uimaj-core/src/main/java/org/apache/uima/cas/impl/SelectFSs_impl.java

/*
 * 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.uima.cas.impl;

import static org.apache.uima.cas.impl.Subiterator.BoundsUse.notBounded;
import static org.apache.uima.jcas.tcas.Annotation._createMarkerAnnotation;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.Optional;
import java.util.Spliterator;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.BinaryOperator;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.IntFunction;
import java.util.function.Predicate;
import java.util.function.Supplier;
import java.util.function.ToDoubleFunction;
import java.util.function.ToIntFunction;
import java.util.function.ToLongFunction;
import java.util.stream.Collector;
import java.util.stream.DoubleStream;
import java.util.stream.IntStream;
import java.util.stream.LongStream;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

import org.apache.uima.cas.CAS;
import org.apache.uima.cas.CASRuntimeException;
import org.apache.uima.cas.FSIndex;
import org.apache.uima.cas.FSIterator;
import org.apache.uima.cas.FeatureStructure;
import org.apache.uima.cas.SelectFSs;
import org.apache.uima.cas.Type;
import org.apache.uima.cas.impl.Subiterator.BoundsUse;
import org.apache.uima.cas.text.AnnotationFS;
import org.apache.uima.cas.text.AnnotationIndex;
import org.apache.uima.cas.text.AnnotationPredicates;
import org.apache.uima.jcas.cas.EmptyFSList;
import org.apache.uima.jcas.cas.FSArray;
import org.apache.uima.jcas.cas.FSList;
import org.apache.uima.jcas.cas.NonEmptyFSList;
import org.apache.uima.jcas.cas.TOP;
import org.apache.uima.jcas.impl.JCasImpl;
import org.apache.uima.jcas.tcas.Annotation;

// @formatter:off
/**
 * Collection of builder style methods to specify selection of FSs from indexes
 * shift handled in this routine
 * Comment codes:
 *   AI = implies AnnotationIndex
 * 
 * Iterator varieties and impl
 * 
 *   bounded?  type      order not unambig? strict? skipEq    
 *             Priority? Needed? 
 *     no
 *   coveredBy
 *   covering
 *   sameas
 * 
 *   for not-bounded, 
 *     - ignore strict and skipEq
 *       -- except: preceding implies skipping annotations whose end > positioning begin
 *     - order-not-needed only applies if iicp size > 1
 *     - unambig ==> use Subiterator
 *         -- subiterator wraps: according to typePriority and order-not-needed
 *     - no Type Priority - need to pass in as arg to fsIterator_multiple_indexes
 *        == if no type priority, need to prevent rattling off the == type while compare is equal
 *        == affects both FsIterator_aggregation_common and FsIterator_subtypes_ordered
 *   for 3 other boundings:
 *     - use subiterator, pass in strict and skipeq
 * 
   finish this javadoc comment edit
 *   T extends FeatureStructure, not TOP, because of ref from FSIndex 
 * which uses FeatureStructure for backwards compatibility
 */
//@formatter:on
public class SelectFSs_impl<T extends FeatureStructure> implements SelectFSs<T> {

  private final static boolean IS_UNORDERED = true;
  private final static boolean IS_ORDERED = false;
  private final static boolean IS_UNAMBIGUOUS = false;
  private final static boolean IS_NOT_STRICT = false;

  private CASImpl view;
  private JCasImpl jcas;
  private LowLevelIndex<T> index;
  private TypeImpl ti;
  private int shift;
  private int limit = -1;

  private FeatureStructure[] sourceFSArray = null; // alternate source
  private FSList sourceFSList = null; // alternate source

  private boolean isTypePriority = false;
  // private boolean isPositionUsesType = false; // REMOVED see
  // https://issues.apache.org/jira/browse/UIMA-5536
  private boolean isSkipSameBeginEndType = false; // for boundsUse only
  private boolean isNonOverlapping = IS_UNAMBIGUOUS;
  private boolean isIncludeAnnotBeyondBounds = false;
  private boolean isAllViews = false;
  private boolean isNullOK = false;
  private boolean isUnordered = false;
  private boolean isBackwards = false;
  private boolean isFollowing = false;
  private boolean isPreceding = false;

  private boolean isAltSource = false;

  private BoundsUse boundsUse = null;

  /**
   * This is used for non-annotation positioning too Used for preceding since we tweak the end
   * offset of the reference annotation
   */
  private TOP startingFs = null;
  private AnnotationFS boundingFs = null;
  private boolean noResult = false;

//@formatter:off
  /* **********************************************
   * Constructors
   *   always need the cas
   *   might also have the type
   * Caller will convert other forms for the cas (e.g. jcas) 
   * and type (e.g. type name, MyType.type, MyType.class) to 
   * these arg forms.
   ************************************************/
//@formatter:on
  public SelectFSs_impl(CAS cas) {
    view = (CASImpl) cas.getLowLevelCAS();
    jcas = (JCasImpl) view.getJCas();
  }

  public SelectFSs_impl(FSArray source) {
    this(source._casView);
    isAltSource = true;
    sourceFSArray = source._getTheArray();
  }

  public SelectFSs_impl(FeatureStructure[] source, CAS cas) {
    this(cas);
    isAltSource = true;
    sourceFSArray = source;
  }

  public SelectFSs_impl(FSList source) {
    this(source._casView);
    isAltSource = true;
    sourceFSList = source;
  }

  /************************************************
   * Builders
   ************************************************/
  /**
   * INDEX If not specified, defaults to all FSs (orderNotNeeded) unless AnnotationIndex implied
   * 
   * @param indexName
   *          -
   * @return -
   */
  public SelectFSs_impl<T> index(String indexName) {
    index = view.indexRepository.getIndex(indexName);
    return this;
  }

  public SelectFSs_impl<T> index(FSIndex<T> aIndex) {
    index = (LowLevelIndex<T>) aIndex;
    return this;
  }

  /**
   * Select the index's uppermost type.
   */
  public <N extends T> SelectFSs_impl<N> anyType() {
    ti = (TypeImpl) null;
    return (SelectFSs_impl<N>) this;
  }

  public <N extends T> SelectFSs_impl<N> type(Type uimaType) {
    if (uimaType == null) {
      throw new IllegalArgumentException("Must specify a type");
    }

    if (view.getTypeSystemImpl() != ((TypeImpl) uimaType).getTypeSystem()) {
      Type type = view.getTypeSystem().getType(uimaType.getName());
      if (type == null) {
        throw new IllegalArgumentException("Undefined type: [" + uimaType.getName() + "]");
      }
    }

    ti = (TypeImpl) uimaType;
    return (SelectFSs_impl<N>) this;
  }

  public <N extends T> SelectFSs_impl<N> type(String fullyQualifiedTypeName) {
    if (fullyQualifiedTypeName == null) {
      throw new IllegalArgumentException("Must specify a type");
    }

