Generate podzol when growing large spruce trees

When 4 spruce saplings are put into a 2×2 square, they may grow into a
large spruce tree. With this patch, dirt group group nodes nearby that
have air above and nodes under the stem are replaced by podzol nodes.

Note that this commit uses a different placement method than MineClone2
commit a83a2e9aba122d0fb4f1c120252a0d4eb07a1f10 to create one connected
podzol patch instead of randomly scattered nodes. It also places podzol
nodes in bulk at once for better performance.

mods/ITEMS/mcl_core/functions.lua

@@ -585,6 +585,69 @@ mcl_core.generate_spruce_tree = function(pos)
 	minetest.place_schematic({ x = pos.x - 3, y = pos.y - 1, z = pos.z - 3 }, path, "0", nil, false)
 end
 
+local vector_distance_xz = function(a, b)
+	return vector.distance(
+		{ x=a.x, y=0, z=a.z },
+		{ x=b.x, y=0, z=b.z }
+	)
+end
+
+local podzol_spread_randomizer = PseudoRandom(minetest.get_mapgen_setting("seed"))
+
+local generate_spruce_podzol = function(pos)
+	local podzol_positions = {}
+	local podzol_positions_xz = {}
+	local add_podzol = function(position)
+		podzol_positions[#podzol_positions+1] = position
+		local x,z = position.x, position.z
+		if nil == podzol_positions_xz[x] then
+		   podzol_positions_xz[x] = {}
+		end
+		podzol_positions_xz[x][z] = true
+	end
+	-- add podzol under tree
+	add_podzol( { x=pos.x+0, y=pos.y, z=pos.z+0 } )
+	add_podzol( { x=pos.x+0, y=pos.y, z=pos.z+1 } )
+	add_podzol( { x=pos.x+1, y=pos.y, z=pos.z+0 } )
+	add_podzol( { x=pos.x+1, y=pos.y, z=pos.z+1 } )
+	local corner_1 = { x=pos.x-5, y=pos.y-3, z=pos.z-5 }
+	local corner_2 = { x=pos.x+5, y=pos.y+3, z=pos.z+5 }
+	local dirt_positions = minetest.find_nodes_in_area_under_air(
+		corner_1,
+		corner_2,
+		{ "group:dirt" }
+	)
+	-- sort dirt positions according to distance from origin
+	table.sort(
+		dirt_positions,
+		function(a, b)
+		   return vector_distance_xz(pos, a) < vector_distance_xz(pos, b)
+		end
+	)
+	-- spread podzol
+	for j=1, #dirt_positions do
+		local dirt_position = dirt_positions[j]
+		local distance = math.ceil(vector_distance_xz(pos, dirt_position) + 0.5)
+		local k = podzol_spread_randomizer:next(0, distance)
+		local x, z = dirt_position.x, dirt_position.z
+		if (
+		   k < (10 - distance) and
+			not (podzol_positions_xz[x] and podzol_positions_xz[x][z]) and (
+				podzol_positions_xz[x] and podzol_positions_xz[x][z-1] or
+				podzol_positions_xz[x] and podzol_positions_xz[x][z+1] or
+				podzol_positions_xz[x-1] and podzol_positions_xz[x-1][z] or
+				podzol_positions_xz[x+1] and podzol_positions_xz[x+1][z]
+			)
+		) then
+			add_podzol(dirt_position)
+		end
+	end
+	minetest.bulk_set_node(
+		podzol_positions,
+		{ name = "mcl_core:podzol" }
+	)
+end
+
 mcl_core.generate_huge_spruce_tree = function(pos)
 	local r1 = math.random(1, 2)
 	local r2 = math.random(1, 4)
@@ -601,6 +664,14 @@ mcl_core.generate_huge_spruce_tree = function(pos)
 		path = minetest.get_modpath("mcl_core") .. "/schematics/mcl_core_spruce_huge_up_"..r2..".mts"
 	end
 	minetest.place_schematic(vector.add(pos, offset), path, "0", nil, false)
+	generate_spruce_podzol(
+		-- position under +x/+z sapling
+		{
+			x = pos.x + offset.x + 4,
+			y = pos.y - 1,
+			z = pos.z + offset.z + 4
+		}
+	)
 end
 
 -- END of spruce tree functions --