commons-numbers-combinatorics/src/test/java/org/apache/commons/numbers/combinatorics/LogBinomialCoefficientTest.java

/*
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 * 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,
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 * See the License for the specific language governing permissions and
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package org.apache.commons.numbers.combinatorics;

import org.junit.Assert;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;

/**
 * Test cases for the {@link LogBinomialCoefficient} class.
 */
public class LogBinomialCoefficientTest {
    /** Verify that b(0,0) = 1 */
    @Test
    public void test0Choose0() {
        Assert.assertEquals(LogBinomialCoefficient.value(0, 0), 0d, 0);
    }

    @Test
    public void testBinomialCoefficient() {
        final long[] bcoef5 = { 1, 5, 10, 10, 5, 1 };
        final long[] bcoef6 = { 1, 6, 15, 20, 15, 6, 1 };

        for (int n = 1; n < 10; n++) {
            for (int k = 0; k <= n; k++) {
                Assert.assertEquals(n + " choose " + k,
                                    Math.log(BinomialCoefficientTest.binomialCoefficient(n, k)),
                                    LogBinomialCoefficient.value(n, k), 1e-12);
            }
        }

        final int[] n = { 34, 66, 100, 1500, 1500 };
        final int[] k = { 17, 33, 10, 1500 - 4, 4 };
        for (int i = 0; i < n.length; i++) {
            final long expected = BinomialCoefficientTest.binomialCoefficient(n[i], k[i]);
            Assert.assertEquals("log(" + n[i] + " choose " + k[i] + ")",
                                Math.log(expected),
                                LogBinomialCoefficient.value(n[i], k[i]),
                                0d);
        }
    }

    @Test()
    public void testBinomialCoefficientFail1() {
        Assertions.assertThrows(CombinatoricsException.class,
            () -> LogBinomialCoefficient.value(4, 5)
        );
    }

    @Test()
    public void testBinomialCoefficientFail2() {
        Assertions.assertThrows(CombinatoricsException.class,
                () -> LogBinomialCoefficient.value(-1, -2)
        );
    }

    /**
     * Tests correctness for large n and sharpness of upper bound in API doc
     * JIRA: MATH-241
     */
    @Test
    public void testBinomialCoefficientLarge() throws Exception {
        // This tests all legal and illegal values for n <= 200.
        for (int n = 0; n <= 200; n++) {
            for (int k = 0; k <= n; k++) {
                long exactResult = -1;
                boolean shouldThrow = false;
                boolean didThrow = false;
                try {
                    BinomialCoefficient.value(n, k);
                } catch (ArithmeticException ex) {
                    didThrow = true;
                }
                try {
                    exactResult = BinomialCoefficientTest.binomialCoefficient(n, k);
                } catch (ArithmeticException ex) {
                    shouldThrow = true;
                }

                if (!shouldThrow && exactResult > 1) {
                    Assert.assertEquals(n + " choose " + k, 1,
                                        LogBinomialCoefficient.value(n, k) / Math.log(exactResult), 1e-10);
                }
            }
        }

        final int n = 10000;
        final double actualOverExpected = LogBinomialCoefficient.value(n, 3) /
            Math.log(BinomialCoefficientTest.binomialCoefficient(n, 3));
        Assert.assertEquals(1, actualOverExpected, 1e-10);
    }
}