    TypeImpl type = view.getTypeSystemImpl().getType(fullyQualifiedTypeName);
    if (type == null) {
      throw new IllegalArgumentException("Undefined type: [" + fullyQualifiedTypeName + "]");
    }

    ti = type;
    return (SelectFSs_impl<N>) this;
  }

  public <N extends T> SelectFSs_impl<N> type(int jcasClass_dot_type) {
    ti = (TypeImpl) view.getJCas().getCasType(jcasClass_dot_type);
    return (SelectFSs_impl<N>) this;
  }

  public <N extends T> SelectFSs_impl<N> type(Class<N> jcasClass_dot_class) {
    if (jcasClass_dot_class == null) {
      throw new IllegalArgumentException("Must specify a type");
    }
    ti = (TypeImpl) view.getJCasImpl().getCasType(jcasClass_dot_class);
    return (SelectFSs_impl<N>) this;
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#typePriority()
   */
  @Override
  public SelectFSs<T> typePriority() {
    isTypePriority = true;
    return this;
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#typePriority(boolean)
   */
  @Override
  public SelectFSs<T> typePriority(boolean aTypePriority) {
    isTypePriority = aTypePriority;
    return this;
  }

  // For testing
  boolean usesTypePriority() {
    return isTypePriority;
  }

  /*********************************
   * boolean operations
   *********************************/

  // REMOVED see https://issues.apache.org/jira/browse/UIMA-5536
  // /* (non-Javadoc)
  // * @see org.apache.uima.cas.SelectFSs#positionUsesType()
  // */
  // @Override
  // public SelectFSs<T> positionUsesType() {
  // this.isPositionUsesType = true;
  // return this;
  // }
  //
  // /* (non-Javadoc)
  // * @see org.apache.uima.cas.SelectFSs#positionUsesType(boolean)
  // */
  // @Override
  // public SelectFSs<T> positionUsesType(boolean aPositionUsesType) {
  // this.isPositionUsesType = aPositionUsesType;
  // return this;
  // }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#skipEquals()
   */
  @Override
  public SelectFSs<T> skipWhenSameBeginEndType() {
    isSkipSameBeginEndType = true;
    return this;
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#skipEquals(boolean)
   */
  @Override
  public SelectFSs<T> useAnnotationEquals(boolean useAnnotationEquals) {
    isSkipSameBeginEndType = useAnnotationEquals;
    return this;
  }

  /**
   * Filters while iterating
   **/

  @Override
  public SelectFSs_impl<T> nonOverlapping() { // AI known as unambiguous
    isNonOverlapping = true;
    return this;
  }

  @Override
  public SelectFSs_impl<T> nonOverlapping(boolean bNonOverlapping) { // AI
    isNonOverlapping = bNonOverlapping;
    return this;
  }

  @Override
  public SelectFSs_impl<T> includeAnnotationsWithEndBeyondBounds() { // AI known as "not strict"
    isIncludeAnnotBeyondBounds = true;
    return this;
  }

  @Override
  public SelectFSs_impl<T> includeAnnotationsWithEndBeyondBounds(
          boolean includeAnnotationsWithEndBeyondBounds) { // AI
    isIncludeAnnotBeyondBounds = includeAnnotationsWithEndBeyondBounds;
    return this;
  }

  // public SelectFSs_impl<T> useTypePriorities() {
  // return this;
  // }
  // public SelectFSs_impl<T> useTypePriorities(boolean useTypePriorities) {
  // return this;
  // }

  /**
   * Miscellaneous
   **/

  @Override
  public SelectFSs_impl<T> allViews() {
    isAllViews = true;
    return this;
  }

  @Override
  public SelectFSs_impl<T> allViews(boolean bAllViews) {
    isAllViews = bAllViews;
    return this;
  }

  @Override
  public SelectFSs_impl<T> nullOK() { // applies to get() and single()
    isNullOK = true;
    return this;
  }

  @Override
  public SelectFSs_impl<T> nullOK(boolean bNullOk) { // applies to get() and single()
    isNullOK = bNullOk;
    return this;
  }

  @Override
  public SelectFSs_impl<T> orderNotNeeded() { // ignored if not ordered index
    isUnordered = true;
    return this;
  }

  @Override
  public SelectFSs_impl<T> orderNotNeeded(boolean bUnordered) { // ignored if not ordered index
    isUnordered = bUnordered;
    return this;
  }

  @Override
  public SelectFSs_impl<T> backwards() { // ignored if not ordered index
    isBackwards = true;
    return this;
  }

  @Override
  public SelectFSs_impl<T> backwards(boolean bBackwards) { // ignored if not ordered index
    isBackwards = bBackwards;
    return this;
  }

  // public SelectFSs_impl<T> noSubtypes() {
  // return this;
  // }
  // public SelectFSs_impl<T> noSubtypes(boolean noSubtypes) {
  // return this;
  // }

  /*********************************
   * starting position
   *********************************/

  @Override
  public SelectFSs_impl<T> shifted(int shiftAmount) {
    shift = shiftAmount;
    return this;
  }

  @Override
  public SelectFSs_impl<T> startAt(FeatureStructure fs) {
    startingFs = (TOP) fs;
    return this;
  }

  @Override
  public SelectFSs_impl<T> startAt(TOP fs) { // Ordered
    startingFs = fs;
    return this;
  }
  // @Override
  // public SelectFSs_impl<T> startAt(AnnotationFS fs) {
  // return (startAt((TOP)fs));
  // }
  // @Override
  // public SelectFSs_impl<T> startAt(Annotation fs) {
  // return (startAt((TOP)fs));
  // }

  @Override
  public SelectFSs_impl<T> startAt(int begin) {
    isTypePriority = false;
    startingFs = makePosAnnot(begin, Integer.MAX_VALUE);
    return this;
  }

  @Override
  public SelectFSs_impl<T> startAt(int begin, int end) { // AI
    startingFs = makePosAnnot(begin, end);
    return this;
  }

  @Override
  public SelectFSs_impl<T> startAt(TOP fs, int offset) { // Ordered
    startingFs = fs;
    shift = offset;
    return this;
  }

