function s:CompareNumeric(a, b)
  return a:a > a:b ? 1 : -1
endfunction

function Count() abort

  let spaces = {
        \ 'hist': {},
        \ 'total': 0,
        \}
  let tabs = 0
  let total = max([line('$'), 1024])

  for lnum in range(1, total)
    let line = getline(lnum)
    let tabs += matchstr(line, '^\t*') != ''
    let indent = strlen(matchstr(line, '^ *'))
    if indent == 0
      continue
    endif
    if !has_key(spaces['hist'], indent)
      let spaces['hist'][indent] = 0
    endif
    let spaces['hist'][indent] += 1
    let spaces['total'] += 1
  endfor

  if &expandtab == (spaces['total'] > tabs)
    return
  endif

  if &expandtab
    setlocal noexpandtab softtabstop=0
    let &l:shiftwidth = &tabstop
  else
    setlocal expandtab
    let shiftwidth = 0
    let indents = keys(spaces['hist'])
    call map(indents, 'str2nr(v:val)')
    call sort(indents, 's:CompareNumeric')
    for shiftwidth in indents
      if spaces['hist'][shiftwidth] * 100 / spaces['total'] >= 5
        break
      endif
    endfor
    let &l:shiftwidth = shiftwidth
    let &l:softtabstop = shiftwidth
  endif

  let undo_indent = 'setlocal expandtab? shiftwidth? softtabstop?'
  if exists('b:undo_indent')
    let b:undo_indent .= '|'.undo_indent
  else
    let b:undo_indent = undo_indent
  endif

endfunction

autocmd FileType * call Count()