/*
 * Copyright 2017-2020 original authors
 *
 * Licensed 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
 *
 * https://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 io.micronaut.core.io.service;

import io.micronaut.core.reflect.ClassUtils;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UncheckedIOException;
import java.net.URL;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Optional;
import java.util.ServiceConfigurationError;
import java.util.function.Predicate;
import java.util.stream.Collectors;

Variation of ServiceLoader that allows soft loading and conditional loading of META-INF/services classes.
Author:
Graeme Rocher
Type parameters:
<S> – The service type
Since:
1.0/**
 * <p>Variation of {@link java.util.ServiceLoader} that allows soft loading and conditional loading of
 * META-INF/services classes.</p>
 *
 * @param <S> The service type
 * @author Graeme Rocher
 * @since 1.0
 */
public final class SoftServiceLoader<S> implements Iterable<ServiceDefinition<S>> {
    public static final String META_INF_SERVICES = "META-INF/services";

    private final Class<S> serviceType;
    private final ClassLoader classLoader;
    private final Map<String, ServiceDefinition<S>> loadedServices = new LinkedHashMap<>();
    private final Iterator<ServiceDefinition<S>> unloadedServices;
    private final Predicate<String> condition;

    private SoftServiceLoader(Class<S> serviceType, ClassLoader classLoader) {
        this(serviceType, classLoader, (String name) -> true);
    }

    private SoftServiceLoader(Class<S> serviceType, ClassLoader classLoader, Predicate<String> condition) {
        this.serviceType = serviceType;
        this.classLoader = classLoader == null ? ClassLoader.getSystemClassLoader() : classLoader;
        this.unloadedServices = new ServiceLoaderIterator();
        this.condition = condition == null ? (String name) -> true : condition;
    }

    
Creates a new SoftServiceLoader using the thread context loader by default. 
Params:
service – The service type
Type parameters:
<S> –     The service generic type
Returns:
A new service loader/**
     * Creates a new {@link SoftServiceLoader} using the thread context loader by default.
     *
     * @param service The service type
     * @param <S>     The service generic type
     * @return A new service loader
     */
    public static <S> SoftServiceLoader<S> load(Class<S> service) {
        return SoftServiceLoader.load(service, SoftServiceLoader.class.getClassLoader());
    }

    
Creates a new SoftServiceLoader using the given type and class loader. 
Params:
service – The service type
loader –  The class loader
Type parameters:
<S> –     The service generic type
Returns:
A new service loader/**
     * Creates a new {@link SoftServiceLoader} using the given type and class loader.
     *
     * @param service The service type
     * @param loader  The class loader
     * @param <S>     The service generic type
     * @return A new service loader
     */
    public static <S> SoftServiceLoader<S> load(Class<S> service,
                                                ClassLoader loader) {
        return new SoftServiceLoader<>(service, loader);
    }

    
Creates a new SoftServiceLoader using the given type and class loader. 
Params:
service –   The service type
loader –    The class loader to use
condition – A Predicate to use to conditionally load the service. The predicate is passed the service class name
Type parameters:
<S> –       The service generic type
Returns:
A new service loader/**
     * Creates a new {@link SoftServiceLoader} using the given type and class loader.
     *
     * @param service   The service type
     * @param loader    The class loader to use
     * @param condition A {@link Predicate} to use to conditionally load the service. The predicate is passed the service class name
     * @param <S>       The service generic type
     * @return A new service loader
     */
    public static <S> SoftServiceLoader<S> load(Class<S> service,
                                                ClassLoader loader,
                                                Predicate<String> condition) {
        return new SoftServiceLoader<>(service, loader, condition);
    }

    
Returns:
Return the first such instance/**
     * @return Return the first such instance
     */
    public Optional<ServiceDefinition<S>> first() {
        Iterator<ServiceDefinition<S>> i = iterator();
        if (i.hasNext()) {
            return Optional.of(i.next());
        }
        return Optional.empty();
    }