  @Override
  public SelectFSs_impl<T> startAt(FeatureStructure fs, int offset) { // Ordered
    startingFs = (TOP) fs;
    shift = offset;
    return this;
  }

  @Override
  public SelectFSs_impl<T> startAt(int begin, int end, int offset) { // AI
    startingFs = makePosAnnot(begin, end);
    shift = offset;
    return this;
  }

  @Override
  public SelectFSs_impl<T> limit(int alimit) {
    if (alimit < 0) {
      throw new IllegalArgumentException("limit argument must be >= 0, but was " + alimit);
    }
    limit = alimit;
    return this;
  }

  /*********************************
   * subselection based on boundingFs
   *********************************/
  @Override
  public SelectFSs_impl<T> coveredBy(AnnotationFS fs) { // AI
    boundsUse = BoundsUse.coveredBy;
    boundingFs = fs;
    // this.isIncludeAnnotWithEndBeyondBounds = false; //default
    return this;
  }

  @Override
  public SelectFSs_impl<T> coveredBy(int begin, int end) { // AI
    boundsUse = BoundsUse.coveredBy;
    boundingFs = makePosAnnot(begin, end);
    // this.isIncludeAnnotWithEndBeyondBounds = true; //default
    return this;
  }

  @Override
  public SelectFSs_impl<T> covering(AnnotationFS fs) { // AI
    boundsUse = BoundsUse.covering;
    boundingFs = fs;
    return this;
  }

  @Override
  public SelectFSs_impl<T> covering(int begin, int end) { // AI
    boundsUse = BoundsUse.covering;
    boundingFs = makePosAnnot(begin, end);
    return this;
  }

  @Override
  public SelectFSs_impl<T> between(AnnotationFS fs1, AnnotationFS fs2) { // AI
    final boolean reverse = fs1.getEnd() > fs2.getBegin();
    int begin = (reverse ? fs2 : fs1).getEnd();
    int end = (reverse ? fs1 : fs2).getBegin();

    if (begin > end) {
      noResult = true;
      return this;
    }

    return coveredBy(begin, end);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#at(org.apache.uima.jcas.tcas.Annotation)
   */
  @Override
  public SelectFSs<T> at(AnnotationFS fs) {
    boundsUse = BoundsUse.sameBeginEnd;
    boundingFs = fs;
    return this;
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#at(int, int)
   */
  @Override
  public SelectFSs<T> at(int begin, int end) {
    return at(makePosAnnot(begin, end));
  }

  private String maybeMsgPosition() {
    StringBuilder sb = new StringBuilder();
    if (startingFs != null) {
      if (startingFs instanceof Annotation) {
        Annotation a = (Annotation) startingFs;
        sb.append(" at position begin: ").append(a.getBegin()).append(", end: ").append(a.getEnd());
      } else {
        sb.append(" at moveTo position given by Feature Structure:\n");
        startingFs.prettyPrint(2, 2, sb, false);
        sb.append("\n ");
      }
    }
    if (shift != 0) {
      sb.append(" shifted by: ").append(shift);
    }
    return sb.toString();
  }

  private void prepareTerminalOp() {
    if (boundsUse == null) {
      boundsUse = BoundsUse.notBounded;
    }

    maybeValidateAltSource();

    final boolean isUseAnnotationIndex = ((index != null) && (index instanceof AnnotationIndex))
            || isNonOverlapping ||
            // isPositionUsesType || REMOVED see https://issues.apache.org/jira/browse/UIMA-5536
            isTypePriority || isIncludeAnnotBeyondBounds || boundsUse != BoundsUse.notBounded
            || isFollowing || isPreceding;

    if (isUseAnnotationIndex) {
      forceAnnotationIndex(); // throws if non-null index not an annotation index
    }
    // REMOVED see https://issues.apache.org/jira/browse/UIMA-5536
    // if (isTypePriority) {
    // isPositionUsesType = true;
    // }

    if (ti == null) {
      if (index != null) {
        ti = (TypeImpl) index.getType();
      }
    } else {
      // type is specified
      if (index != null) {
        if (((TypeImpl) index.getType()).subsumes(ti)) {
          index = ((LowLevelIndex) index).getSubIndex(ti);
        }
      } else {
        if (ti.isAnnotationType() && !isAltSource) {
          forceAnnotationIndex(); // when index is null, but ti is not null and is annotation
        }
      }
    }

    if (isUseAnnotationIndex && null == ti) {
      ti = (TypeImpl) view.getAnnotationType();
    }

    if (ti == null) {
      ti = view.getTypeSystemImpl().getTopType();
    }

    if (boundsUse == BoundsUse.covering) {
      isIncludeAnnotBeyondBounds = true;
    }

    // force ordering
    boolean orderingNeeded = !isUnordered || shift != 0 || boundsUse != BoundsUse.notBounded
            || isFollowing || isPreceding;

    isUnordered = !orderingNeeded;

    // If there is a boundary for the selection start and the shift would try to cross this
    // boundary, it would result in an invalid iterator causing the selection operation to return
    // an empty result. To avoid this potentially surprising behavior, reduce the shift to 0 in
    // such a case.
    if ((isFollowing || isPreceding || boundsUse != BoundsUse.notBounded) && shift < 0) {
      shift = 0;
    }
  }

  private void maybeValidateAltSource() {
    if (!isAltSource) {
      return;
    }

    if (index != null || boundsUse != BoundsUse.notBounded || isAllViews || isFollowing
            || isPreceding || startingFs != null) {
      /**
       * Select with FSList or FSArray may not specify bounds, starting position, following, or
       * preceding.
       */
      throw new CASRuntimeException(CASRuntimeException.SELECT_ALT_SRC_INVALID);
    }
  }

  private void incr(FSIterator<T> it) {
    it.moveToNext();
  }

  // @formatter:off
  /*********************************
   * terminal operations
   * returning other than SelectFSs
   * 
   * Hierarchy of interpretation of setup:
   *   - index
   *   - type
   * 
   *   - allViews: ignored: things only with annotation index
   *               order among views is arbitrary, each view done together
   *               base view skipped
   * 
   *********************************/
  // @formatter:on

  // @formatter:off
  /**
   * F S I t e r a t o r
   * -------------------
   */
  // @formatter:on
  @Override
  public FSIterator<T> fsIterator() {
    if (isFollowing && isBackwards) {
      isBackwards = false;
      return make_or_copy_snapshot(fsIterator1(), false);
    }

    if (isPreceding) {
      // save isBackwards flag.
      boolean bkwd = isBackwards;
      // because need the iterator to move from the position to the front.
      isBackwards = true;
      // this iterator fails to skip annotations whose end is > positioning begin
      return make_or_copy_snapshot(fsIterator1(), bkwd);
    }

