Added option to shoot down hanggliders flying in restricted airspace

Added a /area_flak command to declare areas as closed airspace. Any
hanggliders attempting to fly in such an area will get shot down
within two seconds, which will lead to a massive loss of health to the
occupant (death if flying higher than 6 meters) and the loss of the
hangglider. The closed airspace behaviour depends on the areas mod.

LICENSE.txt

@@ -50,3 +50,4 @@ Textures:
 
 Models and sounds: 
 	CC BY-SA (4.0) by Piezo_.
+	hangglider_flak_shot: tnt_explode sound from minetest_game (CC BY-SA 3.0)

depends.txt

@@ -1,3 +1,4 @@
 default
 wool
 minetest_systemd?
+areas?

init.lua

@@ -39,6 +39,13 @@
 -- Removed airbreak penalty, as any 'advantage' seems minimal after new adjustments
 
 
+-- Modifications by gpcf
+-- 2018-12-09
+-- get shot down while flying over protected areas marked as no-fly-zones (flak, from German Flugabwehrkanone)
+--  set these areas with the /area_flak command
+
+
+
 local HUD_Overlay = true --show glider struts as overlay on HUD
 local debug = false --show debug info in top-center of hud
 local moveModelUp = false
@@ -87,6 +94,31 @@ minetest.register_entity("hangglider:airstopper", { --A one-instant entity that
 	end
 })
 
+if areas then
+	hangglider.flak = true
+	-- chat command definition essentially copied from areas mod.
+	minetest.register_chatcommand("area_flak",{
+		params = "<ID>",
+		description = "Toggle airspace restrictions for area <ID>",
+		func = function(name, param)
+			local id = tonumber(param)
+			if not id then
+				return false, "Invalid usage, see /help area_flak."
+			end
+			
+			if not areas:isAreaOwner(id, name) then
+				return false, "Area "..id.." does not exist"
+					.." or is not owned by you."
+			end
+			local open = not areas.areas[id].flak
+			-- Save false as nil to avoid inflating the DB.
+			areas.areas[id].flak = open or nil
+			areas:save()
+			return true, ("Area's airspace %s."):format(open and "closed" or "opened")
+		end
+	})
+end
+
 if minetestd and minetestd.services.gravityctl.enabled then
 minetestd.gravityctl.register_gravity_effect("hangglider", 
 	function(player) 
@@ -102,6 +134,28 @@ minetestd.gravityctl.register_gravity_effect("hangglider",
 )
 end
 
+hangglider.can_fly = function (pname, pos)
+	-- Checks if the player will get shot down at the position
+	if minetest.is_protected(vector.round(pos), pname) then
+		if hangglider.flak then 
+			for id, area in pairs(areas:getAreasAtPos(pos)) do
+				if area.flak then
+					return false
+				end
+			end
+		end
+	end
+	return true
+end
+
+hangglider.shot_sound = function (pos)
+	minetest.sound_play("hangglider_flak_shot", {
+							pos = pos,
+							max_hear_distance = 30,
+							gain = 10.0,
+	})
+end
+
 local step_v
 minetest.register_entity("hangglider:glider", {
 	visual = "mesh",
@@ -110,7 +164,7 @@ minetest.register_entity("hangglider:glider", {
 	immortal = true,
 	static_save = false,
 	textures = {"wool_white.png","default_wood.png"},
-	on_step = function(self, _)
+	on_step = function(self, dtime)
 		local canExist = false
 		if self.object:get_attach() then
 			local player = self.object:get_attach("parent")
@@ -139,6 +193,20 @@ minetest.register_entity("hangglider:glider", {
 						]]end
 					end
 				end
+				if not hangglider.can_fly(pname,pos) then
+				    if not self.warned then -- warning shot
+						self.warned = 0
+						hangglider.shot_sound(pos)
+						minetest.chat_send_player(pname, "Protected area! You will be shot down in two seconds by anti-aircraft guns!")
+				    end
+				    self.warned = self.warned + dtime
+				    if self.warned > 2 then -- shoot down
+						player:set_hp(1)
+						player:get_inventory():remove_item("main", ItemStack("hangglider:hangglider"))
+						hangglider.shot_sound(pos)
+						canExist = false
+				    end
+				end
 				if not canExist then
 					player:set_physics_override({
 						jump = 1,