Examples of org.apache.sis.math.Statistics

org.apache.sis.math.Statistics
kipedia.org/wiki/Kahan_summation_algorithm">Kahan summation algorithm for reducing the numerical errors; the sample values are never stored in memory.

An instance of {@code Statistics} is initially empty: the {@linkplain #count() count} ofvalues is set to zero, and all above-cited statistical values are set to {@link Double#NaN NaN}. The statistics are updated every time an {@link #accept(double)} method is invoked with a non-NaNvalue.
{@section Examples}The following examples assume that a y=f(x) function is defined. A simple usage is: {@preformat javaStatistics stats = new Statistics("y");}for (int i=0; i

     * @param  name  The name of the value being measured.
     * @param  cache The cache to validate.
     * @return Statistics on the key values of the given map.
     */
    private static Statistics validateStressEntries(final String name, final Map<Integer,Integer> cache) {
        final Statistics statistics = new Statistics(name);
        for (final Map.Entry<Integer,Integer> entry : cache.entrySet()) {
            final int key = entry.getKey();
            final int value = entry.getValue();
            assertEquals(key*key, value);
            statistics.accept(key);
        }
        return statistics;
    }

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        for (int i=0; i<threads.length; i++) threads[i].join();
        TestUtilities.rethrownIfNotNull(failures.get());
        /*
         * Verifies the values.
         */
        final Statistics beforeGC = validateStressEntries("Before GC", cache);
        assertTrue("Should not have more entries than what we put in.", cache.size() <= count);
        assertFalse("Some entries should be retained by strong references.", cache.isEmpty());
        /*
         * If verbose test output is enabled, report the number of cache hits.
         * The numbers below are for tuning the test only. The output is somewhat
         * random so we can not check it in a test suite.  However if the test is
         * properly tuned, most values should be non-zero.
         */
        final PrintWriter out = CacheTest.out;
        TestUtilities.printSeparator("CacheTest.stress() - testing concurrent accesses");
        out.print("There is "); out.print(threads.length); out.print(" threads, each of them"
                + " fetching or creating "); out.print(count); out.println(" values.");
        out.println("Number of times a new value has been created, for each thread:");
        for (int i=0; i<threads.length;) {
            final String n = String.valueOf(threads[i++].addCount);
            out.print(CharSequences.spaces(6 - n.length()));
            out.print(n);
            if ((i % 10) == 0) {
                out.println();
            }
        }
        out.println();
        out.println("Now observe how the background thread cleans the cache.");
        long time = System.nanoTime();
        for (int i=0; i<10; i++) {
            final long t = System.nanoTime();
            out.printf("Cache size: %4d (after %3d ms)%n", cache.size(), round((t - time) / 1E+6));
            time = t;
            Thread.sleep(250);
            if (i >= 2) {
                System.gc();
            }
        }
        out.println();
        /*
         * Gets the statistics of key values after garbage collection. Most values should
         * be higher, because oldest values (which should have been garbage collected first)
         * have lower values. If verbose output is enabled, then we will print the statistics
         * before to perform the actual check in order to allow the developer to have more
         * information in case of failure.
         *
         * The mean value is often greater, but not always. Since we have fewer remaining values
         * (100 instead of 10000), the remaining low values will have a much greater impact on
         * the mean. Only the check on the minimal value is fully reliable.
         */
        final Statistics afterGC = validateStressEntries("After GC", cache);
        out.println("Statistics on the keys before and after garbage collection.");
        out.println("The minimum value should be greater after GC.");
        final StatisticsFormat format = StatisticsFormat.getInstance();
        format.setBorderWidth(1);
        try {
            format.format(new Statistics[] {beforeGC, afterGC}, out);
        } catch (IOException e) {
            throw new AssertionError(e);
        }
        assertTrue("Minimum key value should be greater after garbage collection.",
                afterGC.minimum() >= beforeGC.minimum());
    }

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     * @param  name  The name of the value being measured.
     * @param  cache The cache to validate.
     * @return Statistics on the key values of the given map.
     */
    private static Statistics validateStressEntries(final String name, final Map<Integer,Integer> cache) {
        final Statistics statistics = new Statistics(name);
        for (final Map.Entry<Integer,Integer> entry : cache.entrySet()) {
            final int key = entry.getKey();
            final int value = entry.getValue();
            assertEquals(key*key, value);
            statistics.accept(key);
        }
        return statistics;
    }

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        for (int i=0; i<threads.length; i++) threads[i].join();
        TestUtilities.rethrownIfNotNull(failures.get());
        /*
         * Verifies the values.
         */
        final Statistics beforeGC = validateStressEntries("Before GC", cache);
        assertTrue("Should not have more entries than what we put in.", cache.size() <= count);
        assertFalse("Some entries should be retained by strong references.", cache.isEmpty());
        /*
         * If verbose test output is enabled, report the number of cache hits.
         * The numbers below are for tuning the test only. The output is somewhat
         * random so we can not check it in a test suite.  However if the test is
         * properly tuned, most values should be non-zero.
         */
        final PrintWriter out = CacheTest.out;
        TestUtilities.printSeparator("CacheTest.stress() - testing concurrent accesses");
        out.print("There is "); out.print(threads.length); out.print(" threads, each of them"
                + " fetching or creating "); out.print(count); out.println(" values.");
        out.println("Number of times a new value has been created, for each thread:");
        for (int i=0; i<threads.length;) {
            final String n = String.valueOf(threads[i++].addCount);
            out.print(CharSequences.spaces(6 - n.length()));
            out.print(n);
            if ((i % 10) == 0) {
                out.println();
            }
        }
        out.println();
        out.println("Now observe how the background thread cleans the cache.");
        long time = System.nanoTime();
        for (int i=0; i<10; i++) {
            final long t = System.nanoTime();
            out.printf("Cache size: %4d (after %3d ms)%n", cache.size(), round((t - time) / 1E+6));
            time = t;
            Thread.sleep(250);
            if (i >= 2) {
                System.gc();
            }
        }
        out.println();
        /*
         * Gets the statistics of key values after garbage collection. Most values should
         * be higher, because oldest values (which should have been garbage collected first)
         * have lower values. If verbose output is enabled, then we will print the statistics
         * before to perform the actual check in order to allow the developer to have more
         * information in case of failure.
         *
         * The mean value is often greater, but not always. Since we have fewer remaining values
         * (100 instead of 10000), the remaining low values will have a much greater impact on
         * the mean. Only the check on the minimal value is fully reliable.
         */
        final Statistics afterGC = validateStressEntries("After GC", cache);
        out.println("Statistics on the keys before and after garbage collection.");
        out.println("The minimum value shall always be equals or greater after GC.");
        out.println("The mean value is usually greater too, except by coincidence.");
        final StatisticsFormat format = StatisticsFormat.getInstance();
        format.setBorderWidth(1);
        try {
            format.format(new Statistics[] {beforeGC, afterGC}, out);
        } catch (IOException e) {
            throw new AssertionError(e);
        }
        assertTrue("Minimum key value should be greater after garbage collection.",
                afterGC.minimum() >= beforeGC.minimum());
    }

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Related Classes of org.apache.sis.math.Statistics

org.apache.sis.util.collection.CacheTest

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