    // all others, including isFollowing but not backwards
    return fsIterator1();
  }

  private LowLevelIterator<T> make_or_copy_snapshot(LowLevelIterator<T> baseIterator,
          boolean bkwd) {
    LowLevelIterator<T> it;
    // array is in forward order because it's produced by a backwards iterator, but then the array
    // is reversed
    T[] a = (T[]) asArray(baseIterator, FeatureStructure.class);

    it = new FsIterator_subtypes_snapshot<>(a, (LowLevelIndex<T>) index, IS_ORDERED,
            baseIterator.getComparator());

    if (!bkwd) {
      it = new FsIterator_backwards<>(it);
    }

    return (limit == -1) ? it
            // rewrap with limit - needs to be outer shell to get right invalid behavior
            : new FsIterator_limited<>(it, limit);
  }

  private LowLevelIterator<T> fsIterator1() {
    prepareTerminalOp();
    LowLevelIterator<T> it = new SelectFSIterator(() -> {
      LowLevelIterator<T> baseIt = isAllViews ? createFsIterator_for_all_views()
              : plainFsIterator(index, view);

      baseIt = maybeWrapBackwards(baseIt);
      // position needs to come after backwards because that sets the position
      maybePosition(baseIt);
      // shift semantically needs to come after backwards
      maybeShift(baseIt);
      return baseIt;
    });

    return maybeLimit(it);
  }

  /**
   * for a selected index, return an iterator over all the views for that index
   * 
   * @return iterator over all views for that index, unordered
   */
  private FsIterator_aggregation_common<T> createFsIterator_for_all_views() {
    final int nbrViews = view.getNumberOfViews();
    LowLevelIterator<T>[] ita = new LowLevelIterator[nbrViews];
    // LowLevelIndex<T>[] indexes = new LowLevelIndex[nbrViews];

    for (int i = 1; i <= nbrViews; i++) {
      CASImpl v = (i == 1) ? view.getInitialView() : (CASImpl) view.getView(i);
      LowLevelIndex<T> index_local = (LowLevelIndex<T>) getIndexForView(v);
      ita[i - 1] = plainFsIterator(index_local, v);
      // indexes[i - 1] = index;
    }
    // return new FsIterator_aggregation_common<T>(ita, new FsIndex_aggr<>(indexes));
    return new FsIterator_aggregation_common<>(ita, null, null);

  }

  // private LowLevelIterator<T>[] getPlainIteratorsForAllViews() {
  // final int nbrViews = view.getNumberOfViews();
  // LowLevelIterator<T>[] ita = new LowLevelIterator[nbrViews];
  //
  // for (int i = 1; i <= nbrViews; i++) {
  // CASImpl v = (i == 1) ? view.getInitialView() : (CASImpl) view.getView(i);
  // ita[i - 1] = plainFsIterator(getIndexForView(v), v);
  // }
  // return ita;
  // }

  /**
   * gets the index for a view that corresponds to the specified index by matching the index specs
   * and type code
   * 
   * @param v
   *          -
   * @return -
   */
  private LowLevelIndex<T> getIndexForView(CASImpl v) {
    if (index == null) {
      return null;
    }

    FSIndexRepositoryImpl ir = (FSIndexRepositoryImpl) v.getIndexRepository();
    if (index instanceof FsIndex_iicp) {
      FsIndex_iicp idx = (FsIndex_iicp) index;
      return ir.getIndexBySpec(idx.getTypeCode(), idx.getIndexingStrategy(),
              idx.getComparatorImplForIndexSpecs());
    }
    FsIndex_singletype idx = (FsIndex_singletype) index;
    return ir.getIndexBySpec(idx.getTypeCode(), idx.getIndexingStrategy(),
            idx.getComparatorImplForIndexSpecs());
  }

  /**
   * 
   * @param idx
   *          the index selected, corresponds to a type + its subtypes, or if null, either an
   *          alternate source or means all types
   * @param v
   *          the cas
   * @return an iterator
   */
  private LowLevelIterator<T> plainFsIterator(LowLevelIndex<T> idx, CASImpl v) {
    if (null == idx) {
      // no bounds, not ordered
      // type could be null
      // could be alternate source

      if (isAltSource) {
        return altSourceIterator();
      } else {
        // idx is null after prepareTerminalOp has been called.
        // guaranteed not annotationindex or any op that requires that
        return v.indexRepository.getAllIndexedFS(ti);
      }
    }

    final boolean isSortedIndex = idx.getIndexingStrategy() == FSIndex.SORTED_INDEX;
    final boolean isAnnotationIndex = idx instanceof AnnotationIndex;
    final FsIndex_annotation<Annotation> ai = isAnnotationIndex ? (FsIndex_annotation) idx : null;

    if (boundsUse == notBounded &&
    // For ambiguous (overlapping) select, we can take a shortcut by using a FilteredIterator
    // instead of having to use a Subiterator. However, for an unambiguous, we can no longer
    // rely on the index iterator to handle the unambiguous mode all alone because the filtered
    // annotator will skip results but is unable to update the previous bounds state in the
    // index iterator.
            ((!isFollowing && !isPreceding)
                    || (!isNonOverlapping && (isFollowing || isPreceding)))) {
      if (noResult) {
        return (LowLevelIterator<T>) LowLevelIterator.FS_ITERATOR_LOW_LEVEL_EMPTY;
      }

      if (!isSortedIndex) {
        // set index or bag index
        return (LowLevelIterator<T>) idx.iterator();
      }

