Implemented insertion sort algorithm

insertion_sort.go

@@ -0,0 +1,43 @@
+package algorithms
+
+// InsertionSort is an implementation of insertion sort algorithm.
+// Wikipedia: https://en.wikipedia.org/wiki/Insertion_sort
+//
+// Insertion sort iterates, consuming one input element each repetition, and
+// growing a sorted output list. Each iteration, insertion sort removes one
+// element from the input data, finds the location it belongs within the sorted
+// list, and inserts it there. It repeats until no input elements remain.
+func InsertionSort(a []int) []int {
+	for i := 0; i < len(a); i++ {
+		for j := i; j > 0 && a[j] < a[j-1]; j-- {
+			a[j-1], a[j] = a[j], a[j-1]
+		}
+	}
+
+	return a
+}
+
+// InsertionSortOptimized is an optimized implementation of insertion sort
+// algorithm.
+func InsertionSortOptimized(a []int) []int {
+	// Moving the "sentinel" value to the first position
+	for i := len(a) - 1; i > 0; i-- {
+		if a[i] < a[i-1] {
+			a[i-1], a[i] = a[i], a[i-1]
+		}
+	}
+	if len(a) < 3 {
+		return a
+	}
+
+	for i := 2; i < len(a); i++ {
+		val := a[i]
+		j := i
+		for ; val < a[j-1]; j-- {
+			a[j] = a[j-1]
+		}
+		a[j] = val
+	}
+
+	return a
+}

insertion_sort_test.go

@@ -0,0 +1,65 @@
+package algorithms
+
+import (
+	"testing"
+)
+
+func TestInsertionSort(t *testing.T) {
+	testInsertionSort(t, InsertionSort,
+		[]int{5, 3, 1, 4, 2},
+		[]int{1, 2, 3, 4, 5},
+	)
+}
+
+func TestInsertionSortOptimized(t *testing.T) {
+	testInsertionSort(t, InsertionSortOptimized,
+		[]int{5, 3, 1, 4, 2},
+		[]int{1, 2, 3, 4, 5},
+	)
+}
+
+func TestInsertionSortOptimizedTwoItems(t *testing.T) {
+	testInsertionSort(t, InsertionSortOptimized,
+		[]int{2, 1},
+		[]int{1, 2},
+	)
+}
+
+func BenchmarkInsertionSort(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		InsertionSort(unsortedArray())
+	}
+}
+
+func BenchmarkInsertionSortOptimized(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		InsertionSortOptimized(unsortedArray())
+	}
+}
+
+func testInsertionSort(t *testing.T, sortFun func([]int) []int, a, exp []int) {
+	s := sortFun(a)
+	if len(s) != len(exp) {
+		t.Fatal("Array sizes don't match")
+	}
+	for i := 0; i < len(exp); i++ {
+		if s[i] != exp[i] {
+			t.Fatalf("Expected sorted array to equal %v, got %v", exp, s)
+		}
+	}
+}
+
+func unsortedArray() []int {
+	return []int{
+		57, 64, 83, 25, 26, 10, 55, 22, 76, 61,
+		28, 77, 56, 32, 63, 17, 91, 20, 58, 16,
+		1, 51, 88, 82, 24, 70, 81, 35, 49, 39,
+		89, 30, 46, 6, 41, 19, 43, 67, 53, 97,
+		65, 37, 13, 23, 29, 69, 0, 73, 9, 59,
+		96, 34, 66, 79, 27, 14, 40, 80, 98, 2,
+		5, 45, 50, 4, 85, 18, 86, 7, 87, 31,
+		95, 47, 68, 36, 15, 48, 8, 92, 11, 74,
+		78, 52, 44, 42, 54, 84, 12, 21, 38, 99,
+		72, 33, 71, 93, 60, 62, 90, 94, 3, 75,
+	}
+}