add an API to allow other mods to test positions for the presence of …

…caverns

init.lua

@@ -39,6 +39,7 @@ local modpath = minetest.get_modpath(minetest.get_current_modname())
 dofile(modpath.."/defaults.lua")
 dofile(modpath.."/features.lua") -- some generic cave features useful for a variety of mapgens
 dofile(modpath.."/player_spawn.lua") -- Function for spawning a player in a giant cavern
+dofile(modpath.."/test_pos.lua") -- Function other mapgens can use to test if a position is inside a cavern
 dofile(modpath.."/legacy.lua") -- contains old node definitions and functions, will be removed at some point in the future.
 
 local disable_mapgen_caverns = function()

test_pos.lua

@@ -0,0 +1,41 @@
+-- If pos is located inside a cavern volume, returns the list of cavern definitions that
+-- are responsible for that cavern volume. If not inside a cavern volume returns an empty list.
+
+-- This is a somewhat expensive function, take care not to use it more than necessary
+subterrane.is_in_cavern = function(pos)
+	local results = {}
+
+	for _, cave_layer_def in pairs(subterrane.registered_layers) do
+		local YMIN = cave_layer_def.y_min
+		local YMAX = cave_layer_def.y_max
+		local y = pos.y
+		if y <= YMAX and y >= YMIN then
+			local block_size = mapgen_helper.block_size
+			local BLEND = math.min(cave_layer_def.boundary_blend_range, (YMAX-YMIN)/2)
+			local TCAVE = cave_layer_def.cave_threshold
+			local np_cave = cave_layer_def.perlin_cave
+			local np_wave = cave_layer_def.perlin_wave
+			local y_blend_min = YMIN + BLEND * 1.5
+			local y_blend_max = YMAX - BLEND * 1.5	
+			local nval_cave = minetest.get_perlin(np_cave):get_3d(pos) --cave noise for structure
+			local nval_wave = minetest.get_perlin(np_wave):get_3d(pos) --wavy structure of cavern ceilings and floors
+			nval_cave = (nval_cave + nval_wave)/2
+			local cave_local_threshold
+			if y < y_blend_min then
+				cave_local_threshold = TCAVE + ((y_blend_min - y) / BLEND) ^ 2
+			elseif y > y_blend_max then
+				cave_local_threshold = TCAVE + ((y - y_blend_max) / BLEND) ^ 2
+			else
+				cave_local_threshold = TCAVE
+			end
+			if cave_layer_def.double_frequency and nval_cave < 0 then
+				nval_cave = -nval_cave
+			end
+			if nval_cave > cave_local_threshold then
+				table.insert(results, cave_layer_def)
+			end
+		end
+	end
+
+	return results
+end