/*
 * 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.lucene.util;


import java.util.Collections;
import java.util.Iterator;
import java.util.Map;
import java.util.Objects;
import java.util.LinkedHashMap;
import java.util.Set;
import java.util.ServiceConfigurationError;

Helper class for loading named SPIs from classpath (e.g. Codec, PostingsFormat).
@lucene.internal
/**
 * Helper class for loading named SPIs from classpath (e.g. Codec, PostingsFormat).
 * @lucene.internal
 */
public final class NamedSPILoader<S extends NamedSPILoader.NamedSPI> implements Iterable<S> {

  private volatile Map<String,S> services = Collections.emptyMap();
  private final Class<S> clazz;

  public NamedSPILoader(Class<S> clazz) {
    this(clazz, null);
  }
  
  public NamedSPILoader(Class<S> clazz, ClassLoader classloader) {
    this.clazz = clazz;
    // if clazz' classloader is not a parent of the given one, we scan clazz's classloader, too:
    final ClassLoader clazzClassloader = clazz.getClassLoader();
    if (classloader == null) {
      classloader = clazzClassloader;
    }
    if (clazzClassloader != null && !SPIClassIterator.isParentClassLoader(clazzClassloader, classloader)) {
      reload(clazzClassloader);
    }
    reload(classloader);
  }
  
  
Reloads the internal SPI list from the given ClassLoader. Changes to the service list are visible after the method ends, all iterators (iterator(),...) stay consistent. 
NOTE: Only new service providers are added, existing ones are
never removed or replaced.
This method is expensive and should only be called for discovery
of new service providers on the given classpath/classloader!
/** 
   * Reloads the internal SPI list from the given {@link ClassLoader}.
   * Changes to the service list are visible after the method ends, all
   * iterators ({@link #iterator()},...) stay consistent. 
   * 
   * <p><b>NOTE:</b> Only new service providers are added, existing ones are
   * never removed or replaced.
   * 
   * <p><em>This method is expensive and should only be called for discovery
   * of new service providers on the given classpath/classloader!</em>
   */
  public void reload(ClassLoader classloader) {
    Objects.requireNonNull(classloader, "classloader");
    final LinkedHashMap<String,S> services = new LinkedHashMap<>(this.services);
    final SPIClassIterator<S> loader = SPIClassIterator.get(clazz, classloader);
    while (loader.hasNext()) {
      final Class<? extends S> c = loader.next();
      try {
        final S service = c.newInstance();
        final String name = service.getName();
        // only add the first one for each name, later services will be ignored
        // this allows to place services before others in classpath to make 
        // them used instead of others
        if (!services.containsKey(name)) {
          checkServiceName(name);
          services.put(name, service);
        }
      } catch (Exception e) {
        throw new ServiceConfigurationError("Cannot instantiate SPI class: " + c.getName(), e);
      }
    }
    this.services = Collections.unmodifiableMap(services);
  }
  
  
Validates that a service name meets the requirements of NamedSPI /**
   * Validates that a service name meets the requirements of {@link NamedSPI}
   */
  public static void checkServiceName(String name) {
    // based on harmony charset.java
    if (name.length() >= 128) {
      throw new IllegalArgumentException("Illegal service name: '" + name + "' is too long (must be < 128 chars).");
    }
    for (int i = 0, len = name.length(); i < len; i++) {
      char c = name.charAt(i);
      if (!isLetterOrDigit(c)) {
        throw new IllegalArgumentException("Illegal service name: '" + name + "' must be simple ascii alphanumeric.");
      }
    }
  }
  
  
Checks whether a character is a letter or digit (ascii) which are defined in the spec.
/**
   * Checks whether a character is a letter or digit (ascii) which are defined in the spec.
   */
  private static boolean isLetterOrDigit(char c) {
    return ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') || ('0' <= c && c <= '9');
  }
  
  public S lookup(String name) {
    final S service = services.get(name);
    if (service != null) return service;
    throw new IllegalArgumentException("An SPI class of type "+clazz.getName()+" with name '"+name+"' does not exist."+
     "  You need to add the corresponding JAR file supporting this SPI to your classpath."+
     "  The current classpath supports the following names: "+availableServices());
  }

  public Set<String> availableServices() {
    return services.keySet();
  }
  
  @Override
  public Iterator<S> iterator() {
    return services.values().iterator();
  }
  
  
Interface to support NamedSPILoader.lookup(String) by name. 

Names must be all ascii alphanumeric, and less than 128 characters in length.
/**
   * Interface to support {@link NamedSPILoader#lookup(String)} by name.
   * <p>
   * Names must be all ascii alphanumeric, and less than 128 characters in length.
   */
  public static interface NamedSPI {
    String getName();
  }
  
}