/*
    * 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.commons.math4.legacy.linear;
  
  import java.io.Serializable;
  
  import org.apache.commons.math4.legacy.core.Field;
  import org.apache.commons.math4.legacy.core.FieldElement;
  import org.apache.commons.math4.legacy.exception.DimensionMismatchException;
  import org.apache.commons.math4.legacy.exception.MathArithmeticException;
  import org.apache.commons.math4.legacy.exception.NotPositiveException;
  import org.apache.commons.math4.legacy.exception.NullArgumentException;
  import org.apache.commons.math4.legacy.exception.NumberIsTooSmallException;
  import org.apache.commons.math4.legacy.exception.OutOfRangeException;
  import org.apache.commons.math4.legacy.exception.util.LocalizedFormats;
  import org.apache.commons.math4.legacy.core.MathArrays;
  
  /**
   * This class implements the {@link FieldVector} interface with a {@link OpenIntToFieldHashMap} backing store.
   * <p>
   *  Caveat: This implementation assumes that, for any {@code x},
   *  the equality {@code x * 0d == 0d} holds. But it is is not true for
   *  {@code NaN}. Moreover, zero entries will lose their sign.
   *  Some operations (that involve {@code NaN} and/or infinities) may
   *  thus give incorrect results.
   * </p>
   * @param <T> the type of the field elements
   * @since 2.0
   */
  public class SparseFieldVector<T extends FieldElement<T>> implements FieldVector<T>, Serializable {
      /**  Serialization identifier. */
      private static final long serialVersionUID = 7841233292190413362L;
      /** Field to which the elements belong. */
      private final Field<T> field;
      /** Entries of the vector. */
      private final OpenIntToFieldHashMap<T> entries;
      /** Dimension of the vector. */
      private final int virtualSize;
  
      /**
       * Build a 0-length vector.
       * Zero-length vectors may be used to initialize construction of vectors
       * by data gathering. We start with zero-length and use either the {@link
       * #SparseFieldVector(SparseFieldVector, int)} constructor
       * or one of the {@code append} method ({@link #append(FieldVector)} or
       * {@link #append(SparseFieldVector)}) to gather data into this vector.
       *
       * @param field Field to which the elements belong.
       */
      public SparseFieldVector(Field<T> field) {
          this(field, 0);
      }
  
  
      /**
       * Construct a vector of zeroes.
       *
       * @param field Field to which the elements belong.
       * @param dimension Size of the vector.
       */
      public SparseFieldVector(Field<T> field, int dimension) {
          this.field = field;
          virtualSize = dimension;
          entries = new OpenIntToFieldHashMap<>(field);
      }
  
      /**
       * Build a resized vector, for use with append.
       *
       * @param v Original vector
       * @param resize Amount to add.
       */
      protected SparseFieldVector(SparseFieldVector<T> v, int resize) {
          field = v.field;
          virtualSize = v.getDimension() + resize;
          entries = new OpenIntToFieldHashMap<>(v.entries);
      }
  
  
      /**
       * Build a vector with known the sparseness (for advanced use only).
       *
       * @param field Field to which the elements belong.
       * @param dimension Size of the vector.
       * @param expectedSize Expected number of non-zero entries.
      */
     public SparseFieldVector(Field<T> field, int dimension, int expectedSize) {
         this.field = field;
         virtualSize = dimension;
         entries = new OpenIntToFieldHashMap<>(field,expectedSize);
     }
 
     /**
      * Create from a Field array.
      * Only non-zero entries will be stored.
      *
      * @param field Field to which the elements belong.
      * @param values Set of values to create from.
      * @exception NullArgumentException if values is null
      */
     public SparseFieldVector(Field<T> field, T[] values) throws NullArgumentException {
         NullArgumentException.check(values);
         this.field = field;
         virtualSize = values.length;
         entries = new OpenIntToFieldHashMap<>(field);
         for (int key = 0; key < values.length; key++) {
             T value = values[key];
             entries.put(key, value);
         }
     }
 
