Make jewel ore generation functional again

mods/default/mapgen.lua

@@ -448,6 +448,7 @@ minetest.register_decoration(
 
 minetest.register_decoration(
    {
+      name = "default:gigatree",
       deco_type = "schematic",
       place_on = {"default:dirt_with_grass"},
       sidelen = 16,

mods/jewels/init.lua

@@ -243,20 +243,6 @@ minetest.register_node(
       sounds = default.node_sound_wood_defaults(),
 })
 
--- Ore
-
-minetest.register_ore(
-   {
-      ore_type       = "scatter",
-      ore            = "jewels:jewel_ore",
-      wherein        = "default:tree_birch",
-      clust_scarcity = 11*11*11,
-      clust_num_ores = 3,
-      clust_size     = 6,
-      y_min     = 0,
-      y_max     = 31000,
-})
-
 crafting.register_craft(
    {
       output = "jewels:bench",
@@ -288,5 +274,6 @@ achievements.register_achievement(
 -- The tool jewel definitions
 
 dofile(minetest.get_modpath("jewels").."/jewels.lua")
+dofile(minetest.get_modpath("jewels").."/mapgen.lua")
 
 default.log("mod:jewels", "loaded")

mods/jewels/mapgen.lua

@@ -0,0 +1,100 @@
+-- Generate jewel ore.
+
+-- Jewel ore generated in birch tree nodes in the giga tree decorations
+-- in the Deep Forest biome.
+-- The algorithm uses LVM to imitate Minetest's scatter ores,
+-- since ores in Minetest are generated after decorations.
+
+-- Fields of the original ore definition:
+local clust_scarcity = 11*11*11
+local clust_num_ores = 3
+local clust_size = 6
+local y_min = 0
+
+-- Helper variables
+local gigatree_decoration_id = minetest.get_decoration_id("default:gigatree")
+local biome_y = minetest.registered_biomes["Deep Forest"].y_min
+
+local lvm_buffer = {}
+
+local c_birch = minetest.get_content_id("default:tree_birch")
+local c_jewel_ore = minetest.get_content_id("jewels:jewel_ore")
+
+-- Generation algorithm:
+if gigatree_decoration_id then
+
+	-- Helper function to find a random minimum/maxium range of length clust_size.
+	-- Returned numbers are offsets.
+	local rnd_minmax = function(pr)
+		local min = pr:next(- clust_size + 1, 0)
+		max = min + (clust_size - 1)
+		return min, max
+	end
+
+	minetest.set_gen_notify({decoration=true}, {gigatree_decoration_id})
+	minetest.register_on_generated(function(minp, maxp, blockseed)
+		if maxp.y < y_min then
+			return
+		end
+		local ores_in_mapblock = {}
+		local pr = PseudoRandom(blockseed)
+		local mgobj = minetest.get_mapgen_object("gennotify")
+		local deco = mgobj["decoration#"..gigatree_decoration_id]
+		-- If a giga tree was found anywhere in generated area, activate generation
+		if deco and #deco > 0 then
+			-- This code tries to imitate scatter ores in Minetest
+			local vm, emin, emax = minetest.get_mapgen_object("voxelmanip")
+			local area = VoxelArea:new({MinEdge=emin, MaxEdge=emax})
+			local data = vm:get_data(lvm_buffer)
+			-- Interate through all nodes and place jewel ore in birch tree nodes
+			-- with a low chance (1/clust_scarcity)
+			for z=minp.z, maxp.z do
+			for y=math.max(y_min, minp.y), maxp.y do
+			for x=minp.x, maxp.x do
+				local p_pos = area:index(x,y,z)
+				if data[p_pos] == c_birch then
+					local bdata = minetest.get_biome_data({x=x,y=math.max(y, biome_y),z=z})
+					local bname = minetest.get_biome_name(bdata.biome)
+					if bname == "Deep Forest" and pr:next(1, clust_scarcity) == 1 then
+						data[p_pos] = c_jewel_ore
+						table.insert(ores_in_mapblock, {x=x,y=y,z=z})
+					end
+				end
+			end
+			end
+			end
+			-- If jewel ore was placed in the first phase, also place additional near the initial ore
+			for o=1, #ores_in_mapblock do
+				local start_ore = ores_in_mapblock[o]
+				for n=1, clust_num_ores do
+					local ore = {}
+					local axes = {"z","y","x"}
+					for a=1, #axes do
+						local ax = axes[a]
+						-- New ores are placed within a randomly positioned bounding box
+						-- of size clust_size^3 around the initial ore
+						ore[ax] = start_ore[ax] + pr:next(rnd_minmax(pr))
+						-- Make sure we stay within minp, maxp
+						if ore[ax] < minp[ax] then
+							ore[ax] = minp[ax]
+						elseif ore[ax] > maxp[ax] then
+							ore[ax] = maxp[ax]
+						end
+					end
+					local p_pos = area:index(ore.x, ore.y, ore.z)
+					-- The new random pos must also be a birch tree to generate a jewel ore
+					if data[p_pos] == c_birch then
+						data[p_pos] = c_jewel_ore
+					end
+				end
+			end
+			-- Only write back to map when any ore was actually placed
+			if #ores_in_mapblock > 0 then
+				vm:set_data(data)
+				vm:write_to_map()
+			end
+		end
+	end)
+else
+	minetest.log("error", "[jewels] Decoration ID for gigatree not found!")
+end