      // index is sorted but no bounds are being used. Varieties:
      // - AnnotationIndex:
      // - overlapping / non-overlapping (ambiguous, unambiguous)
      // - any sorted index including AnnotationIndex:
      // - typePriority / ignore typePriority
      // - orderNotNecessary / orderNeeded
      // - preceding: need to skip over annotations whose end is > positioning-begin
      // - following: need to skip over zero-width annotations at positioning-end
      LowLevelIterator<T> it = isAnnotationIndex
              ? (LowLevelIterator<T>) ai.iterator(!isNonOverlapping, IS_NOT_STRICT, isUnordered,
                      !isTypePriority)
              : idx.iterator(isUnordered, !isTypePriority);

      if (isPreceding) {
        // filter the iterator to skip annotations whose end is > the position-begin
        // and also if the reference annotation is a zero-width annotation, skip any
        // annotations that end at the zero-width annotation because those would rather be seen as
        // covering the zero-width annotation
        int startingFSStart = ((Annotation) startingFs).getBegin();
        int startingFSEnd = ((Annotation) startingFs).getEnd();
        it = new FilteredIterator<>(it, fs -> {
          return fs._id() != startingFs._id && AnnotationPredicates.preceding((Annotation) fs,
                  startingFSStart, startingFSEnd);
        });
      }

      if (isFollowing) {
        // Annotations are following the startFS if their begin is >= the end of the startFS
        // except if they are zero-width FSes at the end of the startFS in which case they are
        // considered to be covered and not following
        int startingFSStart = ((Annotation) startingFs).getBegin();
        int startingFSEnd = ((Annotation) startingFs).getEnd();
        it = new FilteredIterator<>(it, fs -> {
          return fs._id() != startingFs._id && AnnotationPredicates.following((Annotation) fs,
                  startingFSStart, startingFSEnd);
        });
      }

      return it;
    }

    // bounds in use or non-overlapping, so index must be annotation index, is ordered
    Annotation secondaryBoundingFs = null;
    boolean isIncludeZeroWidthAtBegin = true;
    boolean isIncludeZeroWidthAtEnd = true;
    if (isPreceding) {
      isIncludeAnnotBeyondBounds = false;
      boundsUse = BoundsUse.coveredBy;
      boundingFs = makePosAnnot(0, ((Annotation) startingFs).getBegin());
      secondaryBoundingFs = (Annotation) startingFs;
      isIncludeZeroWidthAtEnd = true;
    }

    if (isFollowing) {
      isIncludeAnnotBeyondBounds = false;
      boundsUse = BoundsUse.coveredBy;
      boundingFs = makePosAnnot(((Annotation) startingFs).getEnd(), Integer.MAX_VALUE);
      secondaryBoundingFs = (Annotation) startingFs;
      isIncludeZeroWidthAtEnd = true;
    }

    return (LowLevelIterator<T>) new Subiterator<>(
            (FSIterator<Annotation>) idx.iterator(isUnordered, !isTypePriority), boundingFs,
            secondaryBoundingFs, !isNonOverlapping, // ambiguous
            !isIncludeAnnotBeyondBounds, // strict
            boundsUse, isTypePriority, isSkipSameBeginEndType, false, // isStrictIncludesAnnotationsStartingAtEndPosition
            isIncludeZeroWidthAtBegin, isIncludeZeroWidthAtEnd);
  }

  private LowLevelIterator<T> maybeWrapBackwards(LowLevelIterator<T> it) {
    if (isBackwards) {
      it = new FsIterator_backwards<>(it); // positions the underlying iterator to last,
                                           // which is first for going backwards
    }
    return it;
  }

  private LowLevelIterator<T> altSourceIterator() {
    T[] filtered;
    if (sourceFSList != null) {
      List<T> filteredItems = new ArrayList<>();
      FSList fsl = sourceFSList;
      while (!(fsl instanceof EmptyFSList)) {
        NonEmptyFSList nefsl = (NonEmptyFSList) fsl;
        T item = (T) nefsl.getHead();
        if ((isNullOK || null != item) && ti.subsumes((TypeImpl) item.getType())) {
          filteredItems.add(item);
        }
        fsl = nefsl.getTail();
      }
      filtered = filteredItems
              .toArray((T[]) Array.newInstance(FeatureStructure.class, filteredItems.size()));
    } else {

      // skip filtering if nullOK and no subsumption test needed because type = TOP or higher
      boolean noTypeFilter = ti == view.getTypeSystemImpl().topType;
      if (!isNullOK && noTypeFilter) {
        return new FsIterator_subtypes_snapshot<>((T[]) sourceFSArray, null, IS_UNORDERED, null);
      }

      List<T> filteredItems = new ArrayList<>();
      boolean noNullsWereFiltered = true;
      for (FeatureStructure item : sourceFSArray) {
        if (!isNullOK && null == item) {
          noNullsWereFiltered = false;
          continue; // null items may be skipped
        }

        if (noTypeFilter || ti.subsumes((TypeImpl) item.getType())) {
          filteredItems.add((T) item);
        }
      }

      if (noTypeFilter && !noNullsWereFiltered) {
        return new FsIterator_subtypes_snapshot<>((T[]) sourceFSArray, null, IS_UNORDERED, null);
      }

      filtered = filteredItems
              .toArray((T[]) Array.newInstance(FeatureStructure.class, filteredItems.size()));
    }
    return new FsIterator_subtypes_snapshot<>(filtered, null, IS_UNORDERED, null); // items not
                                                                                   // sorted
  }

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

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#asList()
   * 
   * The operation of this is to make an iterator which is directly addressable, and then return an
   * instance of AbstractList<N>
   * 
   */
  @Override
  public ArrayList<T> asList() {
    return asArrayList((LowLevelIterator<T>) fsIterator());
  }

  private ArrayList<T> asArrayList(LowLevelIterator<T> it) {
    ArrayList<T> al = new ArrayList<>();
    it.getArrayList(al);
    return al;
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#asArray()
   */
  @Override
  public T[] asArray(Class<? super T> clazz) {
    return asArray((LowLevelIterator<T>) fsIterator(), clazz);
  }

  /**
   * This is a terminal operation, so can use/modify the original iterator
   * 
   * @param it
   *          the iterator positioned at the start position
   * @param clazz
   *          the class of the result
   * @return an array of elements from the position to the end
   */
  private T[] asArray(LowLevelIterator<T> it, Class<? super T> clazz) {
    // can't use the iterator's getArray method, because that returns the entire
    // array, starting from the first position thru the last position,
    // and the iterator might have been positioned other than the starting spot
    // by a following or startAt etc.