     /**
      * Copy constructor.
      *
      * @param v Instance to copy.
      */
     public SparseFieldVector(SparseFieldVector<T> v) {
         field = v.field;
         virtualSize = v.getDimension();
         entries = new OpenIntToFieldHashMap<>(v.getEntries());
     }
 
     /**
      * Get the entries of this instance.
      *
      * @return the entries of this instance
      */
     private OpenIntToFieldHashMap<T> getEntries() {
         return entries;
     }
 
     /**
      * Optimized method to add sparse vectors.
      *
      * @param v Vector to add.
      * @return {@code this + v}.
      * @throws DimensionMismatchException if {@code v} is not the same size as
      * {@code this}.
      */
     public FieldVector<T> add(SparseFieldVector<T> v)
         throws DimensionMismatchException {
         checkVectorDimensions(v.getDimension());
         SparseFieldVector<T> res = (SparseFieldVector<T>)copy();
         OpenIntToFieldHashMap<T>.Iterator iter = v.getEntries().iterator();
         while (iter.hasNext()) {
             iter.advance();
             int key = iter.key();
             T value = iter.value();
             if (entries.containsKey(key)) {
                 res.setEntry(key, entries.get(key).add(value));
             } else {
                 res.setEntry(key, value);
             }
         }
         return res;
     }
 
     /**
      * Construct a vector by appending a vector to this vector.
      *
      * @param v Vector to append to this one.
      * @return a new vector.
      */
     public FieldVector<T> append(SparseFieldVector<T> v) {
         SparseFieldVector<T> res = new SparseFieldVector<>(this, v.getDimension());
         OpenIntToFieldHashMap<T>.Iterator iter = v.entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             res.setEntry(iter.key() + virtualSize, iter.value());
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> append(FieldVector<T> v) {
         if (v instanceof SparseFieldVector<?>) {
             return append((SparseFieldVector<T>) v);
         } else {
             final int n = v.getDimension();
             FieldVector<T> res = new SparseFieldVector<>(this, n);
             for (int i = 0; i < n; i++) {
                 res.setEntry(i + virtualSize, v.getEntry(i));
             }
             return res;
         }
     }
 
     /** {@inheritDoc}
      * @exception NullArgumentException if d is null
      */
     @Override
     public FieldVector<T> append(T d) throws NullArgumentException {
         NullArgumentException.check(d);
         FieldVector<T> res = new SparseFieldVector<>(this, 1);
         res.setEntry(virtualSize, d);
         return res;
      }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> copy() {
         return new SparseFieldVector<>(this);
     }
 
     /** {@inheritDoc} */
     @Override
     public T dotProduct(FieldVector<T> v) throws DimensionMismatchException {
         checkVectorDimensions(v.getDimension());
         T res = field.getZero();
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             res = res.add(v.getEntry(iter.key()).multiply(iter.value()));
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> ebeDivide(FieldVector<T> v)
         throws DimensionMismatchException, MathArithmeticException {
         checkVectorDimensions(v.getDimension());
         SparseFieldVector<T> res = new SparseFieldVector<>(this);
         OpenIntToFieldHashMap<T>.Iterator iter = res.entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             res.setEntry(iter.key(), iter.value().divide(v.getEntry(iter.key())));
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> ebeMultiply(FieldVector<T> v)
         throws DimensionMismatchException {
         checkVectorDimensions(v.getDimension());
         SparseFieldVector<T> res = new SparseFieldVector<>(this);
         OpenIntToFieldHashMap<T>.Iterator iter = res.entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             res.setEntry(iter.key(), iter.value().multiply(v.getEntry(iter.key())));
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public int getDimension() {
         return virtualSize;
     }
 
     /** {@inheritDoc} */
     @Override
     public T getEntry(int index) throws OutOfRangeException {
         checkIndex(index);
         return entries.get(index);
    }
 
