/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.commons.math4.analysis.integration.gauss;

import java.util.List;
import java.util.ArrayList;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.Callable;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.atomic.AtomicInteger;

import org.apache.commons.math4.util.Pair;
import org.junit.Test;
import org.junit.Assert;

/**
 * Test for {@link BaseRuleFactory}.
 *
 */
public class BaseRuleFactoryTest {
    /**
     * Tests that a given rule rule will be computed and added once to the cache
     * whatever the number of times this rule is called concurrently.
     */
    @Test
        public void testConcurrentCreation() throws InterruptedException,
                                                    ExecutionException {
        // Number of times the same rule will be called.
        final int numTasks = 20;

        final ThreadPoolExecutor exec
            = new ThreadPoolExecutor(3, numTasks, 1, TimeUnit.SECONDS,
                                     new ArrayBlockingQueue<Runnable>(2));

        final List<Future<Pair<double[], double[]>>> results
            = new ArrayList<>();
        for (int i = 0; i < numTasks; i++) {
            results.add(exec.submit(new RuleBuilder()));
        }

        // Ensure that all computations have completed.
        for (Future<Pair<double[], double[]>> f : results) {
            f.get();
        }
        exec.shutdown();

        // Assertion would fail if "getRuleInternal" were not "synchronized".
        final int n = RuleBuilder.getNumberOfCalls();
        Assert.assertEquals("Rule computation was called " + n + " times", 1, n);
    }
}

class RuleBuilder implements Callable<Pair<double[], double[]>> {
    private static final DummyRuleFactory factory = new DummyRuleFactory();

    @Override
    public Pair<double[], double[]> call() {
        final int dummy = 2; // Always request the same rule.
        return factory.getRule(dummy);
    }

    public static int getNumberOfCalls() {
        return factory.getNumberOfCalls();
    }
}

class DummyRuleFactory extends BaseRuleFactory<Double> {
    /** Rule computations counter. */
    private static AtomicInteger nCalls = new AtomicInteger();

    @Override
    protected Pair<Double[], Double[]> computeRule(int order) {
        // Tracks whether this computation has been called more than once.
        nCalls.getAndIncrement();

        try {
            // Sleep to simulate computation time.
            Thread.sleep(20);
        } catch (InterruptedException e) {
            Assert.fail("Unexpected interruption");
        }

         // Dummy rule (but contents must exist).
        final Double[] p = new Double[order];
        final Double[] w = new Double[order];
        for (int i = 0; i < order; i++) {
            p[i] = new Double(i);
            w[i] = new Double(i);
        }
        return new Pair<>(p, w);
    }

    public int getNumberOfCalls() {
        return nCalls.get();
    }
}