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   *  distributed with this work for additional information
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   *  to you under the Apache License, Version 2.0 (the
   *  "License"); you may not use this file except in compliance
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   *    https://www.apache.org/licenses/LICENSE-2.0
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  package org.apache.cayenne.access.jdbc;
  
  import org.apache.cayenne.DataRow;
  import org.apache.cayenne.ResultIterator;
  import org.apache.cayenne.map.DbAttribute;
  import org.apache.cayenne.map.DbEntity;
  import org.apache.cayenne.util.ResultIteratorIterator;
  
  import java.util.ArrayList;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.NoSuchElementException;
  import java.util.Set;
  
  /**
   * A ResultIterator that does in-memory filtering of rows to return only
   * distinct rows. Distinct comparison is done by comparing ObjectIds created
   * from each row. Internally DistinctResultIterator wraps another ResultIterator
   * that provides the actual rows.
   * 
   * @since 3.0
   */
  public class DistinctResultIterator<T> implements ResultIterator<T> {
  
      protected ResultIterator<T> delegate;
      protected Set<Object> fetchedIds;
      protected T nextDataRow;
      protected DbEntity defaultEntity;
      protected boolean compareFullRows;
  
      /**
       * Creates new DistinctResultIterator wrapping another ResultIterator.
       * 
       * @param delegate
       *            actual result iterator, that will be decorated by this DistinctResultIterator
       * @param defaultEntity
       *            an entity needed to build ObjectIds for distinct comparison.
       */
      public DistinctResultIterator(ResultIterator<T> delegate, DbEntity defaultEntity, boolean compareFullRows) {
          if (delegate == null) {
              throw new NullPointerException("Null wrapped iterator.");
          }
  
          if (defaultEntity == null) {
              throw new NullPointerException("Null defaultEntity.");
          }
  
          this.delegate = delegate;
          this.defaultEntity = defaultEntity;
          this.fetchedIds = new HashSet<>();
          this.compareFullRows = compareFullRows;
  
          checkNextRow();
      }
  
      /**
       * @since 4.0
       */
      @Override
      public Iterator<T> iterator() {
          return new ResultIteratorIterator<>(this);
      }
  
      /**
       * Closes underlying ResultIterator.
       */
      @Override
      public void close() {
          delegate.close();
      }
  
      /**
       * @since 3.0
       */
      @Override
      public List<T> allRows() {
          List<T> list = new ArrayList<>();
  
          while (this.hasNextRow()) {
              list.add(nextRow());
          }
          return list;
      }
  
      @Override
      public boolean hasNextRow() {
          return nextDataRow != null;
      }
  
      @Override
      public T nextRow() {
          if (!hasNextRow()) {
              throw new NoSuchElementException("An attempt to read uninitialized row or past the end of the iterator.");
          }
  
          T row = nextDataRow;
          checkNextRow();
          return row;
      }
  
      /**
       * @since 3.0
       */
      @Override
      public void skipRow() {
          if (!hasNextRow()) {
              throw new NoSuchElementException("An attempt to read uninitialized row or past the end of the iterator.");
          }
  
          checkNextRow();
      }
  
      private void checkNextRow() {
          if (this.compareFullRows) {
              checkNextUniqueRow();
          } else {
              checkNextRowWithUniqueId();
          }
      }
  
      private void checkNextUniqueRow() {
          nextDataRow = null;
          while (delegate.hasNextRow()) {
              T next = delegate.nextRow();
  
              if (fetchedIds.add(next)) {
                  this.nextDataRow = next;
                  break;
              }
          }
      }
  
      private void checkNextRowWithUniqueId() {
          nextDataRow = null;
          while (delegate.hasNextRow()) {
              T next = delegate.nextRow();
  
              // create id map...
              // TODO: this can be optimized by creating an array with id keys
              // to avoid iterating over default entity attributes...
  
              Map<String, Object> id = new HashMap<>();
              for (final DbAttribute pk : defaultEntity.getPrimaryKeys()) {
                  id.put(pk.getName(), ((DataRow)next).get(pk.getName()));
              }
  
              if (fetchedIds.add(id)) {
                  this.nextDataRow = next;
                  break;
              }
          }
      }
  
  }
  