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RFC 2445 defines protocols for interoperability between calendar applications, and this library provides java implementations for a number of RFC 2445 primitives including those that describe how events repeat. Start by taking alook at the compat packages.

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// Copyright (C) 2006 Google Inc.
//
// Licensed 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.jresearch.ical.values;

import org.jresearch.ical.util.TimeUtils;

/**
 * days of the week enum.  Names correspond to RFC2445 literals.
 *
 * @author [email protected] (Mike Samuel)
 */
public enum Weekday {
  SU(0),
  MO(1),
  TU(2),
  WE(3),
  TH(4),
  FR(5),
  SA(6),
  ;

  /**
   * agrees with values returned by the javascript builtin Date.getDay and the
   * corresponding ical.js function/method, but is one less than the Java
   * Calendar's DAY_OF_WEEK values.
   */
  public final int jsDayNum;
  /**
   * agrees with the java weekday values as found in java.util.Calendar.
   */
  public final int javaDayNum;

  Weekday(int wDayNum) {
    this.jsDayNum = wDayNum;
    this.javaDayNum = 1 + wDayNum;
  }

  private static Weekday[] VALUES = new Weekday[7];

  static {
    System.arraycopy(values(), 0, VALUES, 0, 7);
  }

  public static Weekday valueOf(DateValue dv) {
    int dayIndex =
      TimeUtils.fixedFromGregorian(dv.year(), dv.month(), dv.day()) % 7;
    if (dayIndex < 0) { dayIndex += 7; }
    return VALUES[dayIndex];
  }

  public static Weekday firstDayOfWeekInMonth(int year, int month) {
    int result = TimeUtils.fixedFromGregorian(year, month, 1) % 7;
    return VALUES[(result >= 0) ? result : result + 7];
  }

  public Weekday successor() {
    return VALUES[(ordinal() + 1) % 7];
  }

  public Weekday predecessor() {
    return VALUES[(ordinal() - 1 + 7) % 7];
  }

}