main/src/main/java/org/apache/poi/ss/formula/atp/WorkdayFunction.java

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   this work for additional information regarding copyright ownership.
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package org.apache.poi.ss.formula.atp;

import org.apache.poi.ss.formula.OperationEvaluationContext;
import org.apache.poi.ss.formula.eval.ErrorEval;
import org.apache.poi.ss.formula.eval.EvaluationException;
import org.apache.poi.ss.formula.eval.NumberEval;
import org.apache.poi.ss.formula.eval.ValueEval;
import org.apache.poi.ss.formula.functions.FreeRefFunction;
import org.apache.poi.ss.usermodel.DateUtil;

/**
 * Implementation of Excel 'Analysis ToolPak' function WORKDAY()<br>
 * Returns the date past a number of workdays beginning at a start date, considering an interval of holidays. A workday is any non
 * saturday/sunday date.
 * <p>
 * <b>Syntax</b><br>
 * <b>WORKDAY</b>(<b>startDate</b>, <b>days</b>, holidays)
 * <p>
 * 
 * @author jfaenomoto@gmail.com
 */
final class WorkdayFunction implements FreeRefFunction {

    public static final FreeRefFunction instance = new WorkdayFunction(ArgumentsEvaluator.instance);

    private ArgumentsEvaluator evaluator;

    private WorkdayFunction(ArgumentsEvaluator anEvaluator) {
        // enforces singleton
        this.evaluator = anEvaluator;
    }

    /**
     * Evaluate for WORKDAY. Given a date, a number of days and a optional date or interval of holidays, determines which date it is past
     * number of parametrized workdays.
     * 
     * @return {@link ValueEval} with date as its value.
     */
    public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
        if (args.length < 2 || args.length > 3) {
            return ErrorEval.VALUE_INVALID;
        }

        int srcCellRow = ec.getRowIndex();
        int srcCellCol = ec.getColumnIndex();

        double start;
        int days;
        double[] holidays;
        try {
            start = this.evaluator.evaluateDateArg(args[0], srcCellRow, srcCellCol);
            days = (int) Math.floor(this.evaluator.evaluateNumberArg(args[1], srcCellRow, srcCellCol));
            ValueEval holidaysCell = args.length == 3 ? args[2] : null;
            holidays = this.evaluator.evaluateDatesArg(holidaysCell, srcCellRow, srcCellCol);
            return new NumberEval(DateUtil.getExcelDate(WorkdayCalculator.instance.calculateWorkdays(start, days, holidays)));
        } catch (EvaluationException e) {
            return ErrorEval.VALUE_INVALID;
        }
    }

}