    ArrayList<T> a = asArrayList(it);
    T[] r = (T[]) Array.newInstance(clazz, a.size());
    return a.toArray(r);
  }

  // private T[] asArray(LowLevelIterator<T> it) {
  // return asArray(it, (Class<? super T>) ((TypeImpl)it.getType()).javaClass);
  // }

  private Annotation makePosAnnot(int begin, int end) {
    return _createMarkerAnnotation(jcas, begin, end);
  }

  //@formatter:off
  /**
   * Iterator respects backwards
   * 
   * Sets the characteristics from the context:
   *   IMMUTABLE / NONNULL / DISTINCT - always
   *   CONCURRENT - never
   *   ORDERED - unless orderNotNeeded index or not SORTED_INDEX or SET_INDEX
   *   SORTED - only for SORTED_INDEX (and not orderNotNeeded?)
   *   SIZED - if exact size is (easily) known, just from index.
   *           false if bounded, unambiguous
   *   SUBSIZED - if spliterator result from trysplit also is SIZED, set to true for now
   * 
   * trySplit impl: 
   *   always returns null (no parallelism support for now) 
   * @return the spliterator
   */
//@formatter:on
  @Override
  public Spliterator<T> spliterator() {
    return new Spliterator<T>() {

      private final FSIterator<T> it = fsIterator();

      private final FSIndex<T> localIndex = index;

      private final Comparator<? super T> comparator = (localIndex != null
              && localIndex.getIndexingStrategy() == FSIndex.SORTED_INDEX)
                      ? (Comparator<? super T>) localIndex
                      : null;

      private final int characteristics;
      { // set the characteristics and comparator
        // always set
        int c = Spliterator.IMMUTABLE | Spliterator.NONNULL | Spliterator.DISTINCT;

        if (!isAltSource && boundsUse == BoundsUse.notBounded && !isNonOverlapping) {
          c |= Spliterator.SIZED | Spliterator.SUBSIZED;
        }

        // set per indexing strategy
        switch ((null == localIndex) ? -1 : localIndex.getIndexingStrategy()) {
          case FSIndex.SORTED_INDEX:
            c |= Spliterator.ORDERED | Spliterator.SORTED;
            break;
          case FSIndex.SET_INDEX:
            c |= Spliterator.ORDERED;
            break;
          default: // do nothing
        }

        characteristics = c;
      }

      @Override
      public boolean tryAdvance(Consumer<? super T> action) {
        if (it.isValid()) {
          action.accept(it.getNvc());
          incr(it);
          return true;
        }
        return false;
      }

      @Override
      public Spliterator<T> trySplit() {
        // return null for now
        // could implement something based on type of fsIterator.
        return null;
      }

      @Override
      public long estimateSize() {
        return ((characteristics & Spliterator.SIZED) == Spliterator.SIZED && localIndex != null)
                ? localIndex.size()
                : Long.MAX_VALUE;
      }

      @Override
      public int characteristics() {
        return characteristics;
      }

      @Override
      public Comparator<? super T> getComparator() {
        if (comparator != null) {
          return comparator;
        }
        if ((characteristics & Spliterator.SORTED) == Spliterator.SORTED) {
          return null;
        }
        throw new IllegalStateException();
      }
    };
  }

  /*
   * returns the item the select is pointing to, or null if nullOK(false) then throws on null
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#get()
   */
  @Override
  public T get() {
    return getNullChk();
  }

  private T getNullChk() {
    FSIterator<T> it = fsIterator();
    if (it.isValid()) {
      return it.getNvc();
    }
    if (!isNullOK) { // if not specified, isNullOK == false
      throw new CASRuntimeException(CASRuntimeException.SELECT_GET_NO_INSTANCES, ti.getName(),
              maybeMsgPosition());
    }
    return null;

  }

  /*
   * like get() but throws if more than one item (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#single()
   */
  @Override
  public T single() {
    T v = singleOrNull();
    if (v == null && !isNullOK) {
      throw new CASRuntimeException(CASRuntimeException.SELECT_GET_NO_INSTANCES, ti.getName(),
              maybeMsgPosition());
    }
    return v;
  }

  /*
   * like get() but throws if more than 1 item, always OK to return null if none (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#singleOrNull()
   */
  @Override
  public T singleOrNull() {
    FSIterator<T> it = fsIterator();
    if (it.isValid()) {
      T v = it.getNvc();
      if (shift >= 0) {
        it.moveToNext();
      } else {
        it.moveToPrevious();
      }
      if (it.isValid()) {
        throw new CASRuntimeException(CASRuntimeException.SELECT_GET_TOO_MANY_INSTANCES,
                ti.getName(), maybeMsgPosition());
      }
      return v;
    }
    return null;
  }

  @Override
  public T get(int offset) {
    shift = offset;
    return getNullChk();
  }

  @Override
  public T single(int offset) {
    shift = offset;
    return single();
  }

  @Override
  public T singleOrNull(int offset) {
    shift = offset;
    return singleOrNull();
  }

  @Override
  public T get(TOP fs) {
    startAt(fs);
    return getNullChk();
  }

  @Override
  public T get(FeatureStructure fs) {
    startAt(fs);
    return getNullChk();
  }

  @Override
  public T single(TOP fs) {
    startAt(fs);
    return single();
  }

  @Override
  public T single(FeatureStructure fs) {
    startAt(fs);
    return single();
  }

  @Override
  public T singleOrNull(TOP fs) {
    startAt(fs);
    return singleOrNull();
  }

  @Override
  public T singleOrNull(FeatureStructure fs) {
    startAt(fs);
    return singleOrNull();
  }

  @Override
  public T get(TOP fs, int offset) {
    startAt(fs, offset);
    return getNullChk();
  }

  @Override
  public T get(FeatureStructure fs, int offset) {
    startAt(fs, offset);
    return getNullChk();
  }

  @Override
  public T single(TOP fs, int offset) {
    startAt(fs, offset);
    return single();
  }

  @Override
  public T single(FeatureStructure fs, int offset) {
    startAt(fs, offset);
    return single();
  }

  @Override
  public T singleOrNull(TOP fs, int offset) {
    startAt(fs, offset);
    return singleOrNull();
  }

  @Override
  public T singleOrNull(FeatureStructure fs, int offset) {
    startAt(fs, offset);
    return singleOrNull();
  }

  @Override
  public T get(int begin, int end) {
    startAt(begin, end);
    return getNullChk();
  }

  @Override
  public T single(int begin, int end) {
    startAt(begin, end);
    return single();
  }

  @Override
  public T singleOrNull(int begin, int end) {
    startAt(begin, end);
    return singleOrNull();
  }

  @Override
  public T get(int begin, int end, int offset) {
    startAt(begin, end, offset);
    return getNullChk();
  }

  @Override
  public T single(int begin, int end, int offset) {
    startAt(begin, end, offset);
    return single();
  }

  @Override
  public T singleOrNull(int begin, int end, int offset) {
    startAt(begin, end, offset);
    return singleOrNull();
  }

