src/main/java/com/google/devtools/build/lib/collect/IterablesChain.java

// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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
//
//    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 com.google.devtools.build.lib.collect;

import com.google.common.base.Joiner;
import com.google.common.collect.AbstractIterator;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterables;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;

/**
 * An immutable chain of immutable Iterables.
 *
 * <p>This class is defined for the sole purpose of being able to check for immutability
 * (using instanceof). Otherwise, we could use a plain Iterable (as returned by
 * {@code Iterables.concat()}).
 *
 * @see CollectionUtils#checkImmutable(Iterable)
 */
public final class IterablesChain<T> implements Iterable<T> {
  public static <T> Iterable<T> concat(Iterable<? extends T> a, Iterable<? extends T> b) {
    return IterablesChain.<T>builder().add(a).add(b).build();
  }

  private final Iterable<T> chain;

  private IterablesChain(Iterable<T> chain) {
    this.chain = chain;
  }

  @Override
  public Iterator<T> iterator() {
    return chain.iterator();
  }

  public static <T> Builder<T> builder() {
    return new Builder<>();
  }

  @Override
  public String toString() {
    return "[" + Joiner.on(", ").join(this) + "]";
  }

  /**
   * Builder for IterablesChain.
   *
 *
   */
  public static class Builder<T> {
    private List<Iterable<? extends T>> iterables = new ArrayList<>();
    private boolean deduplicate;

    private Builder() {
    }

    /**
     * Adds an immutable iterable to the end of the chain.
     *
     * <p>If the iterable can not be confirmed to be immutable, a runtime error is thrown.
     */
    public Builder<T> add(Iterable<? extends T> iterable) {
      CollectionUtils.checkImmutable(iterable);
      if (!Iterables.isEmpty(iterable)) {
        iterables.add(iterable);
      }
      return this;
    }

    /**
     * Adds a single element to the chain.
     */
    public Builder<T> addElement(T element) {
      iterables.add(ImmutableList.of(element));
      return this;
    }

    /**
     * Returns true if the chain is empty.
     */
    public boolean isEmpty() {
      return iterables.isEmpty();
    }

    /**
     * If this is called, the the resulting {@link IterablesChain} object uses a hash set to remove
     * duplicate elements.
     */
    public Builder<T> deduplicate() {
      this.deduplicate = true;
      return this;
    }

    /**
     * Builds an iterable that iterates through all elements in this chain.
     */
    public IterablesChain<T> build() {
      if (isEmpty()) {
        return new IterablesChain<>(ImmutableList.<T>of());
      }
      Iterable<T> concat = Iterables.concat(ImmutableList.copyOf(iterables));
      return new IterablesChain<>(deduplicate ? new Deduper<>(concat) : concat);
    }
  }

  /**
   * An iterable implementation that removes duplicate elements (as determined by equals), using a
   * hash set.
   */
  private static final class Deduper<T> implements Iterable<T> {
    private final Iterable<T> iterable;

    public Deduper(Iterable<T> iterable) {
      this.iterable = iterable;
    }

    @Override
    public Iterator<T> iterator() {
      return new DedupingIterator<T>(iterable.iterator());
    }
  }

  /**
   * An iterator implementation that removes duplicate elements (as determined by equals), using a
   * hash set.
   */
  private static final class DedupingIterator<T> extends AbstractIterator<T> {
    private final HashSet<T> set = new HashSet<>();
    private final Iterator<T> it;

    public DedupingIterator(Iterator<T> it) {
      this.it = it;
    }

    @Override
    protected T computeNext() {
      while (it.hasNext()) {
        T next = it.next();
        if (set.add(next)) {
          return next;
        }
      }
      return endOfData();
    }
  }
}