     /** {@inheritDoc} */
     @Override
     public Field<T> getField() {
         return field;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> getSubVector(int index, int n)
         throws OutOfRangeException, NotPositiveException {
         if (n < 0) {
             throw new NotPositiveException(LocalizedFormats.NUMBER_OF_ELEMENTS_SHOULD_BE_POSITIVE, n);
         }
         checkIndex(index);
         checkIndex(index + n - 1);
         SparseFieldVector<T> res = new SparseFieldVector<>(field,n);
         int end = index + n;
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             int key = iter.key();
             if (key >= index && key < end) {
                 res.setEntry(key - index, iter.value());
             }
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapAdd(T d) throws NullArgumentException {
         return copy().mapAddToSelf(d);
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapAddToSelf(T d) throws NullArgumentException {
         for (int i = 0; i < virtualSize; i++) {
             setEntry(i, getEntry(i).add(d));
         }
         return this;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapDivide(T d)
         throws NullArgumentException, MathArithmeticException {
         return copy().mapDivideToSelf(d);
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapDivideToSelf(T d)
         throws NullArgumentException, MathArithmeticException {
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             entries.put(iter.key(), iter.value().divide(d));
         }
         return this;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapInv() throws MathArithmeticException {
         return copy().mapInvToSelf();
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapInvToSelf() throws MathArithmeticException {
         for (int i = 0; i < virtualSize; i++) {
             setEntry(i, field.getOne().divide(getEntry(i)));
         }
         return this;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapMultiply(T d) throws NullArgumentException {
         return copy().mapMultiplyToSelf(d);
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapMultiplyToSelf(T d) throws NullArgumentException {
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             entries.put(iter.key(), iter.value().multiply(d));
         }
         return this;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapSubtract(T d) throws NullArgumentException {
         return copy().mapSubtractToSelf(d);
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> mapSubtractToSelf(T d) throws NullArgumentException {
         return mapAddToSelf(field.getZero().subtract(d));
     }
 
     /**
      * Optimized method to compute outer product when both vectors are sparse.
      * @param v vector with which outer product should be computed
      * @return the matrix outer product between instance and v
      */
     public FieldMatrix<T> outerProduct(SparseFieldVector<T> v) {
         final int n = v.getDimension();
         SparseFieldMatrix<T> res = new SparseFieldMatrix<>(field, virtualSize, n);
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             OpenIntToFieldHashMap<T>.Iterator iter2 = v.entries.iterator();
             while (iter2.hasNext()) {
                 iter2.advance();
                 res.setEntry(iter.key(), iter2.key(), iter.value().multiply(iter2.value()));
             }
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldMatrix<T> outerProduct(FieldVector<T> v) {
         if (v instanceof SparseFieldVector<?>) {
             return outerProduct((SparseFieldVector<T>)v);
         } else {
             final int n = v.getDimension();
             FieldMatrix<T> res = new SparseFieldMatrix<>(field, virtualSize, n);
             OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
             while (iter.hasNext()) {
                 iter.advance();
                 int row = iter.key();
                 FieldElement<T> value = iter.value();
                 for (int col = 0; col < n; col++) {
                     res.setEntry(row, col, value.multiply(v.getEntry(col)));
                 }
             }
             return res;
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> projection(FieldVector<T> v)
         throws DimensionMismatchException, MathArithmeticException {
         checkVectorDimensions(v.getDimension());
         return v.mapMultiply(dotProduct(v).divide(v.dotProduct(v)));
     }
 
     /** {@inheritDoc}
      * @exception NullArgumentException if value is null
      */
     @Override
     public void set(T value) {
         NullArgumentException.check(value);
         for (int i = 0; i < virtualSize; i++) {
             setEntry(i, value);
         }
     }
 
     /** {@inheritDoc}
      * @exception NullArgumentException if value is null
      */
     @Override
     public void setEntry(int index, T value) throws NullArgumentException, OutOfRangeException {
         NullArgumentException.check(value);
         checkIndex(index);
         entries.put(index, value);
     }
 
