BizHawk - return the number of controllers

-- For BizHawk <https://code.google.com/p/bizhawk/>

--- Return the number of controllers.
-- @return Number of controllers. (nil if no controller number supported)
local function joypad_getcount()
  local keys = joypad.get()

  local count = 0
  for key, value in pairs(keys) do
    controller = key:match("^P(%d) ")
    if controller then
      controller = tonumber(controller)
      if count < controller then
        count = controller
      end
    end
  end

  if count ~= 0 then
    return count
  else
    return nil
  end
end