/*
 * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

import java.util.*;
import java.util.stream.*;

/*
 * @test
 * @bug 8006572
 */
public class TestDoubleSummaryStatisticsSummation {
    public static void main(String... args) {
        double base = 1.0;
        double increment = Math.ulp(base)/2.0;
        int count = 1_000_001;
        /*
         * The exact sum of this stream is 1 + 1e6*ulp(1.0) but a
         * naive summation algorithm will return 1.0 since (1.0 +
         * ulp(1.0)/2) will round to 1.0 again.
         */
        TestDoubleIterator tdi = new TestDoubleIterator(base,
                                                        increment,
                                                        count);
        DoubleSummaryStatistics stats =
            StreamSupport.doubleStream(Spliterators.spliteratorUnknownSize(tdi,
                                                                           Spliterator.IMMUTABLE |
                                                                           Spliterator.NONNULL),
                                       false).collect(DoubleSummaryStatistics::new,
                                                      DoubleSummaryStatistics::accept,
                                                      DoubleSummaryStatistics::combine);

        double expectedSum = base + (increment * (count - 1));
        compareUlpDifference(expectedSum, stats.getSum(), 3);

        double expectedAvg = expectedSum / count;
        compareUlpDifference(expectedAvg, stats.getAverage(), 3);
    }

    private static void compareUlpDifference(double expected, double computed, double threshold) {
        double ulpDifference = Math.abs(expected - computed) / Math.ulp(expected);

        if (ulpDifference > threshold) {
            System.out.printf("Numerical summation error too large, %g ulps rather than %g.%n",
                              ulpDifference, threshold);
            throw new RuntimeException();
        }

    }

    static class TestDoubleIterator implements PrimitiveIterator.OfDouble {
        private double base;
        private double subsequent;
        private long count;
        private long iteration = 0;

        /**
         * Construct a {@code TestDoubleStream} of {@code count}
         * elements whose first value is {@code base} and whose
         * following values are all {@code subsequent}.
         */
        TestDoubleIterator(double base, double subsequent, long count) {
            this.base = base;
            this.subsequent = subsequent;
            this.count = count;
        }

        @Override
        public boolean hasNext() {
            return (iteration < count);
        }

        @Override
        public double nextDouble() {
            if (iteration >= count)
                throw new NoSuchElementException();
            else {
                iteration++;
                return (iteration == 1) ? base : subsequent;
            }
        }
    }
}