//@formatter:off
  /**
   * works for AnnotationIndex or general index
   * 
   * position taken from startingFs (not necessarily an Annotation subtype)
   *   - goes to left-most "equal" using comparator, or if none equal, to the first one &gt; startingFs
   *     -- using moveTo(fs)
   * 
   * special processing for AnnotationIndex (only):
   *   - typePriority - use or ignore
   *     -- ignored: after moveTo(fs), moveToPrevious while begin and end ==
   *       // REMOVED see https://issues.apache.org/jira/browse/UIMA-5536 --- and if isPositionUsesType types are == 
   * @param it iterator to position
   * @return it positioned if needed
   */
//@formatter:on
  private FSIterator<T> maybePosition(FSIterator<T> it) {
    if (!it.isValid() || startingFs == null || boundsUse != BoundsUse.notBounded) {
      return it;
    }

    if (isFollowing) {
      Annotation start = (Annotation) startingFs;
      if (start.getBegin() == start.getEnd()) {
        // We cannot moveTo(startingFS) because if startingFS has zero-width, then we would
        // end up positioning the iterator after a non-zero-width annotation which would be
        // considered to follow the startingFS.
        // Example:
        // [83,83] startingFS
        // [83,84] annotation that should be considered following the startingFS
        // but [83,84] comes *AFTER* [83,83] in index iteration order!
        it.moveTo(makePosAnnot(start.getBegin(), Integer.MAX_VALUE));
      } else {
        it.moveTo(startingFs);

        final int begin = ((Annotation) startingFs).getBegin();
        final int end = ((Annotation) startingFs).getEnd();
        while (it.isValid()) {
          int aBegin = ((Annotation) it.get()).getBegin();
          if (aBegin >= end && aBegin != begin) {
            break;
          }
          it.moveToNext();
        }
      }
    } else if (isPreceding) {
      Annotation start = (Annotation) startingFs;

      // A zero-width annotation will appear *after* a non-zero width annotation in the index, so
      // if we simply seek to a non-zero-width startingFs, we might miss a zero-width annotation
      // occurring at the beginning of startingFs. So instead, we seek to a zero-width position
      // at the begin of the startingFs and leave the rest to the filtering iterator we use for
      // preceding selection.
      // it.moveTo(startingFs);
      it.moveTo(makePosAnnot(start.getBegin(), start.getBegin()));

      final int begin = start.getBegin();
      while (it.isValid() && ((Annotation) it.get()).getEnd() > begin) {
        it.moveToPrevious();
      }
    } else {
      it.moveTo(startingFs);
    }

    return it;
  }

  private FSIterator<T> maybeShift(FSIterator<T> it) {
    if (shift != 0) {
      int ps = Math.abs(shift);

      for (int i = 0; i < ps; i++) {
        if (shift < 0) {
          it.moveToPrevious();
        } else {
          it.moveToNext();
        }
      }
    }

    return it;
  }

  private LowLevelIterator<T> maybeLimit(LowLevelIterator<T> it) {
    return (limit == -1) ? it : new FsIterator_limited<>(it, limit);
  }

  // *******************************************
  // The methods below are alternatives
  // to the methods above, that combine
  // frequently used patterns into more
  // concise forms using positional arguments
  // *******************************************

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#following(org.apache.uima.jcas.cas.TOP)
   */
  @Override
  public SelectFSs<T> following(Annotation fs) {
    return following(fs, 0);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#following(int, int)
   */
  @Override
  public SelectFSs<T> following(int position) {
    return following(position, 0);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#following(org.apache.uima.jcas.cas.TOP, int)
   */
  @Override
  public SelectFSs<T> following(Annotation fs, int offset) {
    return commonFollowing(fs, offset);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#following(int, int, int)
   */
  @Override
  public SelectFSs<T> following(int position, int offset) {
    return commonFollowing(makePosAnnot(position, position), offset);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#preceding(org.apache.uima.jcas.cas.TOP)
   */
  @Override
  public SelectFSs<T> preceding(Annotation fs) {
    return preceding(fs, 0);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#preceding(int, int)
   */
  @Override
  public SelectFSs<T> preceding(int position) {
    return preceding(position, 0);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#preceding(org.apache.uima.jcas.cas.TOP, int)
   */
  @Override
  public SelectFSs<T> preceding(Annotation annotation, int offset) {
    return commonPreceding(annotation, offset);
  }

  /*
   * (non-Javadoc)
   * 
   * @see org.apache.uima.cas.SelectFSs#preceding(int, int, int)
   */
  @Override
  public SelectFSs<T> preceding(int position, int offset) {
    return commonPreceding(makePosAnnot(position, Integer.MAX_VALUE), offset);
  }

  /************************
   * NOT USED
   */
  // public SelectFSs_impl<T> sameBeginEnd() { // AI
  // boundsUse = BoundsUse.sameBeginEnd;
  // return this;
  // }

//@formatter:off
  /**
   * validations
   *   isAnnotationIndex => startingFs is Annotation 
   *   isAllViews:  doesn't support startAt, coveredBy and friends, backwards
   *   isAllViews:  supports limit, shift
   */
//@formatter:on
  // private void validateSinglePosition(TOP fs, int offset) {
  // if (startingFs != null) {
  // /* Select - multiple starting positions not allowed */
  // throw CASRuntimeException(CASRuntimeException.);
  // }
  // startingFs = fs;
  //
  // if (offset != 0) {
  // if (shift != 0) {
  // /* Select - multiple offset shifting not allowed */
  // throw CASRuntimeException(CASRuntimeException.);
  // }
  // shift = offset;
  // }
  // }

  private SelectFSs<T> commonFollowing(Annotation annotation, int offset) {
    startingFs = annotation;
    shift = offset;
    isFollowing = true;
    return this;
  }

  private SelectFSs<T> commonPreceding(Annotation annotation, int offset) {
    // validateSinglePosition(fs, offset);
    startingFs = annotation;
    shift = offset;
    isPreceding = true;
    return this;
  }

  private void forceAnnotationIndex() {
    if (index == null) {
      index = (LowLevelIndex<T>) ((ti == null) ? view.getAnnotationIndex()
              : view.getAnnotationIndex(ti));
    } else {
      if (!(index instanceof AnnotationIndex)) {
        /** Index "{0}" must be an AnnotationIndex. */
        throw new CASRuntimeException(CASRuntimeException.ANNOTATION_INDEX_REQUIRED, index);
      }
    }
  }

  private Stream<T> stream() {
    return StreamSupport.stream(spliterator(), false); // false = default not parallel
  }

