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The Apache Commons Codec package contains simple encoder and decoders for various formats such as Base64 and Hexadecimal. In addition to these widely used encoders and decoders, the codec package also maintains a collection of phonetic encoding utilities.

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/* ====================================================================
   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
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       http://www.apache.org/licenses/LICENSE-2.0

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   distributed under the License is distributed on an "AS IS" BASIS,
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package org.apache.poi.ss.formula.functions;

import org.apache.poi.ss.formula.OperationEvaluationContext;
import org.apache.poi.ss.formula.eval.*;

import java.math.BigInteger;
import java.util.HashMap;

/**
 * Implementation for Excel FACTDOUBLE() function.
 * 

 * Syntax:
 FACTDOUBLE  (number)

 * 

 * Returns the double factorial of a number.
 * 

 * Number is the value for which to return the double factorial. If number is not an integer, it is truncated.
 * 

 * Remarks
 * 

 * If number is nonnumeric, FACTDOUBLE returns the #VALUE! error value.
 * If number is negative, FACTDOUBLE returns the #NUM! error value.
 * 
 * Use a cache for more speed of previously calculated factorial
 *
 * @author cedric dot walter @ gmail dot com
 */
public class FactDouble extends Fixed1ArgFunction implements FreeRefFunction {

    public static final FreeRefFunction instance = new FactDouble();

    //Caching of previously calculated factorial for speed
    static HashMap cache = new HashMap<>();

    public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval numberVE) {
        int number;
        try {
            number = OperandResolver.coerceValueToInt(numberVE);
        } catch (EvaluationException e) {
            return ErrorEval.VALUE_INVALID;
        }

        if (number < 0) {
            return ErrorEval.NUM_ERROR;
        }

        return new NumberEval(factorial(number).longValue());
    }

    public static BigInteger factorial(int n) {
        if (n == 0 || n < 0) {
            return BigInteger.ONE;
        }

        if (cache.containsKey(n))  {
            return cache.get(n);
        }

        BigInteger result = BigInteger.valueOf(n).multiply(factorial(n - 2));
        cache.put(n, result);
        return result;
    }

    public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
        if (args.length != 1) {
            return ErrorEval.VALUE_INVALID;
        }
        return evaluate(ec.getRowIndex(), ec.getColumnIndex(), args[0]);
    }
}