Grace period before reactor meltdown

The meltdown check now doesn't trigger meltdown immediately on reactor
structure being compromised.  Instead, there's a grace period of up to
100 s, during which the reactor can be repaired.  The check doesn't just
look at whether the structure is damaged at all: it looks at how damaged
it is, counting the number of faulty nodes.  The amount of damage is
integrated over time, and the grace period is actually 100 node-seconds,
so greater damage causes meltdown more quickly.  If the active core is
dug then it melts down immediately, preventing the tactic of digging
the core to avert meltdown.

Incidentally move the meltdown check into its own ABM, from the
technic_run callback, so that it applies even when the reactor is not
connected to a switching station.

technic/machines/HV/nuclear_reactor.lua

@@ -54,7 +54,7 @@ local nodebox = {
 	{ -0.303, -0.303, -0.397, 0.303, 0.303, 0.397 },
 }
 
-local check_reactor_structure = function(pos)
+local reactor_structure_badness = function(pos)
 	-- The reactor consists of a 9x9x9 cube structure
 	-- A cross section through the middle:
 	--  CCCC CCCC
@@ -119,19 +119,41 @@ local check_reactor_structure = function(pos)
 	end
 	end
 	end
-	if waterlayer >= 25 and
-	   steellayer >= 96 and
-	   blastlayer >= 216 and
-	   concretelayer >= 384 then
-		return true
-	end
+	if waterlayer > 25 then waterlayer = 25 end
+	if steellayer > 96 then steellayer = 96 end
+	if blastlayer > 216 then blastlayer = 216 end
+	if concretelayer > 384 then concretelayer = 384 end
+	return (25 - waterlayer) + (96 - steellayer) + (216 - blastlayer) + (384 - concretelayer)
 end
 
-local explode_reactor = function(pos)
-	print("A reactor exploded at "..minetest.pos_to_string(pos))
+local function meltdown_reactor(pos)
+	print("A reactor melted down at "..minetest.pos_to_string(pos))
 	minetest.set_node(pos, {name="technic:corium_source"})
 end
 
+minetest.register_abm({
+	nodenames = {"technic:hv_nuclear_reactor_core_active"},
+	interval = 1,
+	chance = 1,
+	action = function (pos, node)
+		local meta = minetest.get_meta(pos)
+		local badness = reactor_structure_badness(pos)
+		local accum_badness = meta:get_int("structure_accumulated_badness")
+		if badness == 0 then
+			if accum_badness ~= 0 then
+				meta:set_int("structure_accumulated_badness", accum_badness - 1)
+			end
+		else
+			accum_badness = accum_badness + badness
+			if accum_badness >= 100 then
+				meltdown_reactor(pos)
+			else
+				meta:set_int("structure_accumulated_badness", accum_badness)
+			end
+		end
+	end,
+})
+
 local run = function(pos, node)
 	local meta = minetest.get_meta(pos)
 	local machine_name = S("Nuclear %s Generator Core"):format("HV")
@@ -152,7 +174,7 @@ local run = function(pos, node)
 			-- Check that the reactor is complete as well
 			-- as the correct number of correct fuel
 			if correct_fuel_count == 6 and
-			   check_reactor_structure(pos) then
+			   reactor_structure_badness(pos) == 0 then
 				meta:set_int("burn_time", 1)
 				technic.swap_node(pos, "technic:hv_nuclear_reactor_core_active") 
 				meta:set_int("HV_EU_supply", power_supply)
@@ -167,10 +189,8 @@ local run = function(pos, node)
 		meta:set_int("burn_time", 0)
 		meta:set_string("infotext", S("%s Idle"):format(machine_name))
 		technic.swap_node(pos, "technic:hv_nuclear_reactor_core")
+		meta:set_int("structure_accumulated_badness", 0)
 	elseif burn_time > 0 then
-		if not check_reactor_structure(pos) then
-			explode_reactor(pos)
-		end
 		burn_time = burn_time + 1
 		meta:set_int("burn_time", burn_time)
 		local percent = math.floor(burn_time / burn_ticks * 100)
@@ -229,6 +249,7 @@ minetest.register_node("technic:hv_nuclear_reactor_core_active", {
 		fixed = nodebox
 	},
 	can_dig = technic.machine_can_dig,
+	after_dig_node = meltdown_reactor,
 	allow_metadata_inventory_put = technic.machine_inventory_put,
 	allow_metadata_inventory_take = technic.machine_inventory_take,
 	allow_metadata_inventory_move = technic.machine_inventory_move,
@@ -249,6 +270,7 @@ minetest.register_node("technic:hv_nuclear_reactor_core_active", {
 			meta:set_int("HV_EU_supply", 0)
 			meta:set_int("burn_time", 0)
 			technic.swap_node(pos, "technic:hv_nuclear_reactor_core")
+			meta:set_int("structure_accumulated_badness", 0)
 			return
 		end