/*
    *    Copyright 2009-2021 the original author or authors.
    *
    *    Licensed under the Apache License, Version 2.0 (the "License");
    *    you may not use this file except in compliance with the License.
    *    You may obtain a copy of the License at
    *
    *       http://www.apache.org/licenses/LICENSE-2.0
    *
   *    Unless required by applicable law or agreed to in writing, software
   *    distributed under the License is distributed on an "AS IS" BASIS,
   *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   *    See the License for the specific language governing permissions and
   *    limitations under the License.
   */
  package org.apache.ibatis.cursor.defaults;
  
  import java.sql.ResultSet;
  import java.sql.SQLException;
  import java.util.Iterator;
  import java.util.NoSuchElementException;
  
  import org.apache.ibatis.cursor.Cursor;
  import org.apache.ibatis.executor.resultset.DefaultResultSetHandler;
  import org.apache.ibatis.executor.resultset.ResultSetWrapper;
  import org.apache.ibatis.mapping.ResultMap;
  import org.apache.ibatis.session.ResultContext;
  import org.apache.ibatis.session.ResultHandler;
  import org.apache.ibatis.session.RowBounds;
  
  /**
   * This is the default implementation of a MyBatis Cursor.
   * This implementation is not thread safe.
   *
   * @author Guillaume Darmont / guillaume@dropinocean.com
   */
  public class DefaultCursor<T> implements Cursor<T> {
  
    // ResultSetHandler stuff
    private final DefaultResultSetHandler resultSetHandler;
    private final ResultMap resultMap;
    private final ResultSetWrapper rsw;
    private final RowBounds rowBounds;
    protected final ObjectWrapperResultHandler<T> objectWrapperResultHandler = new ObjectWrapperResultHandler<>();
  
    private final CursorIterator cursorIterator = new CursorIterator();
    private boolean iteratorRetrieved;
  
    private CursorStatus status = CursorStatus.CREATED;
    private int indexWithRowBound = -1;
  
    private enum CursorStatus {
  
      /**
       * A freshly created cursor, database ResultSet consuming has not started.
       */
      CREATED,
      /**
       * A cursor currently in use, database ResultSet consuming has started.
       */
      OPEN,
      /**
       * A closed cursor, not fully consumed.
       */
      CLOSED,
      /**
       * A fully consumed cursor, a consumed cursor is always closed.
       */
      CONSUMED
    }
  
    public DefaultCursor(DefaultResultSetHandler resultSetHandler, ResultMap resultMap, ResultSetWrapper rsw, RowBounds rowBounds) {
      this.resultSetHandler = resultSetHandler;
      this.resultMap = resultMap;
      this.rsw = rsw;
      this.rowBounds = rowBounds;
    }
  
    @Override
    public boolean isOpen() {
      return status == CursorStatus.OPEN;
    }
  
    @Override
    public boolean isConsumed() {
      return status == CursorStatus.CONSUMED;
    }
  
    @Override
    public int getCurrentIndex() {
      return rowBounds.getOffset() + cursorIterator.iteratorIndex;
    }
  
    @Override
    public Iterator<T> iterator() {
      if (iteratorRetrieved) {
        throw new IllegalStateException("Cannot open more than one iterator on a Cursor");
      }
      if (isClosed()) {
       throw new IllegalStateException("A Cursor is already closed.");
     }
     iteratorRetrieved = true;
     return cursorIterator;
   }
 
   @Override
   public void close() {
     if (isClosed()) {
       return;
     }
 
     ResultSet rs = rsw.getResultSet();
     try {
       if (rs != null) {
         rs.close();
       }
     } catch (SQLException e) {
       // ignore
     } finally {
       status = CursorStatus.CLOSED;
     }
   }
 
   protected T fetchNextUsingRowBound() {
     T result = fetchNextObjectFromDatabase();
     while (objectWrapperResultHandler.fetched && indexWithRowBound < rowBounds.getOffset()) {
       result = fetchNextObjectFromDatabase();
     }
     return result;
   }
 
   protected T fetchNextObjectFromDatabase() {
     if (isClosed()) {
       return null;
     }
 
     try {
       objectWrapperResultHandler.fetched = false;
       status = CursorStatus.OPEN;
       if (!rsw.getResultSet().isClosed()) {
         resultSetHandler.handleRowValues(rsw, resultMap, objectWrapperResultHandler, RowBounds.DEFAULT, null);
       }
     } catch (SQLException e) {
       throw new RuntimeException(e);
     }
 
     T next = objectWrapperResultHandler.result;
     if (objectWrapperResultHandler.fetched) {
       indexWithRowBound++;
     }
     // No more object or limit reached
     if (!objectWrapperResultHandler.fetched || getReadItemsCount() == rowBounds.getOffset() + rowBounds.getLimit()) {
       close();
       status = CursorStatus.CONSUMED;
     }
     objectWrapperResultHandler.result = null;
 
     return next;
   }
 
   private boolean isClosed() {
     return status == CursorStatus.CLOSED || status == CursorStatus.CONSUMED;
   }
 
   private int getReadItemsCount() {
     return indexWithRowBound + 1;
   }
 
   protected static class ObjectWrapperResultHandler<T> implements ResultHandler<T> {
 
     protected T result;
     protected boolean fetched;
 
     @Override
     public void handleResult(ResultContext<? extends T> context) {
       this.result = context.getResultObject();
       context.stop();
       fetched = true;
     }
   }
 
   protected class CursorIterator implements Iterator<T> {
 
     /**
      * Holder for the next object to be returned.
      */
     T object;
 
     /**
      * Index of objects returned using next(), and as such, visible to users.
      */
     int iteratorIndex = -1;
 
     @Override
     public boolean hasNext() {
       if (!objectWrapperResultHandler.fetched) {
         object = fetchNextUsingRowBound();
       }
       return objectWrapperResultHandler.fetched;
     }
 
     @Override
     public T next() {
       // Fill next with object fetched from hasNext()
       T next = object;
 
       if (!objectWrapperResultHandler.fetched) {
         next = fetchNextUsingRowBound();
       }
 
       if (objectWrapperResultHandler.fetched) {
         objectWrapperResultHandler.fetched = false;
         object = null;
         iteratorIndex++;
         return next;
       }
       throw new NoSuchElementException();
     }
 
     @Override
     public void remove() {
       throw new UnsupportedOperationException("Cannot remove element from Cursor");
     }
   }
 }