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A Java library for reading and writing iCalendar (*.ics) files
There is a newer version: 4.0.8
package net.fortuna.ical4j.model;

import net.fortuna.ical4j.model.component.Observance;
import net.fortuna.ical4j.model.component.VTimeZone;
import net.fortuna.ical4j.model.property.DtStart;
import net.fortuna.ical4j.model.property.RRule;
import net.fortuna.ical4j.model.property.TzOffsetFrom;
import net.fortuna.ical4j.model.property.TzOffsetTo;

import java.time.Month;
import java.time.*;
import java.time.zone.ZoneOffsetTransition;
import java.time.zone.ZoneOffsetTransitionRule;
import java.time.zone.ZoneOffsetTransitionRule.TimeDefinition;
import java.time.zone.ZoneRules;
import java.util.*;

/**
 * Construct a {@link java.time.zone.ZoneRules} instance from a {@link net.fortuna.ical4j.model.component.VTimeZone}.
 */
public class ZoneRulesBuilder {

    private VTimeZone vTimeZone;

    public ZoneRulesBuilder vTimeZone(VTimeZone vTimeZone) {
        this.vTimeZone = vTimeZone;
        return this;
    }

    private List buildTransitions(List observances) {
        List transitions = new ArrayList<>();
        for (Observance observance : observances) {
            // ignore transitions that have no effect..
            Optional offsetFrom = observance.getProperties().getFirst(Property.TZOFFSETFROM);
            TzOffsetTo offsetTo = observance.getProperties().getRequired(Property.TZOFFSETTO);

            if (offsetFrom.isPresent() && !offsetFrom.get().getOffset().equals(offsetTo.getOffset())) {
                Optional> startDate = observance.getProperties().getFirst(Property.DTSTART);
                if (startDate.isPresent()) {
                    transitions.add(ZoneOffsetTransition.of(startDate.get().getDate(),
                            offsetFrom.get().getOffset(), offsetTo.getOffset()));
                } else {
                    throw new CalendarException("Missing DTSTART property");
                }
            }
        }
        return transitions;
    }

    private Set buildTransitionRules(List observances, ZoneOffset standardOffset) throws ConstraintViolationException {
        Set transitionRules = new HashSet<>();
        for (Observance observance : observances) {
            Optional> rrule = observance.getProperties().getFirst(Property.RRULE);
            TzOffsetFrom offsetFrom = observance.getProperties().getRequired(Property.TZOFFSETFROM);
            TzOffsetTo offsetTo = observance.getProperties().getRequired(Property.TZOFFSETTO);
            DtStart startDate = observance.getProperties().getRequired(Property.DTSTART);

            // ignore invalid rules
            if (rrule.isPresent() && !rrule.get().getRecur().getMonthList().isEmpty()) {
                Month recurMonth = java.time.Month.of(rrule.get().getRecur().getMonthList().get(0).getMonthOfYear());
                int dayOfMonth = rrule.get().getRecur().getDayList().get(0).getOffset();
                if (dayOfMonth == 0) {
                    dayOfMonth = rrule.get().getRecur().getMonthDayList().get(0);
                }
                DayOfWeek dayOfWeek = WeekDay.getDayOfWeek(rrule.get().getRecur().getDayList().get(0));
                LocalTime time = LocalTime.from(startDate.getDate());
                boolean endOfDay = false;
                TimeDefinition timeDefinition = TimeDefinition.UTC;
                transitionRules.add(ZoneOffsetTransitionRule.of(recurMonth, dayOfMonth, dayOfWeek, time, endOfDay,
                        timeDefinition, standardOffset, offsetFrom.getOffset(), offsetTo.getOffset()));
            }
        }
        return transitionRules;
    }

    public ZoneRules build() throws ConstraintViolationException {
        Observance current = VTimeZone.getApplicableObservance(Instant.now(),
                vTimeZone.getObservances().get(Observance.STANDARD));

        // if no standard time use daylight time..
        if (current == null) {
            current = VTimeZone.getApplicableObservance(Instant.now(),
                    vTimeZone.getObservances().get(Observance.DAYLIGHT));
        }

        TzOffsetFrom offsetFrom = current.getProperties().getRequired(Property.TZOFFSETFROM);
        TzOffsetTo offsetTo = current.getProperties().getRequired(Property.TZOFFSETTO);

        ZoneOffset standardOffset = offsetTo.getOffset();
        ZoneOffset wallOffset = offsetFrom.getOffset();
        List standardOffsetTransitions = buildTransitions(
                vTimeZone.getObservances().get(Observance.STANDARD));
        Collections.sort(standardOffsetTransitions);
        List offsetTransitions = buildTransitions(
                vTimeZone.getObservances().get(Observance.DAYLIGHT));
        Collections.sort(offsetTransitions);
        Set transitionRules = buildTransitionRules(
                vTimeZone.getObservances().getAll(), standardOffset);

        return ZoneRules.of(standardOffset, wallOffset, standardOffsetTransitions, offsetTransitions, new ArrayList<>(transitionRules));
    }
}