Make outer layer of reactor structure optional

As the layers of reactor structure now have a practical purpose,
in attenuating the modelled radiation from the core, it is no longer
necessary to make so much of it mandatory in order to motivate players
to build it.

technic/machines/HV/nuclear_reactor.lua

@@ -97,64 +97,74 @@ local function siren_clear(pos, meta)
 	end
 end
 
+-- The standard reactor structure consists of a 9x9x9 cube.  A cross
+-- section through the middle:
+--
+--     CCCC CCCC
+--     CBBB BBBC
+--     CBSS SSBC
+--     CBSWWWSBC
+--     CBSW#WSBC
+--     CBSW|WSBC
+--     CBSS|SSBC
+--     CBBB|BBBC
+--     CCCC|CCCC
+--     C = Concrete, B = Blast-resistant concrete, S = Stainless Steel,
+--     W = water node, # = reactor core, | = HV cable
+--
+-- The man-hole and the HV cable are only in the middle, and the man-hole
+-- is optional.
+--
+-- For the reactor to operate and not melt down, it insists on the inner
+-- 7x7x7 portion (from the core out to the blast-resistant concrete)
+-- being intact.  Intactness only depends on the number of nodes of the
+-- right type in each layer.  The water layer must have water in all but
+-- at most one node; the steel and blast-resistant concrete layers must
+-- have the right material in all but at most two nodes.  The permitted
+-- gaps are meant for the cable and man-hole, but can actually be anywhere
+-- and contain anything.  For the reactor to be useful, a cable must
+-- connect to the core, but it can go in any direction.
+--
+-- The outer concrete layer of the standard structure is not required
+-- for the reactor to operate.  It is noted here because it used to
+-- be mandatory, and for historical reasons (that it predates the
+-- implementation of radiation) it needs to continue being adequate
+-- shielding of legacy reactors.  If it ever ceases to be adequate
+-- shielding for new reactors, legacy ones should be grandfathered.
 local reactor_structure_badness = function(pos)
-	-- The reactor consists of a 9x9x9 cube structure
-	-- A cross section through the middle:
-	--  CCCC CCCC
-	--  CBBB BBBC
-	--  CBSS SSBC
-	--  CBSWWWSBC
-	--  CBSW#WSBC
-	--  CBSW|WSBC
-	--  CBSS|SSBC
-	--  CBBB|BBBC
-	--  CCCC|CCCC
-	--  C = Concrete, B = Blast resistant concrete, S = Stainless Steel,
-	--  W = water node, # = reactor core, | = HV cable
-	--  The man-hole and the HV cable is only in the middle
-	--  The man-hole is optional
-
 	local vm = VoxelManip()
-	local pos1 = vector.subtract(pos, 4)
-	local pos2 = vector.add(pos, 4)
+	local pos1 = vector.subtract(pos, 3)
+	local pos2 = vector.add(pos, 3)
 	local MinEdge, MaxEdge = vm:read_from_map(pos1, pos2)
 	local data = vm:get_data()
 	local area = VoxelArea:new({MinEdge=MinEdge, MaxEdge=MaxEdge})
 
-	local c_concrete = minetest.get_content_id("technic:concrete")
 	local c_blast_concrete = minetest.get_content_id("technic:blast_resistant_concrete")
 	local c_stainless_steel = minetest.get_content_id("technic:stainless_steel_block")
 	local c_water_source = minetest.get_content_id("default:water_source")
 	local c_water_flowing = minetest.get_content_id("default:water_flowing")
 
-	local concretelayer, blastlayer, steellayer, waterlayer = 0, 0, 0, 0
+	local blastlayer, steellayer, waterlayer = 0, 0, 0
 
 	for z = pos1.z, pos2.z do
 	for y = pos1.y, pos2.y do
 	for x = pos1.x, pos2.x do
-		-- If the position is in the outer layer
+		local cid = data[area:index(x, y, z)]
 		if x == pos1.x or x == pos2.x or
 		   y == pos1.y or y == pos2.y or
 		   z == pos1.z or z == pos2.z then
-			if data[area:index(x, y, z)] == c_concrete then
-				concretelayer = concretelayer + 1
+			if cid == c_blast_concrete then
+				blastlayer = blastlayer + 1
 			end
 		elseif x == pos1.x+1 or x == pos2.x-1 or
 		   y == pos1.y+1 or y == pos2.y-1 or
 		   z == pos1.z+1 or z == pos2.z-1 then
-			if data[area:index(x, y, z)] == c_blast_concrete then
-				blastlayer = blastlayer + 1
+			if cid == c_stainless_steel then
+				steellayer = steellayer + 1
 			end
 		elseif x == pos1.x+2 or x == pos2.x-2 or
 		   y == pos1.y+2 or y == pos2.y-2 or
 		   z == pos1.z+2 or z == pos2.z-2 then
-			if data[area:index(x, y, z)] == c_stainless_steel then
-				steellayer = steellayer + 1
-			end
-		elseif x == pos1.x+3 or x == pos2.x-3 or
-		   y == pos1.y+3 or y == pos2.y-3 or
-		   z == pos1.z+3 or z == pos2.z-3 then
-		   	local cid = data[area:index(x, y, z)]
 			if cid == c_water_source or cid == c_water_flowing then
 				waterlayer = waterlayer + 1
 			end
@@ -165,8 +175,7 @@ local reactor_structure_badness = function(pos)
 	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)
+	return (25 - waterlayer) + (96 - steellayer) + (216 - blastlayer)
 end
 
 local function meltdown_reactor(pos)