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/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * Copyright (c) 2009-2012, Stephen Colebourne & Michael Nascimento Santos

 *

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions are met:

 *

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 *    this list of conditions and the following disclaimer.

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package java.time.zone;

import java.security.AccessController;

import java.security.PrivilegedAction;

import java.time.ZoneId;

import java.time.ZonedDateTime;

import java.util.ArrayList;

import java.util.HashSet;

import java.util.Iterator;

import java.util.List;

import java.util.NavigableMap;

import java.util.Objects;

import java.util.ServiceConfigurationError;

import java.util.ServiceLoader;

import java.util.Set;

import java.util.concurrent.ConcurrentHashMap;

import java.util.concurrent.ConcurrentMap;

import java.util.concurrent.CopyOnWriteArrayList;

import java.util.Collections;

/**

 * Provider of time-zone rules to the system.

 * <p>

 * This class manages the configuration of time-zone rules.

 * The static methods provide the public API that can be used to manage the providers.

 * The abstract methods provide the SPI that allows rules to be provided.

 * <p>

 * ZoneRulesProvider may be installed in an instance of the Java Platform as

 * extension classes, that is, jar files placed into any of the usual extension

 * directories. Installed providers are loaded using the service-provider loading

 * facility defined by the {@link ServiceLoader} class. A ZoneRulesProvider

 * identifies itself with a provider configuration file named

 * {@code java.time.zone.ZoneRulesProvider} in the resource directory

 * {@code META-INF/services}. The file should contain a line that specifies the

 * fully qualified concrete zonerules-provider class name.

 * Providers may also be made available by adding them to the class path or by

 * registering themselves via {@link #registerProvider} method.

 * <p>

 * The Java virtual machine has a default provider that provides zone rules

 * for the time-zones defined by IANA Time Zone Database (TZDB). If the system

 * it is taken to be the fully-qualified name of a concrete ZoneRulesProvider

 * class to be loaded as the default provider, using the system class loader.

 * If this system property is not defined, a system-default provider will be

 * loaded to serve as the default provider.

 * <p>

 * Rules are looked up primarily by zone ID, as used by {@link ZoneId}.

 * Only zone region IDs may be used, zone offset IDs are not used here.

 * <p>

 * Time-zone rules are political, thus the data can change at any time.

 * Each provider will provide the latest rules for each zone ID, but they

 * may also provide the history of how the rules changed.

 *

 * @implSpec

 * This interface is a service provider that can be called by multiple threads.

 * Implementations must be immutable and thread-safe.

 * <p>

 * Providers must ensure that once a rule has been seen by the application, the

 * rule must continue to be available.

 * <p>

 * Providers are encouraged to implement a meaningful {@code toString} method.

 * <p>

 * Many systems would like to update time-zone rules dynamically without stopping the JVM.

 * When examined in detail, this is a complex problem.

 * Providers may choose to handle dynamic updates, however the default provider does not.

 *

 * @since 1.8

 */

public abstract class ZoneRulesProvider {

    /**

     * The set of loaded providers.

     */

    private static final CopyOnWriteArrayList<ZoneRulesProvider> PROVIDERS = new CopyOnWriteArrayList<>();

    /**

     * The lookup from zone ID to provider.

     */

    private static final ConcurrentMap<String, ZoneRulesProvider> ZONES = new ConcurrentHashMap<>(512, 0.75f, 2);

    /**

     * The zone ID data

     */

    private static volatile Set<String> ZONE_IDS;

    static {

        // if the property java.time.zone.DefaultZoneRulesProvider is

        // set then its value is the class name of the default provider

        final List<ZoneRulesProvider> loaded = new ArrayList<>();

        AccessController.doPrivileged(new PrivilegedAction<>() {

            public Object run() {

                String prop = System.getProperty("java.time.zone.DefaultZoneRulesProvider");

                if (prop != null) {

                    try {

                        Class<?> c = Class.forName(prop, true, ClassLoader.getSystemClassLoader());

                        @SuppressWarnings("deprecation")

                        ZoneRulesProvider provider = ZoneRulesProvider.class.cast(c.newInstance());

                        registerProvider(provider);

                        loaded.add(provider);

                    } catch (Exception x) {

                        throw new Error(x);

                    }

                } else {

                    registerProvider(new TzdbZoneRulesProvider());

                }

                return null;

            }

        });

        ServiceLoader<ZoneRulesProvider> sl = ServiceLoader.load(ZoneRulesProvider.class, ClassLoader.getSystemClassLoader());

        Iterator<ZoneRulesProvider> it = sl.iterator();

        while (it.hasNext()) {

            ZoneRulesProvider provider;

            try {

                provider = it.next();

            } catch (ServiceConfigurationError ex) {

                if (ex.getCause() instanceof SecurityException) {

                    continue;  // ignore the security exception, try the next provider

                }

                throw ex;

            }

            boolean found = false;

            for (ZoneRulesProvider p : loaded) {

                if (p.getClass() == provider.getClass()) {

                    found = true;

                }

            }

            if (!found) {

                registerProvider0(provider);

                loaded.add(provider);

            }

        }

        // CopyOnWriteList could be slow if lots of providers and each added individually

        PROVIDERS.addAll(loaded);

    }

    //-------------------------------------------------------------------------

    /**

     * Gets the set of available zone IDs.