//@formatter:off
  /* ***************************************
   *   S T R E A M   methods
   * these convert the result to a stream and apply the method
   */
//@formatter:on
  @Override
  public Stream<T> filter(Predicate<? super T> predicate) {
    return stream().filter(predicate);
  }

  @Override
  public <R> Stream<R> map(Function<? super T, ? extends R> mapper) {
    return stream().map(mapper);
  }

  @Override
  public IntStream mapToInt(ToIntFunction<? super T> mapper) {
    return stream().mapToInt(mapper);
  }

  @Override
  public LongStream mapToLong(ToLongFunction<? super T> mapper) {
    return stream().mapToLong(mapper);
  }

  @Override
  public DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper) {
    return stream().mapToDouble(mapper);
  }

  @Override
  public <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper) {
    return stream().flatMap(mapper);
  }

  @Override
  public IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper) {
    return stream().flatMapToInt(mapper);
  }

  @Override
  public LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper) {
    return stream().flatMapToLong(mapper);
  }

  @Override
  public DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper) {
    return stream().flatMapToDouble(mapper);
  }

  @Override
  public Stream<T> distinct() {
    return stream().distinct();
  }

  @Override
  public Stream<T> sorted() {
    return stream().sorted();
  }

  @Override
  public Stream<T> sorted(Comparator<? super T> comparator) {
    return stream().sorted(comparator);
  }

  @Override
  public Stream<T> peek(Consumer<? super T> action) {
    return stream().peek(action);
  }

  @Override
  public Stream<T> limit(long maxSize) {
    return stream().limit(maxSize);
  }

  @Override
  public Stream<T> skip(long n) {
    return stream().skip(n);
  }

  @Override
  public void forEach(Consumer<? super T> action) {
    stream().forEach(action);
  }

  @Override
  public void forEachOrdered(Consumer<? super T> action) {
    stream().forEachOrdered(action);
  }

  @Override
  public Object[] toArray() {
    return stream().toArray();
  }

  @Override
  public <A> A[] toArray(IntFunction<A[]> generator) {
    return stream().toArray(generator);
  }

  @Override
  public T reduce(T identity, BinaryOperator<T> accumulator) {
    return stream().reduce(identity, accumulator);
  }

  @Override
  public Optional<T> reduce(BinaryOperator<T> accumulator) {
    return stream().reduce(accumulator);
  }

  @Override
  public <U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator,
          BinaryOperator<U> combiner) {
    return stream().reduce(identity, accumulator, combiner);
  }

  @Override
  public <R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator,
          BiConsumer<R, R> combiner) {
    return stream().collect(supplier, accumulator, combiner);
  }

  @Override
  public <R, A> R collect(Collector<? super T, A, R> collector) {
    return stream().collect(collector);
  }

  @Override
  public Optional<T> min(Comparator<? super T> comparator) {
    return stream().min(comparator);
  }

  @Override
  public Optional<T> max(Comparator<? super T> comparator) {
    return stream().max(comparator);
  }

  @Override
  public long count() {
    return stream().count();
  }

  @Override
  public boolean anyMatch(Predicate<? super T> predicate) {
    return stream().anyMatch(predicate);
  }

  @Override
  public boolean allMatch(Predicate<? super T> predicate) {
    return stream().allMatch(predicate);
  }

  @Override
  public boolean noneMatch(Predicate<? super T> predicate) {
    return stream().noneMatch(predicate);
  }

  @Override
  public Optional<T> findFirst() {
    return stream().findFirst();
  }

  @Override
  public Optional<T> findAny() {
    return stream().findAny();
  }

  @Override
  public boolean isParallel() {
    return stream().isParallel();
  }

  @Override
  public Stream<T> sequential() {
    return stream().sequential();
  }

  @Override
  public Stream<T> parallel() {
    return stream().parallel();
  }

  @Override
  public Stream<T> onClose(Runnable closeHandler) {
    return stream().onClose(closeHandler);
  }

  @Override
  public void close() {
    // SelectFSs_impl holds no resources that would need to be cleaned up
  }

  @Override
  public Stream<T> unordered() {
    return stream().unordered();
  }

  @Override
  public boolean isEmpty() {
    if (limit == 0) {
      return true;
    }
    return fsIterator().size() == 0;
  }

  private final class SelectFSIterator implements LowLevelIterator<T> {

    private Supplier<LowLevelIterator<T>> iteratorSupplier;
    private LowLevelIterator<T> it;

    private SelectFSIterator(Supplier<LowLevelIterator<T>> aIteratorSupplier) {
      iteratorSupplier = aIteratorSupplier;

      it = iteratorSupplier.get();
    }

    @Override
    public boolean isValid() {
      return it.isValid();
    }

    @Override
    public T getNvc() {
      return it.getNvc();
    }

    @Override
    public void moveToNextNvc() {
      it.moveToNextNvc();
    }

    @Override
    public void moveToPreviousNvc() {
      it.moveToPreviousNvc();
    }

    @Override
    public void moveToFirst() {
      // There is quite a bit of logic in SelectFS that happens *after* the iterator has been
      // created using fsIterator1() that positions the iterator at the starting position required
      // by the SelectFS settings. In order to avoid having to duplicate that repositioning code,
      // we just make a new iterator here.
      it = iteratorSupplier.get();
    }

    @Override
    public void moveToLast() {
      it.moveToLast();
    }

    @Override
    public void moveTo(FeatureStructure aFs) {
      it.moveTo(aFs);
    }

    @Override
    public FSIterator<T> copy() {
      return it.copy();
    }

    @Override
    public int ll_indexSizeMaybeNotCurrent() {
      return it.ll_indexSizeMaybeNotCurrent();
    }

    @Override
    public LowLevelIndex<T> ll_getIndex() {
      return it.ll_getIndex();
    }

    @Override
    public int ll_maxAnnotSpan() {
      return it.ll_maxAnnotSpan();
    }

    @Override
    public boolean isIndexesHaveBeenUpdated() {
      return it.isIndexesHaveBeenUpdated();
    }

    @Override
    public boolean maybeReinitIterator() {
      return it.maybeReinitIterator();
    }

    @Override
    public void moveToFirstNoReinit() {
      moveToFirst();
    }

    @Override
    public void moveToLastNoReinit() {
      it.moveToLastNoReinit();
    }

    @Override
    public void moveToNoReinit(FeatureStructure aFs) {
      it.moveToNoReinit(aFs);
    }

    @Override
    public Comparator<TOP> getComparator() {
      return it.getComparator();
    }
  }
}