@Test
public void testImpurityLeftAndRightPart() {

    long[] left = new long[]{3, 0, 0};
    long[] right = new long[]{0, 3, 0};
    GiniImpurityMeasure impurity = new GiniImpurityMeasure(left, right);
    assertEquals(-6, impurity.impurity(), 1e-10);
}