shell sort

sorting/shell.go

@@ -0,0 +1,37 @@
+package sorting
+
+type shell struct{}
+
+func (*shell) Sort(items Sortable) {
+	len := items.Len()
+	gap := 1
+
+	// Calculating gap maximum value.
+	// This is for "Sedgewick gap sequence" variation.
+	// Another sequences can be used
+	for gap < len/3 {
+		gap = gap*3 + 1
+	}
+
+	// This loop needed to progressively degrease gap until siple insertion
+	// sort will be used
+	for gap >= 1 {
+
+		// Insertion sort loop
+		for i := gap; i < len; i++ {
+
+			// Instead of comparing adjacent elements we compare
+			// gap distance elements and swap them
+			for j := i; j >= gap && items.Less(j, j-gap); j -= gap {
+				items.Swap(j, j-gap)
+			}
+		}
+
+		// "Sedgewick gap sequence"
+		gap = gap / 3
+	}
+}
+
+func NewShell() Sorter {
+	return &shell{}
+}

sorting/sort_test.go

@@ -23,3 +23,13 @@ func BenchmarkInsertion(b *testing.B) {
 		BenchmarkSort(10000, NewInsertion())
 	}
 }
+
+func TestShell(t *testing.T) {
+	CheckSorter(NewShell())
+}
+
+func BenchmarkShell(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		BenchmarkSort(10000, NewShell())
+	}
+}