    
Params:
alternative – An alternative type to use if the this type is not present
classLoader – The classloader
Returns:
Return the first such instance/**
     * @param alternative An alternative type to use if the this type is not present
     * @param classLoader The classloader
     * @return Return the first such instance
     */
    public Optional<ServiceDefinition<S>> firstOr(String alternative, ClassLoader classLoader) {
        Iterator<ServiceDefinition<S>> i = iterator();
        if (i.hasNext()) {
            return Optional.of(i.next());
        }

        Optional<Class> alternativeClass = ClassUtils.forName(alternative, classLoader);
        if (alternativeClass.isPresent()) {
            return Optional.of(newService(alternative, alternativeClass));
        }
        return Optional.empty();
    }

    
Returns:
The iterator/**
     * @return The iterator
     */
    @Override
    public Iterator<ServiceDefinition<S>> iterator() {
        return new Iterator<ServiceDefinition<S>>() {
            Iterator<ServiceDefinition<S>> loaded = loadedServices.values().iterator();

            @Override
            public boolean hasNext() {
                if (loaded.hasNext()) {
                    return true;
                }
                return unloadedServices.hasNext();
            }

            @Override
            public ServiceDefinition<S> next() {
                if (!hasNext()) {
                    throw new NoSuchElementException();
                }

                if (loaded.hasNext()) {
                    return loaded.next();
                }
                if (unloadedServices.hasNext()) {
                    ServiceDefinition<S> nextService = unloadedServices.next();
                    loadedServices.put(nextService.getName(), nextService);
                    return nextService;
                }
                // should not happen
                throw new Error("Bug in iterator");
            }
        };
    }

    
Params:
name –        The name
loadedClass – The loaded class
Returns:
The service definition/**
     * @param name        The name
     * @param loadedClass The loaded class
     * @return The service definition
     */
    @SuppressWarnings("unchecked")
    protected ServiceDefinition<S> newService(String name, Optional<Class> loadedClass) {
        return new DefaultServiceDefinition(name, loadedClass);
    }

    
A service loader iterator implementation.
/**
     * A service loader iterator implementation.
     */
    private final class ServiceLoaderIterator implements Iterator<ServiceDefinition<S>> {
        private Enumeration<URL> serviceConfigs = null;
        private Iterator<String> unprocessed = null;

        @Override
        public boolean hasNext() {

            if (serviceConfigs == null) {
                String name = serviceType.getName();
                try {
                    serviceConfigs = classLoader.getResources(META_INF_SERVICES + '/' + name);
                } catch (IOException e) {
                    throw new ServiceConfigurationError("Failed to load resources for service: " + name, e);
                }
            }
            while (unprocessed == null || !unprocessed.hasNext()) {
                if (!serviceConfigs.hasMoreElements()) {
                    return false;
                }
                URL url = serviceConfigs.nextElement();
                try {
                    try (BufferedReader reader = new BufferedReader(new InputStreamReader(url.openStream()))) {
                        List<String> lines = reader.lines()
                            .filter(line -> line.length() != 0 && line.charAt(0) != '#')
                            .filter(condition)
                            .map(line -> {
                                int i = line.indexOf('#');
                                if (i > -1) {
                                    line = line.substring(0, i);
                                }
                                return line;
                            })
                            .collect(Collectors.toList());
                        unprocessed = lines.iterator();

                    }
                } catch (IOException | UncheckedIOException e) {
                    // ignore, can't do anything here and can't log because class used in compiler
                }
            }
            return unprocessed.hasNext();
        }

        @Override
        public ServiceDefinition<S> next() {
            if (!hasNext()) {
                throw new NoSuchElementException();
            }

            String nextName = unprocessed.next();
            try {
                final Class<?> loadedClass = Class.forName(nextName, false, classLoader);
                return newService(nextName, Optional.of(loadedClass));
            } catch (NoClassDefFoundError | ClassNotFoundException e) {
                return newService(nextName, Optional.empty());
            }
        }
    }
}