merge sort

sorting/merge.go

@@ -0,0 +1,61 @@
+package sorting
+
+func merge[T any](items []T, low, mid, hig int, less func(a, b T) bool, aux []T) {
+	i, j := low, mid+1
+
+	for k := low; k <= hig; k++ {
+		aux[k] = items[k]
+	}
+
+	for k := low; k <= hig; k++ {
+		if i > mid {
+			items[k] = aux[j]
+			j++
+		} else if j > hig {
+			items[k] = aux[i]
+			i++
+		} else if less(aux[i], aux[j]) {
+			items[k] = aux[i]
+			i++
+		} else {
+			items[k] = aux[j]
+			j++
+		}
+	}
+
+}
+
+func doSort[T any](items []T, low, hig int, less func(a, b T) bool, aux []T) {
+
+	// Optimization 2: if array length is less then 15
+	// it's imperically prooven that insertion sort
+	// for such arrays can speed up merge sort up to 15%
+	if hig-low <= 15 {
+		Insertion(items[low:hig+1], less)
+		return
+	}
+
+	if hig <= low {
+		return
+	}
+
+	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 Merge[T any](items []T, less func(a, b T) bool) {
+
+	len := len(items)
+
+	doSort(items, 0, len-1, less, make([]T, len))
+}

sorting/sort_test.go

@@ -33,3 +33,13 @@ func BenchmarkShell(b *testing.B) {
 		BenchmarkSort(10000, Shell[int])
 	}
 }
+
+func TestMerge(t *testing.T) {
+	CheckSliceSorter(Merge[int])
+}
+
+func BenchmarkMerge(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BenchmarkSort(10000, Merge[int])
+	}
+}