     * <p>

     * These IDs are the string form of a {@link ZoneId}.

     *

     * @return the unmodifiable set of zone IDs, not null

     */

    public static Set<String> getAvailableZoneIds() {

        return ZONE_IDS;

    }

    /**

     * Gets the rules for the zone ID.

     * <p>

     * This returns the latest available rules for the zone ID.

     * <p>

     * This method relies on time-zone data provider files that are configured.

     * These are loaded using a {@code ServiceLoader}.

     * <p>

     * The caching flag is designed to allow provider implementations to

     * prevent the rules being cached in {@code ZoneId}.

     * Under normal circumstances, the caching of zone rules is highly desirable

     * as it will provide greater performance. However, there is a use case where

     * the caching would not be desirable, see {@link #provideRules}.

     *

     * @param zoneId the zone ID as defined by {@code ZoneId}, not null

     * @param forCaching whether the rules are being queried for caching,

     * true if the returned rules will be cached by {@code ZoneId},

     * false if they will be returned to the user without being cached in {@code ZoneId}

     * @return the rules, null if {@code forCaching} is true and this

     * is a dynamic provider that wants to prevent caching in {@code ZoneId},

     * otherwise not null

     * @throws ZoneRulesException if rules cannot be obtained for the zone ID

     */

    public static ZoneRules getRules(String zoneId, boolean forCaching) {

        Objects.requireNonNull(zoneId, "zoneId");

        return getProvider(zoneId).provideRules(zoneId, forCaching);

    }

    /**

     * Gets the history of rules for the zone ID.

     * <p>

     * Time-zones are defined by governments and change frequently.

     * This method allows applications to find the history of changes to the

     * rules for a single zone ID. The map is keyed by a string, which is the

     * version string associated with the rules.

     * <p>

     * The exact meaning and format of the version is provider specific.

     * The version must follow lexicographical order, thus the returned map will

     * be order from the oldest known rules to the newest available rules.

     * The default 'TZDB' group uses version numbering consisting of the year

     * followed by a letter, such as '2009e' or '2012f'.

     * <p>

     * Implementations must provide a result for each valid zone ID, however

     * they do not have to provide a history of rules.

     * Thus the map will always contain one element, and will only contain more

     * than one element if historical rule information is available.

     *

     * @param zoneId  the zone ID as defined by {@code ZoneId}, not null

     * @return a modifiable copy of the history of the rules for the ID, sorted

     *  from oldest to newest, not null

     * @throws ZoneRulesException if history cannot be obtained for the zone ID

     */

    public static NavigableMap<String, ZoneRules> getVersions(String zoneId) {

        Objects.requireNonNull(zoneId, "zoneId");

        return getProvider(zoneId).provideVersions(zoneId);

    }

    /**

     * Gets the provider for the zone ID.

     *

     * @param zoneId  the zone ID as defined by {@code ZoneId}, not null

     * @return the provider, not null

     * @throws ZoneRulesException if the zone ID is unknown

     */

    private static ZoneRulesProvider getProvider(String zoneId) {

        ZoneRulesProvider provider = ZONES.get(zoneId);

        if (provider == null) {

            if (ZONES.isEmpty()) {

                throw new ZoneRulesException("No time-zone data files registered");

            }

            throw new ZoneRulesException("Unknown time-zone ID: " + zoneId);

        }

        return provider;

    }

    //-------------------------------------------------------------------------

    /**

     * Registers a zone rules provider.

     * <p>

     * This adds a new provider to those currently available.

     * A provider supplies rules for one or more zone IDs.

     * A provider cannot be registered if it supplies a zone ID that has already been

     * registered. See the notes on time-zone IDs in {@link ZoneId}, especially

     * the section on using the concept of a "group" to make IDs unique.

     * <p>

     * To ensure the integrity of time-zones already created, there is no way

     * to deregister providers.

     *

     * @param provider  the provider to register, not null

     * @throws ZoneRulesException if a zone ID is already registered

     */

    public static void registerProvider(ZoneRulesProvider provider) {

        Objects.requireNonNull(provider, "provider");

        registerProvider0(provider);

        PROVIDERS.add(provider);

    }

    /**

     * Registers the provider.

     *

     * @param provider  the provider to register, not null

     * @throws ZoneRulesException if unable to complete the registration

     */

    private static synchronized void registerProvider0(ZoneRulesProvider provider) {

        for (String zoneId : provider.provideZoneIds()) {

            Objects.requireNonNull(zoneId, "zoneId");

            ZoneRulesProvider old = ZONES.putIfAbsent(zoneId, provider);

            if (old != null) {

                throw new ZoneRulesException(

                    "Unable to register zone as one already registered with that ID: " + zoneId +

                    ", currently loading from provider: " + provider);

            }

        }

        Set<String> combinedSet = new HashSet<String>(ZONES.keySet());

        ZONE_IDS = Collections.unmodifiableSet(combinedSet);

    }

    /**

     * Refreshes the rules from the underlying data provider.

