package Radar is

   type Friend_Or_Foe is (Friend, Foe, Unknown);
   type Class_Of_Ship is
     (Not_A_Ship, Coastal_Patrol, Fast_Attack_Craft, Corvette, Frigate,
      Destroyer, Cruiser, Pocket_Battleship, Battlecruiser, Helicopter_Carrier,
      Battleship, Dreadnought, Aircraft_Carrier, Supercarrier);
   type Kilometer is new Natural;

   --  Every ship detected by the radar is assigned a unique identifier that
   --  allows tracking it until it espaces the radar's scope.
   type Ship is record
      Quality   : Friend_Or_Foe := Unknown;
      Category  : Class_Of_Ship := Not_A_Ship;
      Distance  : Kilometer     := 0;
      Unique_ID : Natural       := 0;
   end record;

   function "=" (X, Y : Ship) return Boolean;

   type Ship_Array is array (Positive range <>) of Ship;

   --  Detect when too many ships are sailing in the combat zone, which makes
   --  it very likely to be hit by enemy fire or friendly fire. This triggers
   --  a different combat mode where the ship principally attempts to escape
   --  the crowded zone.
   Saturated_Combat_Zone : Boolean := False;

   --  Track ships closer than this distance
   Short_Distance : constant Kilometer := 20;

   --  Short list of nearest ships that should be dealt with higher priority,
   --  whether it is to engage combat for enemies or to agree on maneuvers
   --  for friends.
   protected type Short_List (Max_Tracked_Ships : Positive) is
      --  Add a ship when entering the short range
      procedure Add_Ship (E : Ship);
      --  Remove a ship when leaving the short range
      procedure Remove_Ship (E : Ship);
      --  Return the biggest ship in the short range
      function Biggest_Ship return Ship;
   private
      Num_Tracked_Ships   : Natural := 0;
      Num_Untracked_Ships : Natural := 0;
      --  Only tracked ships are included in Ships
      Ships               : Ship_Array (1 .. Max_Tracked_Ships);
   end Short_List;

   Friends : Short_List (50);  --  50 friendly ships tracked for maneuvers
   Foes    : Short_List (18);  --  18 enemy ships tracked (# of weapon systems)

   --  Track a ship entering and leaving the short range
   task type Track_Ship is
      entry New_Ship (E : Ship);
      entry Update_Position (Distance : Kilometer);
   end Track_Ship;

end Radar;

package body Radar is

   function "=" (X, Y : Ship) return Boolean is
   begin
      return X.Unique_ID = Y.Unique_ID;
   end "=";

   protected body Short_List is

      procedure Add_Ship (E : Ship) is
      begin
         if Num_Tracked_Ships < Max_Tracked_Ships then
            Ships (Num_Tracked_Ships) := E;
            Num_Tracked_Ships         := Num_Tracked_Ships + 1;
         else
            Num_Untracked_Ships       := Num_Untracked_Ships + 1;
            Saturated_Combat_Zone     := True;
         end if;
      end Add_Ship;

      procedure Remove_Ship (E : Ship) is
      begin
         for J in Ships'First .. Num_Tracked_Ships loop
            if Ships (J) = E then
               --  Ship E was found. First remove it.
               for K in J + 1 .. Num_Tracked_Ships loop
                  Ships (K - 1) := Ships (K);
               end loop;
               --  Then update the number of tracked ships and return.
               Num_Tracked_Ships := Num_Tracked_Ships - 1;
               return;
            end if;
         end loop;

         --  Ship E was not found, so it is an untracked ship.
         --  Update the number of untracked ships and possibly leave the mode
         --  where combat zone is saturated.
         if Num_Untracked_Ships = 0 then
            Saturated_Combat_Zone := False;
         end if;
         Num_Untracked_Ships := Num_Untracked_Ships - 1;
      end Remove_Ship;

      function Biggest_Ship return Ship is
         Current : Class_Of_Ship := Not_A_Ship;
         Index   : Integer;
      begin
         for J in Ships'First .. Num_Tracked_Ships loop
            if Current < Ships (J).Category then
               --  Found a bigger ship. Update Current and Index.
               Current := Ships (J).Category;
               Index   := J;
            end if;
         end loop;

         return Ships (Index);
      end Biggest_Ship;

   end Short_List;

   task body Track_Ship is
      Current : Ship;
   begin
      --  Define the ship being tracked.
      accept New_Ship (E : Ship)
      do
         Current := E;
      end New_Ship;

      loop
         --  Following a movement of the ship entering or leaving the short
         --  range, add or remove it from the corresponding short list.
         accept Update_Position (Distance : Kilometer)
         do
            Current.Distance := Distance;
            if Distance < Short_Distance then
               case Current.Quality is
                  when Friend =>
                     Friends.Add_Ship (Current);
                  when Foe =>
                     Foes.Add_Ship (Current);
                  when Unknown =>
                     null;
               end case;
            else
               case Current.Quality is
                  when Friend =>
                     Friends.Remove_Ship (Current);
                  when Foe =>
                     Foes.Remove_Ship (Current);
                  when Unknown =>
                     null;
               end case;
            end if;
         end;
      end loop;
   end Track_Ship;

end Radar;