ABM "inversion" API

This is useful when we want to perform an
action on a commonly-occuring node (like
sand or item stacks) with a requirement of
a neighbor of a less common node type (like
igniters).  Checking for B with A nearby can
be a lot faster than A with B nearby if B is
less common, so fewer neighbor checks
actually need to happen.

This was already done "manually" for the
flammable/igniter interaction, but now this is
a general API for all ABMs, that allows the
inversion to be done nearly transparently
with just a flag in the definition, and so the
ABM rules can be written more simply, with
the inversion handled elsewhere.

TODO: Rewrite current manual-inversions,
including the ones for flammable/igniter and
lava/stone hardening/softening.

mods/nc_api_active/abminvert.lua

@@ -0,0 +1,43 @@
+-- LUALOCALS < ---------------------------------------------------------
+local ipairs, minetest, nodecore, pairs
+    = ipairs, minetest, nodecore, pairs
+-- LUALOCALS > ---------------------------------------------------------
+
+local hash = minetest.hash_node_position
+
+local oldreg = minetest.register_abm
+function minetest.register_abm(def, ...)
+	if not def.neighbors_invert then return oldreg(def, ...) end
+
+	local nnames = def.nodenames
+	def.nodenames = def.neighbors
+	def.neighbors = nnames
+
+	local oldact = def.action
+
+	local queue
+	local function process()
+		for _, v in pairs(queue) do
+			local nnode = minetest.get_node(v.pos)
+			if nnode.name == v.node.name then
+				oldact(v.pos, nnode)
+			end
+		end
+		queue = nil
+	end
+
+	function def.action(pos)
+		if not queue then
+			queue = {}
+			minetest.after(0, process)
+		end
+		for _, npos in ipairs(nodecore.find_nodes_around(pos, nnames, 1)) do
+			queue[hash(pos)] = {
+				pos = npos,
+				node = minetest.get_node(npos)
+			}
+		end
+	end
+
+	return oldreg(def, ...)
+end

mods/nc_api_active/init.lua

@@ -8,6 +8,7 @@ nodecore.amcoremod()
 nodecore.register_limited_abm = function(...) return minetest.register_abm(...) end
 
 include("abmmux")
+include("abminvert")
 include("stasis")
 include("dnts")
 include("aism")