[modified] clean up source of stable_sort.lua

stable_sort.lua

@@ -103,21 +103,22 @@ function _sort_core.merge_sort_impl(array, workspace, low, high, less)
 	end
 end
 
---inline common setup stuff
+function default_less(a, b)
-function _sort_core.sort_setup(array, less)
-	local n = #array
-	local trivial = false
-	--trivial cases; empty or 1 element
-	if n <= 1 then
-		trivial = true
-	else
-		--default less
-		less = less or function (a, b)
 	return a < b
 end
+
+--inline common setup stuff
+function _sort_core.sort_setup(array, less)
+	--default less
+	less = less or default_less
+	--
+	local n = #array
+	--trivial cases; empty or 1 element
+	local trivial = (n <= 1)
+	if not trivial then
 		--check less
 		if less(array[1], array[1]) then
-		  error("invalid order function for sorting")
+			error("invalid order function for sorting; less(v, v) should not be true for any v.")
 		end
 	end
 	--setup complete
@@ -128,9 +129,10 @@ function _sort_core.stable_sort(array, less)
 	--setup
 	local trivial, n, less = _sort_core.sort_setup(array, less)
 	if not trivial then
-		--temp storage
+		--temp storage; allocate ahead of time
 		local workspace = {}
-		workspace[ math.floor( (n+1)/2 ) ] = array[1]
+		local middle = math.ceil(n / 2)
+		workspace[middle] = array[1]
 		--dive in
 		_sort_core.merge_sort_impl( array, workspace, 1, n, less )
 	end