/*
    * 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.internal.transformation;
  
  import java.io.IOException;
  import java.io.UncheckedIOException;
  import java.nio.file.Files;
  import java.nio.file.Path;
  import java.util.Objects;
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.function.BiConsumer;
  import java.util.function.Function;
  import java.util.function.Supplier;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Keeps transformed file up-to-date relative to its source file. It manages state (i.e. hashing the content) using
   * passed in stateFunction, and transforms when needed using passed in transformer bi-consumer.
   * <p>
   * Covered cases:
   * <ul>
   *     <li>when source supplier returns {@code null}, this class will return {@code null}.</li>
   *     <li>when source supplier returns non existing path, this class will return non existing path.</li>
   *     <li>when source supplier returns existing path, this class will ensure transformation is in sync.</li>
   * </ul>
   *
   * @since TBD
   */
  final class OnChangeTransformer implements Supplier<Path> {
  
      private final Supplier<Path> source;
  
      private final Path target;
  
      private final Function<Path, String> stateFunction;
  
      private final BiConsumer<Path, Path> transformerConsumer;
  
      private final AtomicReference<String> sourceState;
  
      OnChangeTransformer(
              Supplier<Path> source,
              Path target,
              Function<Path, String> stateFunction,
              BiConsumer<Path, Path> transformerConsumer) {
          this.source = requireNonNull(source);
          this.target = requireNonNull(target);
          this.stateFunction = requireNonNull(stateFunction);
          this.transformerConsumer = requireNonNull(transformerConsumer);
          this.sourceState = new AtomicReference<>(null);
      }
  
      @Override
      public synchronized Path get() {
          String state = mayUpdate();
          if (state == null) {
              return null;
          }
          return target;
      }
  
      private String mayUpdate() {
          String result;
          try {
              Path src = source.get();
              if (src == null) {
                  Files.deleteIfExists(target);
                  result = null;
              } else if (!Files.exists(src)) {
                  Files.deleteIfExists(target);
                  result = "";
              } else {
                  String current = stateFunction.apply(src);
                  String existing = sourceState.get();
                  if (!Objects.equals(current, existing)) {
                      transformerConsumer.accept(src, target);
                      Files.setLastModifiedTime(target, Files.getLastModifiedTime(src));
                  }
                  result = current;
              }
          } catch (IOException e) {
              throw new UncheckedIOException(e);
         }
         sourceState.set(result);
         return result;
     }
 }