     /** {@inheritDoc} */
     @Override
     public void setSubVector(int index, FieldVector<T> v)
         throws OutOfRangeException {
         checkIndex(index);
         checkIndex(index + v.getDimension() - 1);
         final int n = v.getDimension();
         for (int i = 0; i < n; i++) {
             setEntry(i + index, v.getEntry(i));
         }
     }
 
     /**
      * Optimized method to compute {@code this} minus {@code v}.
      * @param v vector to be subtracted
      * @return {@code this - v}
      * @throws DimensionMismatchException if {@code v} is not the same size as
      * {@code this}.
      */
     public SparseFieldVector<T> subtract(SparseFieldVector<T> v)
         throws DimensionMismatchException {
         checkVectorDimensions(v.getDimension());
         SparseFieldVector<T> res = (SparseFieldVector<T>)copy();
         OpenIntToFieldHashMap<T>.Iterator iter = v.getEntries().iterator();
         while (iter.hasNext()) {
             iter.advance();
             int key = iter.key();
             if (entries.containsKey(key)) {
                 res.setEntry(key, entries.get(key).subtract(iter.value()));
             } else {
                 res.setEntry(key, field.getZero().subtract(iter.value()));
             }
         }
         return res;
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> subtract(FieldVector<T> v)
         throws DimensionMismatchException {
         if (v instanceof SparseFieldVector<?>) {
             return subtract((SparseFieldVector<T>)v);
         } else {
             final int n = v.getDimension();
             checkVectorDimensions(n);
             SparseFieldVector<T> res = new SparseFieldVector<>(this);
             for (int i = 0; i < n; i++) {
                 if (entries.containsKey(i)) {
                     res.setEntry(i, entries.get(i).subtract(v.getEntry(i)));
                 } else {
                     res.setEntry(i, field.getZero().subtract(v.getEntry(i)));
                 }
             }
             return res;
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public T[] toArray() {
         T[] res = MathArrays.buildArray(field, virtualSize);
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             res[iter.key()] = iter.value();
         }
         return res;
     }
 
     /**
      * Check whether an index is valid.
      *
      * @param index Index to check.
      * @throws OutOfRangeException if the index is not valid.
      */
     private void checkIndex(final int index) throws OutOfRangeException {
         if (index < 0 || index >= getDimension()) {
             throw new OutOfRangeException(index, 0, getDimension() - 1);
         }
     }
 
     /**
      * Checks that the indices of a subvector are valid.
      *
      * @param start the index of the first entry of the subvector
      * @param end the index of the last entry of the subvector (inclusive)
      * @throws OutOfRangeException if {@code start} of {@code end} are not valid
      * @throws NumberIsTooSmallException if {@code end < start}
      * @since 3.3
      */
     private void checkIndices(final int start, final int end)
         throws NumberIsTooSmallException, OutOfRangeException {
         final int dim = getDimension();
         if (start < 0 || start >= dim) {
             throw new OutOfRangeException(LocalizedFormats.INDEX, start, 0,
                                           dim - 1);
         }
         if (end < 0 || end >= dim) {
             throw new OutOfRangeException(LocalizedFormats.INDEX, end, 0,
                                           dim - 1);
         }
         if (end < start) {
             throw new NumberIsTooSmallException(LocalizedFormats.INITIAL_ROW_AFTER_FINAL_ROW,
                                                 end, start, false);
         }
     }
 
     /**
      * Check if instance dimension is equal to some expected value.
      *
      * @param n Expected dimension.
      * @throws DimensionMismatchException if the dimensions do not match.
      */
     protected void checkVectorDimensions(int n)
         throws DimensionMismatchException {
         if (getDimension() != n) {
             throw new DimensionMismatchException(getDimension(), n);
         }
     }
 
