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f61efe3fe7
Problem: calling `vim.filetype.match()` has performance bottleneck in
that it has to match a lot of Lua patterns against several versions of
input file name. This might be the problem if users need to call it
synchronously a lot of times.
Solution: add "parent pattern pre-matching" which can be used to quickly
reject several potential pattern matches at (usually rare) cost of
adding time for one extra Lua pattern match.
"Parent pattern" is a manually added/tracked grouping of filetype
patterns which should have two properties:
- Match at least the same set of strings as its filetype patterns.
But not too much more.
- Be fast to match.
For them to be effective, group should consist from at least three
filetype patterns.
Example: for a filetpye pattern ".*/etc/a2ps/.*%.cfg", both "/etc/"
and "%.cfg" are good parent patterns (prefer the one which can group
more filetype patterns).
After this commit, `vim.filetype.match()` on most inputs runs ~3.4
times faster (while some inputs may see less impact if they match
many parent patterns).