poi/src/java/org/apache/poi/ss/formula/functions/FactDouble.java

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   this work for additional information regarding copyright ownership.
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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.<p/>
 * <p/>
 * <b>Syntax</b>:<br/> <b>FACTDOUBLE  </b>(<b>number</b>)<br/>
 * <p/>
 * Returns the double factorial of a number.
 * <p/>
 * Number is the value for which to return the double factorial. If number is not an integer, it is truncated.
 * <p/>
 * Remarks
 * <ul>
 * <li>If number is nonnumeric, FACTDOUBLE returns the #VALUE! error value.</li>
 * <li>If number is negative, FACTDOUBLE returns the #NUM! error value.</li>
 * </ul>
 * 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<Integer, BigInteger> cache = new HashMap<Integer, BigInteger>();

    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]);
    }
}