/*
    * 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.maven.index.reader;
  
  import java.io.IOException;
  import java.io.InputStream;
  import java.io.OutputStream;
  import java.util.HashSet;
  import java.util.Set;
  
  import org.apache.maven.index.reader.WritableResourceHandler.WritableResource;
  
  /**
   * A trivial caching {@link ResourceHandler} that caches forever during single session (existence of the instance).
   */
  public class CachingResourceHandler implements ResourceHandler {
      private static final Resource NOT_EXISTING_RESOURCE = () -> null;
  
      private final WritableResourceHandler local;
  
      private final ResourceHandler remote;
  
      private final Set<String> notFoundResources;
  
      public CachingResourceHandler(final WritableResourceHandler local, final ResourceHandler remote) {
          if (local == null || remote == null) {
              throw new NullPointerException("null resource handler");
          }
          this.local = local;
          this.remote = remote;
          this.notFoundResources = new HashSet<>();
      }
  
      @Override
      public Resource locate(final String name) {
          if (notFoundResources.contains(name)) {
              return NOT_EXISTING_RESOURCE;
          } else {
              return new CachingResource(name);
          }
      }
  
      private class CachingResource implements Resource {
          private final String name;
  
          private CachingResource(final String name) {
              this.name = name;
          }
  
          @Override
          public InputStream read() throws IOException {
              InputStream inputStream = local.locate(name).read();
              if (inputStream != null) {
                  return inputStream;
              }
              if (cacheLocally(name)) {
                  return local.locate(name).read();
              }
              notFoundResources.add(name);
              return null;
          }
  
          private boolean cacheLocally(final String name) throws IOException {
              final Resource remoteResource = remote.locate(name);
              try (WritableResource localResource = local.locate(name);
                      final InputStream inputStream = remoteResource.read();
                      final OutputStream outputStream = localResource.write()) {
                  if (inputStream != null) {
                      int read;
                      byte[] bytes = new byte[8192];
                      while ((read = inputStream.read(bytes)) != -1) {
                          outputStream.write(bytes, 0, read);
                      }
                      outputStream.flush();
                      return true;
                  }
                  return false;
              }
          }
      }
  
      @Override
      public void close() throws IOException {
         remote.close();
         local.close();
     }
 }