minetest.register_tool("ham_radio:handheld_receiver", {
  description = "Handheld Radio Receiver",
  wield_image = "ham_radio_receiver_handheld.png",
  inventory_image = "ham_radio_receiver_handheld.png",
  groups = { disable_repair = 1 },
  -- left click - change frequency
  on_use = function(itemstack, user, pointed_thing)
    local meta = itemstack:get_meta()
    local frequency = meta:get_string("frequency")
    minetest.show_formspec(user:get_player_name(), "ham_radio:configure_handheld_receiver",
      table.concat({
        "size[3,4]",
        "image[1,0;1,1;ham_radio_receiver_handheld.png]",
        "field[0.25,2;3,1;frequency;Frequency;",tostring(frequency),"]",
        "tooltip[frequency;Integer number ",
          ham_radio.settings.frequency.min,"-",
          ham_radio.settings.frequency.max, "]",
        "button_exit[0,3.5;3,1;;Done]"
      },'')
    )
    return itemstack
  end,
  -- right click - RDS on/off
  on_secondary_use = function(itemstack, user, pointed_thing)
    local meta = itemstack:get_meta()
    local is_rds_disabled = meta:get_string("rds_disabled")
    if is_rds_disabled == "" then
      meta:set_string("rds_disabled", "true")
    else
      meta:set_string("rds_disabled", "")
    end
    return itemstack
  end
})

minetest.register_on_player_receive_fields(function(player, formname, fields)
  if formname ~= "ham_radio:configure_handheld_receiver" or not minetest.is_player(player) then
    return false
  end
  if fields.frequency == nil then
    -- form is not sent
    return
  end
  local is_frequency_valid = ham_radio.validate_frequency(fields.frequency, true)
  if is_frequency_valid.result == false then
    ham_radio.errormsg(player, is_frequency_valid.message)
    return false
  end
  local item = player:get_wielded_item()
  local meta = item:get_meta()
  meta:set_string("frequency", fields.frequency)
  -- play radio sound
  ham_radio.play_tuning_sound(player)
   -- replace wielded item with new metadata
  player:set_wielded_item(item)
   -- reset rds messages
  ham_radio.player_rds[player:get_player_name()] = nil
  return true
end)


function ham_radio:locate_transmitter(player, transmitter_pos)
  local player_pos = player:get_pos()
  local player_look_vector = player:get_look_dir()
  local player_direction = vector.add(player_pos, player_look_vector)

  local coeff = 0.9
  local distance_to_target = 0

  local distance = vector.distance(player_pos, transmitter_pos)
  if distance < 3 then
    distance_to_target = 100
    coeff = 0.99
  else
    distance_to_target = -0.0000000001*math.pow(distance,3)+0.00000145*math.pow(distance,2)-0.03*distance+100
    if distance_to_target < 3 then
      distance_to_target = 3
    end
  end

  local distance2 = vector.distance(player_direction, transmitter_pos)
  local signal_power = distance - distance2;

  -- 0-100
  return distance_to_target * coeff + distance_to_target * (1 - coeff) * signal_power;
end