Add Mergesort

Author: appgurueu

.spec/sorting/sort_spec.lua

@@ -23,6 +23,9 @@ end
 describe("Heapsort", function()
 	check_sort(require("sorting.heapsort"))
 end)
+describe("Mergesort", function()
+	check_sort(require("sorting.mergesort"))
+end)
 describe("Quicksort", function()
 	check_sort(require("sorting.quicksort")())
 end)

src/sorting/mergesort.lua

@@ -0,0 +1,52 @@
+return function(
+	-- list to be sorted in-place
+	list,
+	-- function(a, b) -> truthy value if a < b
+	less_than
+)
+	less_than = less_than or function(a, b)
+		return a < b
+	end
+	-- Merges two sorted lists; elements of a come before those of b
+	local function merge(result, list, other_list)
+		local result_index = 1
+		local index = 1
+		local other_index = 1
+		while index <= #list and other_index <= #other_list do
+			-- Compare "head" element, insert "winner"
+			if less_than(other_list[other_index], list[index]) then
+				result[result_index] = other_list[other_index]
+				other_index = other_index + 1
+			else
+				result[result_index] = list[index]
+				index = index + 1
+			end
+			result_index = result_index + 1
+		end
+		-- Add remaining elements of either list or other_list
+		for offset = 0, #list - index do
+			result[result_index + offset] = list[index + offset]
+		end
+		for offset = 0, #other_list - other_index do
+			result[result_index + offset] = other_list[other_index + offset]
+		end
+	end
+
+	local function mergesort(list_to_sort, lower_index, upper_index)
+		if lower_index == upper_index then
+			list_to_sort[1] = list[lower_index]
+		end
+		if lower_index >= upper_index then
+			return
+		end
+		local middle_index = math.floor((upper_index + lower_index) / 2)
+
+		local left = {}
+		mergesort(left, lower_index, middle_index)
+		local right = {}
+		mergesort(right, middle_index + 1, upper_index)
+
+		merge(list_to_sort, left, right)
+	end
+	mergesort(list, 1, #list)
+end