From 27b73543e028e013138e4ae2d2b1968019bffab5 Mon Sep 17 00:00:00 2001
From: Max Cahill <1bardesign@gmail.com>
Date: Tue, 10 Mar 2020 13:14:26 +1100
Subject: [PATCH] [modified] clean up source of stable_sort.lua

---
 stable_sort.lua | 54 +++++++++++++++++++++++++------------------------
 1 file changed, 28 insertions(+), 26 deletions(-)

diff --git a/stable_sort.lua b/stable_sort.lua
index 0f6686b..09e9a8d 100644
--- a/stable_sort.lua
+++ b/stable_sort.lua
@@ -43,13 +43,13 @@ local _sort_core = {}
 --tunable size for
 _sort_core.max_chunk_size = 24
 
-function _sort_core.insertion_sort_impl( array, first, last, less )
+function _sort_core.insertion_sort_impl(array, first, last, less)
 	for i = first + 1, last do
 		local k = first
 		local v = array[i]
 		for j = i, first + 1, -1 do
-			if less( v, array[j-1] ) then
-				array[j] = array[j-1]
+			if less(v, array[j - 1]) then
+				array[j] = array[j - 1]
 			else
 				k = j
 				break
@@ -59,12 +59,12 @@ function _sort_core.insertion_sort_impl( array, first, last, less )
 	end
 end
 
-function _sort_core.merge( array, workspace, low, middle, high, less )
+function _sort_core.merge(array, workspace, low, middle, high, less)
 	local i, j, k
 	i = 1
 	-- copy first half of array to auxiliary array
 	for j = low, middle do
-		workspace[ i ] = array[ j ]
+		workspace[i] = array[j]
 		i = i + 1
 	end
 	-- sieve through
@@ -75,18 +75,18 @@ function _sort_core.merge( array, workspace, low, middle, high, less )
 		if (k >= j) or (j > high) then
 			break
 		end
-		if less( array[ j ], workspace[ i ] )  then
-			array[ k ] = array[ j ]
+		if less(array[j], workspace[i])  then
+			array[k] = array[j]
 			j = j + 1
 		else
-			array[ k ] = workspace[ i ]
+			array[k] = workspace[i]
 			i = i + 1
 		end
 		k = k + 1
 	end
 	-- copy back any remaining elements of first half
-	for k = k, j-1 do
-		array[ k ] = workspace[ i ]
+	for k = k, j - 1 do
+		array[k] = workspace[i]
 		i = i + 1
 	end
 end
@@ -94,30 +94,31 @@ end
 
 function _sort_core.merge_sort_impl(array, workspace, low, high, less)
 	if high - low <= _sort_core.max_chunk_size then
-		_sort_core.insertion_sort_impl( array, low, high, less )
+		_sort_core.insertion_sort_impl(array, low, high, less)
 	else
-		local middle = math.floor((low + high)/2)
-		_sort_core.merge_sort_impl( array, workspace, low, middle, less )
-		_sort_core.merge_sort_impl( array, workspace, middle + 1, high, less )
-		_sort_core.merge( array, workspace, low, middle, high, less )
+		local middle = math.floor((low + high) / 2)
+		_sort_core.merge_sort_impl(array, workspace, low, middle, less)
+		_sort_core.merge_sort_impl(array, workspace, middle + 1, high, less)
+		_sort_core.merge(array, workspace, low, middle, high, less)
 	end
 end
 
+function default_less(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
-	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
+	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