function on_ai_command_completed(event)
  log(serpent.block(event))

  if event.result == defines.behavior_result.success then
    local c = storage.player.character
    log("attack: " .. c.position.x .. "," ..  c.position.y)
    storage.group.set_command{
      type = defines.command.attack,
      target = c
    }
  end
end

function on_player_joined_game(event)
  local player = game.get_player(event.player_index)
  if not player then return end
  storage.player = player

  local surface = player.surface

  local position = {
    x = 0,
    y = -40
  }

  local biter = surface.create_entity{
    name = "small-biter",
    position = position,
  }

  local group = surface.create_unit_group{
    position = position,
  }
  group.add_member(biter)
  storage.group = group

  local destination = {
    x = 0,
    y = -70
  }

  group.set_command{
    type = defines.command.go_to_location,
    destination = destination,
    radius = 0
  }
end

script.on_event(defines.events.on_ai_command_completed, on_ai_command_completed)
script.on_event(defines.events.on_player_joined_game, on_player_joined_game)