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International Component for Unicode for Java (ICU4J) is a mature, widely used Java library providing Unicode and Globalization support

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/*
 *******************************************************************************
 * Copyright (C) 2009,2016 International Business Machines Corporation and         *
 * others. All Rights Reserved.                                                *
 *******************************************************************************
 */
package com.ibm.icu.impl;

import java.util.MissingResourceException;

import com.ibm.icu.util.ULocale;
import com.ibm.icu.util.UResourceBundle;

/**
 * Calendar utilities.
 * 
 * Date/time format service classes in com.ibm.icu.text packages
 * sometimes need to access calendar internal APIs.  But calendar
 * classes are in com.ibm.icu.util package, so the package local
 * cannot be used.  This class is added in com.ibm.icu.impl
 * package for sharing some calendar internal code for calendar
 * and date format.
 */
public class CalendarUtil {

    private static ICUCache CALTYPE_CACHE = new SimpleCache();

    private static final String CALKEY = "calendar";
    private static final String DEFCAL = "gregorian";

    /**
     * Returns a calendar type for the given locale.
     * When the given locale has calendar keyword, the
     * value of calendar keyword is returned.  Otherwise,
     * the default calendar type for the locale is returned.
     * @param loc The locale
     * @return Calendar type string, such as "gregorian"
     */
    public static String getCalendarType(ULocale loc) {
        String calType = null;

        calType = loc.getKeywordValue(CALKEY);
        if (calType != null) {
            return calType;
        }

        // Canonicalize, so grandfathered variant will be transformed to keywords
        ULocale canonical = ULocale.createCanonical(loc.toString());
        calType = canonical.getKeywordValue(CALKEY);
        if (calType != null) {
            return calType;
        }

        // When calendar keyword is not available, use the locale's
        // region to get the default calendar type
        String region = ULocale.getRegionForSupplementalData(canonical, true);

        // Check the cache (now we cache by region, not base locale)
        calType = CALTYPE_CACHE.get(region);
        if (calType != null) {
            return calType;
        }

        // Read supplementalData to get the default calendar type for
        // the locale's region
        try {
            UResourceBundle rb = UResourceBundle.getBundleInstance(
                                    ICUResourceBundle.ICU_BASE_NAME,
                                    "supplementalData",
                                    ICUResourceBundle.ICU_DATA_CLASS_LOADER);
            UResourceBundle calPref = rb.get("calendarPreferenceData");
            UResourceBundle order = null;
            try {
                order = calPref.get(region);
            } catch (MissingResourceException mre) {
                // use "001" as fallback
                order = calPref.get("001");
            }
            // the first calendar type is the default for the region
            calType = order.getString(0);
        } catch (MissingResourceException mre) {
            // fall through
        }

        if (calType == null) {
            // Use "gregorian" as the last resort fallback.
            calType = DEFCAL;
        }

        // Cache the resolved value for the next time
        CALTYPE_CACHE.put(region, calType);

        return calType;
    }
}