fix(filetype): only check first 100 and last line of buffer (#19819)

fix(filetype): only pass first 100 and last lines to contents check

sufficient for current content checks and avoids performance issues for
buffers with a large number of lines

fixes #19817

1 files changed, 9 insertions, 1 deletions

diff --git a/runtime/lua/vim/filetype.lua b/runtime/lua/vim/filetype.lua
index 872106b20d..99c98764dd 100644
--- a/runtime/lua/vim/filetype.lua
+++ b/runtime/lua/vim/filetype.lua
@@ -2489,7 +2489,15 @@ function M.match(args)
 
   -- Finally, check file contents
   if contents or bufnr then
-    contents = contents or M.getlines(bufnr)
+    if contents == nil then
+      if api.nvim_buf_line_count(bufnr) > 101 then
+        -- only need first 100 and last line for current checks
+        contents = M.getlines(bufnr, 1, 100)
+        contents[#contents + 1] = M.getlines(bufnr, -1)
+      else
+        contents = M.getlines(bufnr)
+      end
+    end
     -- If name is nil, catch any errors from the contents filetype detection function.
     -- If the function tries to use the filename that is nil then it will fail,
     -- but this enables checks which do not need a filename to still work.