@Test
public void testFitRegression() {

    int size = 100;
    double[] xs = new double[size];
    double[] ys = new double[size];
    double from = -5.0;
    double to = 5.0;
    double step = Math.abs(from - to) / size;
    Map<Integer, double[]> learningSample = new HashMap<>();
    for (int i = 0; i < size; i++) {
        xs[i] = from + step * i;
        ys[i] = 2 * xs[i];
        learningSample.put(i, new double[] {xs[i], ys[i]});
    }
    GDBTrainer trainer = new GDBRegressionOnTreesTrainer(1.0, 400, 3, 0.0)
        .withUsingIdx(true);
    IgniteModel<Vector, Double> mdl = trainer.fit(
        learningSample, 1,
        new DoubleArrayVectorizer<Integer>().labeled(Vectorizer.LabelCoordinate.LAST)
    );
    double mse = 0.0;
    for (int j = 0; j < size; j++) {
        double x = xs[j];
        double y = ys[j];
        double p = mdl.predict(VectorUtils.of(x));
        mse += Math.pow(y - p, 2);
    }
    mse /= size;
    assertEquals(0.0, mse, 0.0001);
    ModelsComposition composition = (ModelsComposition)mdl;
    assertTrue(!composition.toString().isEmpty());
    assertTrue(!composition.toString(true).isEmpty());
    assertTrue(!composition.toString(false).isEmpty());
    composition.getModels().forEach(m -> assertTrue(m instanceof DecisionTreeConditionalNode));
    assertEquals(400, composition.getModels().size());
    assertTrue(composition.getPredictionsAggregator() instanceof WeightedPredictionsAggregator);
    trainer = trainer.withCheckConvergenceStgyFactory(new MeanAbsValueConvergenceCheckerFactory(0.1));
    assertTrue(trainer.fit(learningSample, 1, new DoubleArrayVectorizer<Integer>().labeled(1)).getModels().size() < 2000);
}