     /** {@inheritDoc} */
     @Override
     public FieldVector<T> add(FieldVector<T> v) throws DimensionMismatchException {
         if (v instanceof SparseFieldVector<?>) {
             return add((SparseFieldVector<T>) v);
         } else {
             final int n = v.getDimension();
             checkVectorDimensions(n);
             SparseFieldVector<T> res = new SparseFieldVector<>(field,
                                                                 getDimension());
             for (int i = 0; i < n; i++) {
                 res.setEntry(i, v.getEntry(i).add(getEntry(i)));
             }
             return res;
         }
     }
 
     /**
      * Visits (but does not alter) all entries of this vector in default order
      * (increasing index).
      *
      * @param visitor the visitor to be used to process the entries of this
      * vector
      * @return the value returned by {@link FieldVectorPreservingVisitor#end()}
      * at the end of the walk
      * @since 3.3
      */
     public T walkInDefaultOrder(final FieldVectorPreservingVisitor<T> visitor) {
         final int dim = getDimension();
         visitor.start(dim, 0, dim - 1);
         for (int i = 0; i < dim; i++) {
             visitor.visit(i, getEntry(i));
         }
         return visitor.end();
     }
 
     /**
      * Visits (but does not alter) some entries of this vector in default order
      * (increasing index).
      *
      * @param visitor visitor to be used to process the entries of this vector
      * @param start the index of the first entry to be visited
      * @param end the index of the last entry to be visited (inclusive)
      * @return the value returned by {@link FieldVectorPreservingVisitor#end()}
      * at the end of the walk
      * @throws NumberIsTooSmallException if {@code end < start}.
      * @throws OutOfRangeException if the indices are not valid.
      * @since 3.3
      */
     public T walkInDefaultOrder(final FieldVectorPreservingVisitor<T> visitor,
                                 final int start, final int end)
         throws NumberIsTooSmallException, OutOfRangeException {
         checkIndices(start, end);
         visitor.start(getDimension(), start, end);
         for (int i = start; i <= end; i++) {
             visitor.visit(i, getEntry(i));
         }
         return visitor.end();
     }
 
     /**
      * Visits (but does not alter) all entries of this vector in optimized
      * order. The order in which the entries are visited is selected so as to
      * lead to the most efficient implementation; it might depend on the
      * concrete implementation of this abstract class.
      *
      * @param visitor the visitor to be used to process the entries of this
      * vector
      * @return the value returned by {@link FieldVectorPreservingVisitor#end()}
      * at the end of the walk
      * @since 3.3
      */
     public T walkInOptimizedOrder(final FieldVectorPreservingVisitor<T> visitor) {
         return walkInDefaultOrder(visitor);
     }
 
     /**
      * Visits (but does not alter) some entries of this vector in optimized
      * order. The order in which the entries are visited is selected so as to
      * lead to the most efficient implementation; it might depend on the
      * concrete implementation of this abstract class.
      *
      * @param visitor visitor to be used to process the entries of this vector
      * @param start the index of the first entry to be visited
      * @param end the index of the last entry to be visited (inclusive)
      * @return the value returned by {@link FieldVectorPreservingVisitor#end()}
      * at the end of the walk
      * @throws NumberIsTooSmallException if {@code end < start}.
      * @throws OutOfRangeException if the indices are not valid.
      * @since 3.3
      */
     public T walkInOptimizedOrder(final FieldVectorPreservingVisitor<T> visitor,
                                   final int start, final int end)
         throws NumberIsTooSmallException, OutOfRangeException {
         return walkInDefaultOrder(visitor, start, end);
     }
 
     /**
      * Visits (and possibly alters) all entries of this vector in default order
      * (increasing index).
      *
      * @param visitor the visitor to be used to process and modify the entries
      * of this vector
      * @return the value returned by {@link FieldVectorChangingVisitor#end()}
      * at the end of the walk
      * @since 3.3
      */
     public T walkInDefaultOrder(final FieldVectorChangingVisitor<T> visitor) {
         final int dim = getDimension();
         visitor.start(dim, 0, dim - 1);
         for (int i = 0; i < dim; i++) {
             setEntry(i, visitor.visit(i, getEntry(i)));
         }
         return visitor.end();
     }
 
