/*
 * @notice
 * 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.elasticsearch.plugins.spi;

import org.apache.lucene.util.IOUtils;
import org.apache.lucene.util.SuppressForbidden;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;
import java.net.URLConnection;
import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.Locale;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.ServiceConfigurationError;

/**
 * Helper class for loading SPI classes from classpath (META-INF files).
 * This is a light impl of {@link java.util.ServiceLoader} but is guaranteed to
 * be bug-free regarding classpath order and does not instantiate or initialize
 * the classes found.
 */
@SuppressForbidden(reason = "Copied from Lucene")
public final class SPIClassIterator<S> implements Iterator<Class<? extends S>> {
    private static final String META_INF_SERVICES = "META-INF/services/";

    private final Class<S> clazz;
    private final ClassLoader loader;
    private final Enumeration<URL> profilesEnum;
    private Iterator<String> linesIterator;

    /** Creates a new SPI iterator to lookup services of type {@code clazz} using
     * the same {@link ClassLoader} as the argument. */
    public static <S> SPIClassIterator<S> get(Class<S> clazz) {
        return new SPIClassIterator<>(clazz, Objects.requireNonNull(clazz.getClassLoader(), () -> clazz + " has no classloader."));
    }

    /** Creates a new SPI iterator to lookup services of type {@code clazz} using the given classloader. */
    public static <S> SPIClassIterator<S> get(Class<S> clazz, ClassLoader loader) {
        return new SPIClassIterator<>(clazz, loader);
    }

    /**
     * Utility method to check if some class loader is a (grand-)parent of or the same as another one.
     * This means the child will be able to load all classes from the parent, too.
     * <p>
     * If caller's codesource doesn't have enough permissions to do the check, {@code false} is returned
     * (this is fine, because if we get a {@code SecurityException} it is for sure no parent).
     */
    public static boolean isParentClassLoader(final ClassLoader parent, final ClassLoader child) {
        try {
            ClassLoader cl = child;
            while (cl != null) {
                if (cl == parent) {
                    return true;
                }
                cl = cl.getParent();
            }
            return false;
        } catch (SecurityException se) {
            return false;
        }
    }

    private SPIClassIterator(Class<S> clazz, ClassLoader loader) {
        this.clazz = Objects.requireNonNull(clazz, "clazz");
        this.loader = Objects.requireNonNull(loader, "loader");
        try {
            final String fullName = META_INF_SERVICES + clazz.getName();
            this.profilesEnum = loader.getResources(fullName);
        } catch (IOException ioe) {
            throw new ServiceConfigurationError("Error loading SPI profiles for type " + clazz.getName() + " from classpath", ioe);
        }
        this.linesIterator = Collections.<String>emptySet().iterator();
    }

    private boolean loadNextProfile() {
        ArrayList<String> lines = null;
        while (profilesEnum.hasMoreElements()) {
            if (lines != null) {
                lines.clear();
            } else {
                lines = new ArrayList<>();
            }
            final URL url = profilesEnum.nextElement();
            try {
                URLConnection urlc = url.openConnection();
                urlc.setUseCaches(false); // prevents retaining a handle to the underlying jar file, when the stream is closed
                final InputStream in = urlc.getInputStream();
                boolean success = false;
                try {
                    final BufferedReader reader = new BufferedReader(new InputStreamReader(in, StandardCharsets.UTF_8));
                    String line;
                    while ((line = reader.readLine()) != null) {
                        final int pos = line.indexOf('#');
                        if (pos >= 0) {
                            line = line.substring(0, pos);
                        }
                        line = line.trim();
                        if (line.length() > 0) {
                            lines.add(line);
                        }
                    }
                    success = true;
                } finally {
                    if (success) {
                        IOUtils.close(in);
                    } else {
                        IOUtils.closeWhileHandlingException(in);
                    }
                }
            } catch (IOException ioe) {
                throw new ServiceConfigurationError("Error loading SPI class list from URL: " + url, ioe);
            }
            if (lines.isEmpty() == false) {
                this.linesIterator = lines.iterator();
                return true;
            }
        }
        return false;
    }

    @Override
    public boolean hasNext() {
        return linesIterator.hasNext() || loadNextProfile();
    }

    @Override
    public Class<? extends S> next() {
        // hasNext() implicitely loads the next profile, so it is essential to call this here!
        if (hasNext() == false) {
            throw new NoSuchElementException();
        }
        assert linesIterator.hasNext();
        final String c = linesIterator.next();
        try {
            // don't initialize the class (pass false as 2nd parameter):
            return Class.forName(c, false, loader).asSubclass(clazz);
        } catch (ClassNotFoundException cnfe) {
            throw new ServiceConfigurationError(
                String.format(
                    Locale.ROOT,
                    "An SPI class of type %s with classname %s does not exist, " + "please fix the file '%s%1$s' in your classpath.",
                    clazz.getName(),
                    c,
                    META_INF_SERVICES
                )
            );
        }
    }

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }

}
