add kline window tail test

bbgo/types/kline.go

@@ -320,9 +320,14 @@ func (k KLineWindow) Take(size int) KLineWindow {
 
 func (k KLineWindow) Tail(size int) KLineWindow {
 	if len(k) <= size {
-		return k[:]
+		win := make(KLineWindow, len(k))
+		copy(win, k)
+		return win
 	}
-	return k[len(k)-size:]
+
+	win := make(KLineWindow, size)
+	copy(win, k[len(k)-size:])
+	return win
 }
 
 func (k *KLineWindow) Truncate(size int) {

bbgo/types/kline_test.go

@@ -0,0 +1,22 @@
+package types
+
+import ("testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestKLineWindow_Tail(t *testing.T) {
+	var win = KLineWindow{
+		{ Open: "11600.0", Close: "11600.0", High: "11600.0", Low: "11600.0"},
+		{ Open: "11600.0", Close: "11600.0", High: "11600.0", Low: "11600.0"},
+	}
+
+	var win2 = win.Tail(1)
+	assert.Len(t, win2, 1)
+
+	var win3 = win.Tail(2)
+	assert.Len(t, win3, 2)
+
+	var win4 = win.Tail(3)
+	assert.Len(t, win4, 2)
+}