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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
 *    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.poi.ss.formula.functions;

import java.util.Calendar;

import org.apache.poi.ss.formula.eval.BlankEval;
import org.apache.poi.ss.formula.eval.ErrorEval;
import org.apache.poi.ss.formula.eval.EvaluationException;
import org.apache.poi.ss.formula.eval.MissingArgEval;
import org.apache.poi.ss.formula.eval.NumberEval;
import org.apache.poi.ss.formula.eval.OperandResolver;
import org.apache.poi.ss.formula.eval.ValueEval;
import org.apache.poi.ss.usermodel.DateUtil;


/**
 * Implementation for the Excel function WEEKDAY
 *
 * @author Thies Wellpott
 */
public final class WeekdayFunc implements Function {
//or:  extends Var1or2ArgFunction {

	public static final Function instance = new WeekdayFunc();

	private WeekdayFunc() {
		// no fields to initialise
	}

	/* for Var1or2ArgFunction:
	@Override
	public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval arg0) {
	}

	@Override
	public ValueEval evaluate(int srcRowIndex, int srcColumnIndex, ValueEval arg0, ValueEval arg1) {
	}
	*/


	/**
	 * Perform WEEKDAY(date, returnOption) function.
	 * Note: Parameter texts are from German EXCEL-2010 help.
	 * Parameters in args[]:
	 *  args[0] serialDate
	 * EXCEL-date value
	 * Standardmaessig ist der 1. Januar 1900 die fortlaufende Zahl 1 und
	 * der 1. Januar 2008 die fortlaufende Zahl 39.448, da dieser Tag nach 39.448 Tagen
	 * auf den 01.01.1900 folgt.
	 * @return Option (optional)
	 * Bestimmt den Rueckgabewert:
		1	oder nicht angegeben Zahl 1 (Sonntag) bis 7 (Samstag). Verhaelt sich wie fruehere Microsoft Excel-Versionen.
		2	Zahl 1 (Montag) bis 7 (Sonntag).
		3	Zahl 0 (Montag) bis 6 (Sonntag).
		11	Die Zahlen 1 (Montag) bis 7 (Sonntag)
		12	Die Zahlen 1 (Dienstag) bis 7 (Montag)
		13	Die Zahlen 1 (Mittwoch) bis 7 (Dienstag)
		14	Die Zahlen 1 (Donnerstag) bis 7 (Mittwoch)
		15	Die Zahlen 1 (Freitag) bis 7 (Donnerstag)
		16	Die Zahlen 1 (Samstag) bis 7 (Freitag)
		17	Die Zahlen 1 (Sonntag) bis 7 (Samstag)
	 */
	public ValueEval evaluate(ValueEval[] args, int srcRowIndex, int srcColumnIndex) {
		try {
			if (args.length < 1  ||  args.length > 2) {
				return ErrorEval.VALUE_INVALID;
			}

			// extract first parameter
			ValueEval serialDateVE = OperandResolver.getSingleValue(args[0], srcRowIndex, srcColumnIndex);
			double serialDate = OperandResolver.coerceValueToDouble(serialDateVE);
			if (!DateUtil.isValidExcelDate(serialDate)) {
				return ErrorEval.NUM_ERROR;						// EXCEL uses this and no VALUE_ERROR
			}
			Calendar date = DateUtil.getJavaCalendar(serialDate, false);		// (XXX 1904-windowing not respected)
			int weekday = date.get(Calendar.DAY_OF_WEEK);		// => sunday = 1, monday = 2, ..., saturday = 7

			// extract second parameter
			int returnOption = 1;					// default value
			if (args.length == 2) {
				ValueEval ve = OperandResolver.getSingleValue(args[1], srcRowIndex, srcColumnIndex);
				if (ve == MissingArgEval.instance  ||  ve == BlankEval.instance) {
					return ErrorEval.NUM_ERROR;		// EXCEL uses this and no VALUE_ERROR
				}
				returnOption = OperandResolver.coerceValueToInt(ve);
				if (returnOption == 2) {
					returnOption = 11;				// both mean the same
				}
			} // if

			// perform calculation
			double result;
			if (returnOption == 1) {
				result = weekday;
			// value 2 is handled above (as value 11)
			} else if (returnOption == 3) {
				result = (weekday + 6 - 1) % 7;
			} else if (returnOption >= 11  &&  returnOption <= 17) {
				result = (weekday + 6 - (returnOption - 10)) % 7 + 1;		// rotate in the value range 1 to 7
			} else {
				return ErrorEval.NUM_ERROR;		// EXCEL uses this and no VALUE_ERROR
			}

			return new NumberEval(result);
		} catch (EvaluationException e) {
			return e.getErrorEval();
		}
	} // evaluate()

}