add heapify

slice/slice.go

@@ -160,3 +160,35 @@ func Align[T any](k []T, v []T, less func(T, T) bool) []Pair[*T, *T] {
 	return lis
 
 }
+
+func Heapify[T any](arr []T, n, i int, less func(T, T) bool) {
+	largest := i
+	l := 2*i + 1
+	r := 2*i + 2
+
+	if l < n && less(arr[largest], arr[l]) {
+		largest = l
+	}
+	if r < n && less(arr[largest], arr[r]) {
+		largest = r
+	}
+	if largest != i {
+		arr[i], arr[largest] = arr[largest], arr[i]
+		Heapify(arr, n, largest, less)
+	}
+}
+
+func BuildMaxHeap[T any](arr []T, less func(T, T) bool) {
+	n := len(arr)
+	for i := (n / 2) - 1; i >= 0; i-- {
+		Heapify(arr, n, i, less)
+	}
+}
+
+func HeapSort[T any](arr []T, less func(T, T) bool) {
+	BuildMaxHeap(arr, less)
+	for i := len(arr) - 1; i > 0; i-- {
+		arr[0], arr[i] = arr[i], arr[0]
+		Heapify(arr, i, 0, less)
+	}
+}

slice/slice_test.go

@@ -51,3 +51,21 @@ func TestAlign(t *testing.T) {
 }
 
 func ptr[T any](v T) *T { return &v }
+
+func TestHeapSort(t *testing.T) {
+	is := is.New(t)
+
+	arr := []int{9, 4, 3, 8, 10, 2, 5}
+	slice.HeapSort(arr, func(l, r int) bool { return l < r })
+
+	is.Equal(arr, []int{2, 3, 4, 5, 8, 9, 10})
+}
+
+func TestBuildHeap(t *testing.T) {
+	is := is.New(t)
+
+	arr := []int{1, 3, 5, 4, 6, 13, 10, 9, 8, 15, 17}
+	slice.BuildMaxHeap(arr, func(l, r int) bool { return l < r })
+
+	is.Equal(arr, []int{17, 15, 13, 9, 6, 5, 10, 4, 8, 3, 1})
+}