[modified] unique_mapping memoised compare function and cleanup

unique_mapping.lua

@@ -1,6 +1,10 @@
---generate a mapping from unique values to plain numbers
+--[[
---useful for arbitrarily ordering things that don't have
+	unique mapping
---a natural ordering implied (eg textures for batching)
+
+	generate a mapping from unique values to plain numbers
+	useful for arbitrarily ordering things that don't have
+	a natural ordering in lua (eg textures for batching)
+]]
 
 local unique_mapping = {}
 unique_mapping._mt = {
@@ -13,6 +17,7 @@ local _MAP_VARS = setmetatable({}, {
 	__mode = "k" --only keys are weak
 })
 
+--create a new unique mapping
 function unique_mapping:new()
 	local r =  setmetatable({}, self._mt)
 	--set up the actual vars
@@ -22,12 +27,15 @@ function unique_mapping:new()
 	return r
 end
 
+--private;
+--get the next index for this mapping
 function unique_mapping:_increment()
 	local vars = _MAP_VARS[self]
 	vars.current_index = vars.current_index + 1
 	return vars.current_index
 end
 
+--get or build a mapping for a passed value
 function unique_mapping:map(value)
 	local val = self[value]
 	if val then
@@ -38,4 +46,17 @@ function unique_mapping:map(value)
 	return i
 end
 
+--get a function representing an a < b comparision that can be used
+--with table.sort and friends, like `table.sort(values, mapping:compare())`
+function unique_mapping:compare()
+	--	memoised so it doesn't generate garbage, but also doesn't
+	--	allocate until it's actually used
+	if not self._compare then
+		self._compare = function(a, b)
+			return self:map(a) < self:map(b)
+		end
+	end
+	return self._compare
+end
+
 return unique_mapping