add memoize util

init.lua

@@ -8,6 +8,7 @@ building_lib = {
 }
 
 local MP = minetest.get_modpath("building_lib")
+dofile(MP .. "/memoize.lua")
 dofile(MP .. "/entity.lua")
 dofile(MP .. "/preview.lua")
 dofile(MP .. "/api.lua")

memoize.lua

@@ -0,0 +1,24 @@
+
+local cache = {}
+
+function building_lib.memoize(fn, keyfn)
+    return function(...)
+        local key = keyfn(...)
+        if not cache[key] then
+            cache[key] = { fn(...) }
+        end
+        return unpack(cache[key])
+    end
+end
+
+function building_lib.memoize_invalidate(key)
+    cache[key] = nil
+end
+
+-- periodic cleanup
+local function purge_cache()
+    cache = {}
+    minetest.after(300, purge_cache)
+end
+
+minetest.after(10, purge_cache)