add basic api support for loose nodes.

expose an api function to register "loose" versions of existing nodes,
and implement some behaviour for such nodes; they will fall down if
there is air below them, and will eventually become their non-loose
counterparts if there is not air below them.

mods/bg_api/init.lua

@@ -17,5 +17,6 @@ load_file("util_node")
 load_file("wrappers")
 
 load_file("random_tick")
+load_file("loose_nodes")
 
 load_file("cleanup")

mods/bg_api/loose_nodes.lua

@@ -0,0 +1,63 @@
+-- TODO: collapse nodes when the node under them is removed.
+
+local function attempt_collapse_at (pos)
+	local below = pos + blockgame.vector.dirs.down
+	if minetest.get_node(below).name ~= "air" then return end
+
+	minetest.spawn_falling_node(pos)
+end
+
+local function place_loose_node (itemstack, placer, pointed)
+	local result = minetest.item_place(itemstack, placer, pointed)
+	attempt_collapse_at(pointed.above)
+	return result
+end
+
+-- collapse nodes that somehow managed to stay in the air.
+blockgame.register_abm({
+	label = "collapse collapsible nodes",
+	nodenames = {"group:loose"},
+	neighbors = {"air"},
+	interval = 15,
+	chance = 1,
+	action = attempt_collapse_at,
+})
+
+local function attempt_settle_at (pos)
+	local below = pos + blockgame.vector.dirs.down
+	if minetest.get_node(below).name == "air" then return end
+
+	local def = minetest.registered_nodes[minetest.get_node(pos).name]
+	minetest.set_node(pos, {name = def.stable_version})
+end
+
+-- settle nodes that are still (in the future, it will take some time for loose nodes that are still to settle).
+blockgame.register_abm({
+	label = "settle collapsible nodes",
+	nodenames = {"group:loose"},
+	interval = 30,
+	chance = 10,
+	action = attempt_settle_at,
+})
+
+
+
+function blockgame.register_loose_version (name)
+	local def = minetest.registered_nodes[name]
+	if not def then return false end
+
+	local groups = {}
+	for key, value in pairs(def.groups) do
+		groups[key] = value
+	end
+	groups.loose = 1
+
+	blockgame.register_node(name .. "_loose", {
+		description = "Loose " .. def.description,
+		tiles = def.tiles, -- temporary
+		groups = groups,
+		on_place = place_loose_node,
+		stable_version = name,
+	})
+	return true
+end