How to test if the current file is in some directory?

Question

I want to have a piece of vimscript (a custom formatter) only apply if the current file is inside some directory. How can I tell whether the active buffer is within a particular directory?

Answer 1

  fu! StartsWith(longer, shorter) abort
    return a:longer[0:len(a:shorter)-1] ==# a:shorter
  endfunction

  fu! FileIsInDirectory(directory) abort
    let l:fname = expand('%:p')
    return StartsWith(l:fname, a:directory)
  endfunction

  function YourFunction() abort
    if !FileIsInDirectory(expand("$HOME") . "/project/dir/")
      return
    endif
    (...)

I believe you can also put a gitignored / committed .vim/something.vim inside the project directory, and it will only apply for buffers in that directory, but I'm not sure how it works.