ISequence

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 * Portions of this software was developed by employees of the National Institute
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 * being made available as a public service. Pursuant to title 17 United States
 * Code Section 105, works of NIST employees are not subject to copyright
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 * copyright. Permission in the United States and in foreign countries, to the
 * extent that NIST may hold copyright, to use, copy, modify, create derivative
 * works, and distribute this software and its documentation without fee is hereby
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 * of warranty appears in all copies.
 *
 * THE SOFTWARE IS PROVIDED 'AS IS' WITHOUT ANY WARRANTY OF ANY KIND, EITHER
 * EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTY
 * THAT THE SOFTWARE WILL CONFORM TO SPECIFICATIONS, ANY IMPLIED WARRANTIES OF
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package gov.nist.secauto.metaschema.core.metapath;

import gov.nist.secauto.metaschema.core.metapath.item.IItem;
import gov.nist.secauto.metaschema.core.util.ObjectUtils;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.function.BinaryOperator;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;
import java.util.stream.Stream;

import edu.umd.cs.findbugs.annotations.NonNull;
import edu.umd.cs.findbugs.annotations.Nullable;

public interface ISequence<ITEM_TYPE extends IItem> extends Iterable<ITEM_TYPE> {
  @SuppressWarnings("rawtypes")
  ISequence EMPTY = new EmptyListImpl<>();

  /**
   * Get an empty sequence.
   *
   * @param <ITEM_TYPE>
   *          the item type
   * @return the empty sequence
   */
  @SuppressWarnings({ "unchecked", "null" })
  @NonNull
  static <ITEM_TYPE extends IItem> ISequence<ITEM_TYPE> empty() {
    return EMPTY;
  }

  /**
   * Construct a new sequence containing the provided {@code item}.
   * <p>
   * If the item is {@code null} and empty sequence will be created.
   *
   * @param <ITEM_TYPE>
   *          the type of items contained in the sequence.
   * @param item
   *          the item to add to the sequence
   * @return the new sequence
   */
  @NonNull
  public static <ITEM_TYPE extends IItem> ISequence<ITEM_TYPE> of( // NOPMD - intentional
      @Nullable ITEM_TYPE item) {
    ISequence<ITEM_TYPE> retval;
    if (item == null) {
      retval = empty();
    } else {
      retval = new SingletonSequenceImpl<>(item);
    }
    return retval;
  }

  /**
   * Construct a new sequence containing the provided {@code items}.
   *
   * @param <ITEM_TYPE>
   *          the type of items contained in the sequence.
   * @param items
   *          the items to add to the sequence
   * @return the new sequence
   */
  @NonNull
  public static <ITEM_TYPE extends IItem> ISequence<ITEM_TYPE> of( // NOPMD - intentional
      @NonNull List<ITEM_TYPE> items) {
    ISequence<ITEM_TYPE> retval;
    if (items.isEmpty()) {
      retval = empty();
    } else {
      retval = new ListSequenceImpl<>(items);
    }
    return retval;
  }

  /**
   * Construct a new sequence containing the provided {@code items}.
   *
   * @param <ITEM_TYPE>
   *          the type of items contained in the sequence.
   * @param items
   *          the items to add to the sequence
   * @return the new sequence
   */
  // TODO: remove null check on callers
  @NonNull
  public static <ITEM_TYPE extends IItem> ISequence<ITEM_TYPE> of( // NOPMD - intentional
      Stream<ITEM_TYPE> items) {
    return items == null ? empty() : new StreamSequenceImpl<>(items);
  }

  @Override
  default Iterator<ITEM_TYPE> iterator() {
    return asList().listIterator();
  }

  /**
   * Get the items in this sequence as a {@link List}.
   *
   * @return a list containing all the items of the sequence
   */
  @NonNull
  List<ITEM_TYPE> asList();

  /**
   * Get the items in this sequence as a {@link Stream}.
   *
   * @return a stream containing all the items of the sequence
   */
  // TODO: rename to "stream"
  @NonNull
  Stream<ITEM_TYPE> asStream();

  /**
   * This optional operation ensures that a list is used to back this sequence.
   * <p>
   * If a stream is currently backing this sequence, the stream will be collected
   * into a list. This ensures the sequence can be visited multiple times.
   */
  void collect();

  /**
   * Determine if this sequence is empty.
   *
   * @return {@code true} if the sequence contains no items, or {@code false}
   *         otherwise
   */
  boolean isEmpty();

  /**
   * Get the count of items in this sequence.
   *
   * @return the count of items
   */
  int size();

  /**
   * Iterate over each item in the sequence using the provided {@code action}.
   *
   * @param action
   *          code to execute for each item
   */
  @Override
  void forEach(Consumer<? super ITEM_TYPE> action);

  @NonNull
  static <ITEM_TYPE extends IItem> Collector<ITEM_TYPE, ?, ISequence<ITEM_TYPE>> toSequence() {

    return new Collector<ITEM_TYPE, List<ITEM_TYPE>, ISequence<ITEM_TYPE>>() {

      @Override
      public Supplier<List<ITEM_TYPE>> supplier() {
        return ArrayList::new;
      }

      @Override
      public BiConsumer<List<ITEM_TYPE>, ITEM_TYPE> accumulator() {
        return (list, value) -> list.add(value);
      }

      @Override
      public BinaryOperator<List<ITEM_TYPE>> combiner() {
        return (list1, list2) -> {
          list1.addAll(list2);
          return list1;
        };
      }

      @Override
      public Function<List<ITEM_TYPE>, ISequence<ITEM_TYPE>> finisher() {
        return list -> {
          ISequence<ITEM_TYPE> retval;
          if (list.isEmpty()) {
            retval = empty();
          } else if (list.size() == 1) {
            retval = new SingletonSequenceImpl<>(ObjectUtils.notNull(list.iterator().next()));
          } else {
            retval = new ListSequenceImpl<>(list, false);
          }
          return retval;
        };
      }

      @Override
      public Set<Characteristics> characteristics() {
        return Collections.emptySet();
      }

    };
  }
}