content: /*
   * 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.calcite.plan;
  
  import org.apache.calcite.config.CalciteConnectionConfig;
  
  import com.google.common.collect.ImmutableList;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.ArrayList;
  import java.util.List;
  
  import static java.util.Objects.requireNonNull;
  
  /**
   * Utilities for {@link Context}.
   */
  public class Contexts {
    public static final EmptyContext EMPTY_CONTEXT = new EmptyContext();
  
    private Contexts() {}
  
    /** Returns a context that contains a
     * {@link org.apache.calcite.config.CalciteConnectionConfig}.
     *
     * @deprecated Use {@link #of}
     */
    @Deprecated // to be removed before 2.0
    public static Context withConfig(CalciteConnectionConfig config) {
      return of(config);
    }
  
    /** Returns a context that returns null for all inquiries. */
    public static Context empty() {
      return EMPTY_CONTEXT;
    }
  
    /** Returns a context that wraps an object.
     *
     * <p>A call to {@code unwrap(C)} will return {@code target} if it is an
     * instance of {@code C}.
     */
    public static Context of(Object o) {
      return new WrapContext(o);
    }
  
    /** Returns a context that wraps an array of objects, ignoring any nulls. */
    public static Context of(@Nullable Object... os) {
      final List<Context> contexts = new ArrayList<>();
      for (Object o : os) {
        if (o != null) {
          contexts.add(of(o));
        }
      }
      return chain(contexts);
    }
  
    /** Returns a context that wraps a list of contexts.
     *
     * <p>A call to {@code unwrap(C)} will return the first object that is an
     * instance of {@code C}.
     *
     * <p>If any of the contexts is a {@link Context}, recursively looks in that
     * object. Thus this method can be used to chain contexts.
     */
    public static Context chain(Context... contexts) {
      return chain(ImmutableList.copyOf(contexts));
    }
  
    private static Context chain(Iterable<? extends Context> contexts) {
      // Flatten any chain contexts in the list, and remove duplicates
      final List<Context> list = new ArrayList<>();
      for (Context context : contexts) {
        build(list, context);
      }
      switch (list.size()) {
      case 0:
        return empty();
      case 1:
        return list.get(0);
      default:
        return new ChainContext(ImmutableList.copyOf(list));
      }
    }
  
    /** Recursively populates a list of contexts. */
    private static void build(List<Context> list, Context context) {
      if (context == EMPTY_CONTEXT || list.contains(context)) {
        return;
      }
      if (context instanceof ChainContext) {
        ChainContext chainContext = (ChainContext) context;
        for (Context child : chainContext.contexts) {
          build(list, child);
        }
      } else {
        list.add(context);
      }
    }
  
    /** Context that wraps an object. */
    private static class WrapContext implements Context {
      final Object target;
  
      WrapContext(Object target) {
        this.target = requireNonNull(target, "target");
      }
  
      @Override public <T extends Object> @Nullable T unwrap(Class<T> clazz) {
        if (clazz.isInstance(target)) {
          return clazz.cast(target);
        }
        return null;
      }
    }
  
    /** Empty context. */
    static class EmptyContext implements Context {
      @Override public <T extends Object> @Nullable T unwrap(Class<T> clazz) {
        return null;
      }
    }
  
    /** Context that wraps a chain of contexts. */
    private static final class ChainContext implements Context {
      final ImmutableList<Context> contexts;
  
      ChainContext(ImmutableList<Context> contexts) {
        this.contexts = requireNonNull(contexts, "contexts");
        for (Context context : contexts) {
          assert !(context instanceof ChainContext) : "must be flat";
        }
      }
  
      @Override public <T extends Object> @Nullable T unwrap(Class<T> clazz) {
        for (Context context : contexts) {
          final T t = context.unwrap(clazz);
          if (t != null) {
            return t;
          }
        }
        return null;
      }
    }
  }
  