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Apache Commons Lang, a package of Java utility classes for the classes that are in java.lang's hierarchy, or are considered to be so standard as to justify existence in java.lang.

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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.commons.lang3.time;

import java.util.TimeZone;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Faster methods to produce custom time zones.
 *
 * @since 3.7
 */
public class FastTimeZone {

    private static final Pattern GMT_PATTERN = Pattern.compile("^(?:(?i)GMT)?([+-])?(\\d\\d?)?(:?(\\d\\d?))?$");

    private static final TimeZone GREENWICH = new GmtTimeZone(false, 0, 0);

    /**
     * Gets the GMT TimeZone.
     * @return A TimeZone with a raw offset of zero.
     */
    public static TimeZone getGmtTimeZone() {
        return GREENWICH;
    }

    /**
     * Gets a TimeZone with GMT offsets.  A GMT offset must be either 'Z', or 'UTC', or match
     * (GMT)? hh?(:?mm?)?, where h and m are digits representing hours and minutes.
     *
     * @param pattern The GMT offset
     * @return A TimeZone with offset from GMT or null, if pattern does not match.
     */
    public static TimeZone getGmtTimeZone(final String pattern) {
        if ("Z".equals(pattern) || "UTC".equals(pattern)) {
            return GREENWICH;
        }

        final Matcher m = GMT_PATTERN.matcher(pattern);
        if (m.matches()) {
            final int hours = parseInt(m.group(2));
            final int minutes = parseInt(m.group(4));
            if (hours == 0 && minutes == 0) {
                return GREENWICH;
            }
            return new GmtTimeZone(parseSign(m.group(1)), hours, minutes);
        }
        return null;
    }

    /**
     * Gets a TimeZone, looking first for GMT custom ids, then falling back to Olson ids.
     * A GMT custom id can be 'Z', or 'UTC', or has an optional prefix of GMT,
     * followed by sign, hours digit(s), optional colon(':'), and optional minutes digits.
     * i.e. [GMT] (+|-) Hours [[:] Minutes]
     *
     * @param id A GMT custom id (or Olson id
     * @return A timezone
     */
    public static TimeZone getTimeZone(final String id) {
        final TimeZone tz = getGmtTimeZone(id);
        if (tz != null) {
            return tz;
        }
        return TimeZone.getTimeZone(id);
    }

    private static int parseInt(final String group) {
        return group != null ? Integer.parseInt(group) : 0;
    }

    private static boolean parseSign(final String group) {
        return group != null && group.charAt(0) == '-';
    }

    // do not instantiate
    private FastTimeZone() {
    }

}