     /**
      * Visits (and possibly alters) some entries of this vector in default order
      * (increasing index).
      *
      * @param visitor visitor to be used to process the entries of this vector
      * @param start the index of the first entry to be visited
      * @param end the index of the last entry to be visited (inclusive)
      * @return the value returned by {@link FieldVectorChangingVisitor#end()}
      * at the end of the walk
      * @throws NumberIsTooSmallException if {@code end < start}.
      * @throws OutOfRangeException if the indices are not valid.
      * @since 3.3
      */
     public T walkInDefaultOrder(final FieldVectorChangingVisitor<T> visitor,
                                 final int start, final int end)
         throws NumberIsTooSmallException, OutOfRangeException {
         checkIndices(start, end);
         visitor.start(getDimension(), start, end);
         for (int i = start; i <= end; i++) {
             setEntry(i, visitor.visit(i, getEntry(i)));
         }
         return visitor.end();
     }
 
     /**
      * Visits (and possibly alters) all entries of this vector in optimized
      * order. The order in which the entries are visited is selected so as to
      * lead to the most efficient implementation; it might depend on the
      * concrete implementation of this abstract class.
      *
      * @param visitor the visitor to be used to process the entries of this
      * vector
      * @return the value returned by {@link FieldVectorChangingVisitor#end()}
      * at the end of the walk
      * @since 3.3
      */
     public T walkInOptimizedOrder(final FieldVectorChangingVisitor<T> visitor) {
         return walkInDefaultOrder(visitor);
     }
 
     /**
      * Visits (and possibly change) some entries of this vector in optimized
      * order. The order in which the entries are visited is selected so as to
      * lead to the most efficient implementation; it might depend on the
      * concrete implementation of this abstract class.
      *
      * @param visitor visitor to be used to process the entries of this vector
      * @param start the index of the first entry to be visited
      * @param end the index of the last entry to be visited (inclusive)
      * @return the value returned by {@link FieldVectorChangingVisitor#end()}
      * at the end of the walk
      * @throws NumberIsTooSmallException if {@code end < start}.
      * @throws OutOfRangeException if the indices are not valid.
      * @since 3.3
      */
     public T walkInOptimizedOrder(final FieldVectorChangingVisitor<T> visitor,
                                   final int start, final int end)
         throws NumberIsTooSmallException, OutOfRangeException {
         return walkInDefaultOrder(visitor, start, end);
     }
 
     /** {@inheritDoc} */
     @Override
     public int hashCode() {
         final int prime = 31;
         int result = 1;
         result = prime * result + ((field == null) ? 0 : field.hashCode());
         result = prime * result + virtualSize;
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             int temp = iter.value().hashCode();
             result = prime * result + temp;
         }
         return result;
     }
 
 
     /** {@inheritDoc} */
     @Override
     public boolean equals(Object obj) {
 
         if (this == obj) {
             return true;
         }
 
         if (!(obj instanceof SparseFieldVector<?>)) {
             return false;
         }
 
         @SuppressWarnings("unchecked") // OK, because "else if" check below ensures that
                                        // other must be the same type as this
         SparseFieldVector<T> other = (SparseFieldVector<T>) obj;
         if (field == null) {
             if (other.field != null) {
                 return false;
             }
         } else if (!field.equals(other.field)) {
             return false;
         }
         if (virtualSize != other.virtualSize) {
             return false;
         }
 
         OpenIntToFieldHashMap<T>.Iterator iter = entries.iterator();
         while (iter.hasNext()) {
             iter.advance();
             T test = other.getEntry(iter.key());
             if (!test.equals(iter.value())) {
                 return false;
             }
         }
         iter = other.getEntries().iterator();
         while (iter.hasNext()) {
             iter.advance();
             T test = iter.value();
             if (!test.equals(getEntry(iter.key()))) {
                 return false;
             }
         }
         return true;
     }
 }