061c9fb8..2899612dce 100644
--- a/runtime/plugin/matchparen.vim
+++ b/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\(.\=\)')
-  if empty(matches)
-    let [c_before, c] = ['', '']
-  else
-    let [c_before, c] = matches[1:2]
-  endif
+  let c_before = text->strpart(0, c_col - 1)->slice(-1)
+  let c = text->strpart(c_col - 1)->slice(0, 1)
   let plist = split(&matchpairs, '.\zs[:,]')
   let i = index(plist, c)
   if i < 0
diff --git a/test/functional/legacy/matchparen_spec.lua b/test/functional/legacy/matchparen_spec.lua
index 3841761515..df0d80f0ab 100644
--- a/test/functional/legacy/matchparen_spec.lua
+++ b/test/functional/legacy/matchparen_spec.lua
@@ -120,8 +120,7 @@ describe('matchparen', function()
     ]])
 
     feed('i<C-X><C-N><C-N>')
-    screen:expect {
-      grid = [[
+    screen:expect([[
       aa                            |
       aaa                           |
       aaaa                          |
@@ -131,7 +130,79 @@ describe('matchparen', function()
       {4: aaaa           }{1:              }|
       {1:~                             }|
       {5:-- }{6:match 2 of 3}               |
-    ]],
-    }
+    ]])
+  end)
+
+  -- oldtest: Test_matchparen_mbyte()
+  it("works with multibyte chars in 'matchpairs'", function()
+    local screen = Screen.new(30, 10)
+    screen:set_default_attr_ids({
+      [0] = { bold = true, foreground = Screen.colors.Blue },
+      [1] = { background = Screen.colors.Cyan },
+      [2] = { bold = true },
+    })
+    screen:attach()
+
+    exec([[
+      source $VIMRUNTIME/plugin/matchparen.vim
+      call setline(1, ['aaaaaaaa（', 'bbbb）cc'])
+      set matchpairs+=（:）
+    ]])
+
+    screen:expect([[
+      ^aaaaaaaa（                    |
+      bbbb）cc                      |
+      {0:~                             }|*7
+                                    |
+    ]])
+    feed('$')
+    screen:expect([[
+      aaaaaaaa{1:^（}                    |
+      bbbb{1:）}cc                      |
+      {0:~                             }|*7
+                                    |
+    ]])
+    feed('j')
+    screen:expect([[
+      aaaaaaaa（                    |
+      bbbb）c^c                      |
+      {0:~                             }|*7
+                                    |
+    ]])
+    feed('2h')
+    screen:expect([[
+      aaaaaaaa{1:（}                    |
+      bbbb{1:^）}cc                      |
+      {0:~                             }|*7
+                                    |
+    ]])
+    feed('0')
+    screen:expect([[
+      aaaaaaaa（                    |
+      ^bbbb）cc                      |
+      {0:~                             }|*7
+                                    |
+    ]])
+    feed('kA')
+    screen:expect([[
+      aaaaaaaa{1:（}^                    |
+      bbbb{1:）}cc                      |
+      {0:~                             }|*7
+      {2:-- INSERT --}                  |
+    ]])
+    feed('<Down>')
+    screen:expect([[
+      aaaaaaaa（                    |
+      bbbb）cc^                      |
+      {0:~                             }|*7
+      {2:-- INSERT --}                  |
+    ]])
+    feed('<C-W>')
+    screen:expect([[
+      aaaaaaaa{1:（}                    |
+      bbbb{1:）}^                        |
+      {0:~                             }|*7
+      {2:-- INSERT --}                  |
+    ]])
   end)
 end)
diff --git a/test/old/testdir/test_matchparen.vim b/test/old/testdir/test_matchparen.vim
index ab425b046a..7d80e43046 100644
--- a/test/old/testdir/test_matchparen.vim
+++ b/test/old/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