     * <p>

     * This method allows an application to request that the providers check

     * for any updates to the provided rules.

     * After calling this method, the offset stored in any {@link ZonedDateTime}

     * may be invalid for the zone ID.

     * <p>

     * Dynamic update of rules is a complex problem and most applications

     * should not use this method or dynamic rules.

     * To achieve dynamic rules, a provider implementation will have to be written

     * as per the specification of this class.

     * In addition, instances of {@code ZoneRules} must not be cached in the

     * application as they will become stale. However, the boolean flag on

     * {@link #provideRules(String, boolean)} allows provider implementations

     * to control the caching of {@code ZoneId}, potentially ensuring that

     * all objects in the system see the new rules.

     * Note that there is likely to be a cost in performance of a dynamic rules

     * provider. Note also that no dynamic rules provider is in this specification.

     *

     * @return true if the rules were updated

     * @throws ZoneRulesException if an error occurs during the refresh

     */

    public static boolean refresh() {

        boolean changed = false;

        for (ZoneRulesProvider provider : PROVIDERS) {

            changed |= provider.provideRefresh();

        }

        return changed;

    }

    /**

     * Constructor.

     */

    protected ZoneRulesProvider() {

    }

    //-----------------------------------------------------------------------

    /**

     * SPI method to get the available zone IDs.

     * <p>

     * This obtains the IDs that this {@code ZoneRulesProvider} provides.

     * A provider should provide data for at least one zone ID.

     * <p>

     * The returned zone IDs remain available and valid for the lifetime of the application.

     * A dynamic provider may increase the set of IDs as more data becomes available.

     *

     * @return the set of zone IDs being provided, not null

     * @throws ZoneRulesException if a problem occurs while providing the IDs

     */

    protected abstract Set<String> provideZoneIds();

    /**

     * SPI method to get the rules for the zone ID.

     * <p>

     * This loads the rules for the specified zone ID.

     * The provider implementation must validate that the zone ID is valid and

     * available, throwing a {@code ZoneRulesException} if it is not.

     * The result of the method in the valid case depends on the caching flag.

     * <p>

     * If the provider implementation is not dynamic, then the result of the

     * method must be the non-null set of rules selected by the ID.

     * <p>

     * If the provider implementation is dynamic, then the flag gives the option

     * of preventing the returned rules from being cached in {@link ZoneId}.

     * When the flag is true, the provider is permitted to return null, where

     * null will prevent the rules from being cached in {@code ZoneId}.

     * When the flag is false, the provider must return non-null rules.

     *

     * @param zoneId the zone ID as defined by {@code ZoneId}, not null

     * @param forCaching whether the rules are being queried for caching,

     * true if the returned rules will be cached by {@code ZoneId},

     * false if they will be returned to the user without being cached in {@code ZoneId}

     * @return the rules, null if {@code forCaching} is true and this

     * is a dynamic provider that wants to prevent caching in {@code ZoneId},

     * otherwise not null

     * @throws ZoneRulesException if rules cannot be obtained for the zone ID

     */

    protected abstract ZoneRules provideRules(String zoneId, boolean forCaching);

    /**

     * SPI method to get the history of rules for the zone ID.

     * <p>

     * This returns a map of historical rules keyed by a version string.

     * The exact meaning and format of the version is provider specific.

     * The version must follow lexicographical order, thus the returned map will

     * be order from the oldest known rules to the newest available rules.

     * The default 'TZDB' group uses version numbering consisting of the year

     * followed by a letter, such as '2009e' or '2012f'.

     * <p>

     * Implementations must provide a result for each valid zone ID, however

     * they do not have to provide a history of rules.

     * Thus the map will contain at least one element, and will only contain

     * more than one element if historical rule information is available.

     * <p>

     * The returned versions remain available and valid for the lifetime of the application.

     * A dynamic provider may increase the set of versions as more data becomes available.

     *

     * @param zoneId  the zone ID as defined by {@code ZoneId}, not null

     * @return a modifiable copy of the history of the rules for the ID, sorted

     *  from oldest to newest, not null

     * @throws ZoneRulesException if history cannot be obtained for the zone ID

     */

    protected abstract NavigableMap<String, ZoneRules> provideVersions(String zoneId);

    /**

     * SPI method to refresh the rules from the underlying data provider.

     * <p>

     * This method provides the opportunity for a provider to dynamically

     * recheck the underlying data provider to find the latest rules.

     * This could be used to load new rules without stopping the JVM.

     * Dynamic behavior is entirely optional and most providers do not support it.

     * <p>

     * This implementation returns false.

     *

     * @return true if the rules were updated

     * @throws ZoneRulesException if an error occurs during the refresh

     */

    protected boolean provideRefresh() {

        return false;

    }

}