floats: add Truncate method support to floats slice

pkg/datatype/floats/slice.go

@@ -191,14 +191,18 @@ func (s Slice) Last(i int) float64 {
 	return s[length-1-i]
 }
 
+func (s Slice) Truncate(size int) Slice {
+	if len(s) < size {
+		return s
+	}
+
+	return s[len(s)-size:]
+}
+
 // Index fetches the element from the end of the slice
 // WARNING: it does not start from 0!!!
 func (s Slice) Index(i int) float64 {
-	length := len(s)
-	if i < 0 || length-1-i < 0 {
-		return 0.0
-	}
-	return s[length-1-i]
+	return s.Last(i)
 }
 
 func (s Slice) Length() int {

pkg/datatype/floats/slice_test.go

@@ -15,6 +15,14 @@ func TestSub(t *testing.T) {
 	assert.Equal(t, 5, c.Length())
 }
 
+func TestTruncate(t *testing.T) {
+	a := New(1, 2, 3, 4, 5)
+	for i := 5; i > 0; i-- {
+		a = a.Truncate(i)
+		assert.Equal(t, i, a.Length())
+	}
+}
+
 func TestAdd(t *testing.T) {
 	a := New(1, 2, 3, 4, 5)
 	b := New(1, 2, 3, 4, 5)