From 4431b6638f8957db0a0d1021acc59c67b5c26722 Mon Sep 17 00:00:00 2001
From: Gregory <neogen570@gmail.com>
Date: Thu, 16 Dec 2021 22:23:18 +0200
Subject: [PATCH] bottom up merge sort

---
 sorting/merge.go     | 55 +++++++++++++++++++++++++++++++++++++++++---
 sorting/sort_test.go | 20 ++++++++++++++++
 2 files changed, 72 insertions(+), 3 deletions(-)

diff --git a/sorting/merge.go b/sorting/merge.go
index 144c992..c56595c 100644
--- a/sorting/merge.go
+++ b/sorting/merge.go
@@ -1,6 +1,8 @@
 package sorting
 
-import "sync"
+import (
+	"sync"
+)
 
 func merge[T any](items []T, low, mid, hig int, less func(a, b T) bool, aux []T) {
 	i, j := low, mid+1
@@ -42,6 +44,30 @@ func doSort[T any](items []T, low, hig int, less func(a, b T) bool, aux []T) {
 
 	mid := low + (hig-low)/2
 
+	doSort(items, low, mid, less, aux)
+	doSort(items, mid+1, hig, less, aux)
+
+	// Optimization 1: if two subarrays already sorted
+	// we can skip merging them
+	if less(items[mid], items[mid+1]) {
+		return
+	}
+
+	merge(items, low, mid, hig, less, aux)
+}
+
+func doParallelSort[T any](items []T, low, hig int, less func(a, b T) bool, aux []T) {
+	if hig <= low {
+		return
+	}
+
+	if hig-low <= 15 {
+		Insertion(items[low:hig+1], less)
+		return
+	}
+
+	mid := low + (hig-low)/2
+
 	// Optimization 3: we can run sorting in parallel.
 	// This is costly optimization because we increase memory
 	// usage by creating WaitGroup instances
@@ -51,8 +77,6 @@ func doSort[T any](items []T, low, hig int, less func(a, b T) bool, aux []T) {
 	go func() { doSort(items, mid+1, hig, less, aux); wg.Done() }()
 	wg.Wait()
 
-	// Optimization 1: if two subarrays already sorted
-	// we can skip merging them
 	if less(items[mid], items[mid+1]) {
 		return
 	}
@@ -66,3 +90,28 @@ func Merge[T any](items []T, less func(a, b T) bool) {
 
 	doSort(items, 0, len-1, less, make([]T, len))
 }
+
+func ParallelMerge[T any](items []T, less func(a, b T) bool) {
+
+	len := len(items)
+
+	doParallelSort(items, 0, len-1, less, make([]T, len))
+}
+
+func BottomUpMerge[T any](items []T, less func(a, b T) bool) {
+
+	len := len(items)
+	aux := make([]T, len)
+	min := func(x, y int) int {
+		if x < y {
+			return x
+		}
+		return y
+	}
+
+	for sz := 1; sz < len; sz = sz + sz {
+		for low := 0; low < len-sz; low += sz + sz {
+			merge(items, low, low+sz-1, min(low+sz+sz-1, len-1), less, aux)
+		}
+	}
+}
diff --git a/sorting/sort_test.go b/sorting/sort_test.go
index 07bafc4..2513cfb 100644
--- a/sorting/sort_test.go
+++ b/sorting/sort_test.go
@@ -43,3 +43,23 @@ func BenchmarkMerge(b *testing.B) {
 		BenchmarkSort(10000, Merge[int])
 	}
 }
+
+func TestParallelMerge(t *testing.T) {
+	CheckSliceSorter(ParallelMerge[int])
+}
+
+func BenchmarkParallelMerge(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BenchmarkSort(10000, ParallelMerge[int])
+	}
+}
+
+func TestBottomUpMerge(t *testing.T) {
+	CheckSliceSorter(BottomUpMerge[int])
+}
+
+func BenchmarkBottomUpMerge(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BenchmarkSort(10000, BottomUpMerge[int])
+	}
+}