/*
Copyright (c) 2013 James Ahlborn

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 com.healthmarketscience.jackcess.impl;

import java.io.IOException;

import com.healthmarketscience.jackcess.impl.TableImpl.RowState;


/**
 * Simple un-indexed cursor.
 *
 * @author James Ahlborn
 */
public class TableScanCursor extends CursorImpl 
{
  /** first position for the TableScanCursor */
  private static final ScanPosition FIRST_SCAN_POSITION =
    new ScanPosition(RowIdImpl.FIRST_ROW_ID);
  /** last position for the TableScanCursor */
  private static final ScanPosition LAST_SCAN_POSITION =
    new ScanPosition(RowIdImpl.LAST_ROW_ID);


  /** ScanDirHandler for forward traversal */
  private final ScanDirHandler _forwardDirHandler =
    new ForwardScanDirHandler();
  /** ScanDirHandler for backward traversal */
  private final ScanDirHandler _reverseDirHandler =
    new ReverseScanDirHandler();
  /** Cursor over the pages that this table owns */
  private final UsageMap.PageCursor _ownedPagesCursor;
    
  public TableScanCursor(TableImpl table) {
    super(new IdImpl(table, null), table,
          FIRST_SCAN_POSITION, LAST_SCAN_POSITION);
    _ownedPagesCursor = table.getOwnedPagesCursor();
  }

  @Override
  protected ScanDirHandler getDirHandler(boolean moveForward) {
    return (moveForward ? _forwardDirHandler : _reverseDirHandler);
  }

  @Override
  protected boolean isUpToDate() {
    return(super.isUpToDate() && _ownedPagesCursor.isUpToDate());
  }
    
  @Override
  protected void reset(boolean moveForward) {
    _ownedPagesCursor.reset(moveForward);
    super.reset(moveForward);
  }

  @Override
  protected void restorePositionImpl(PositionImpl curPos, PositionImpl prevPos)
    throws IOException
  {
    if(!(curPos instanceof ScanPosition) ||
       !(prevPos instanceof ScanPosition)) {
      throw new IllegalArgumentException(
          "Restored positions must be scan positions");
    }
    _ownedPagesCursor.restorePosition(curPos.getRowId().getPageNumber(),
                                      prevPos.getRowId().getPageNumber());
    super.restorePositionImpl(curPos, prevPos);
  }

  @Override
  protected PositionImpl getRowPosition(RowIdImpl rowId) throws IOException
  {
    return new ScanPosition(rowId);
  }

  @Override
  protected PositionImpl findAnotherPosition(
      RowState rowState, PositionImpl curPos, boolean moveForward)
    throws IOException
  {
    ScanDirHandler handler = getDirHandler(moveForward);
      
    // figure out how many rows are left on this page so we can find the
    // next row
    RowIdImpl curRowId = curPos.getRowId();
    TableImpl.positionAtRowHeader(rowState, curRowId);
    int currentRowNumber = curRowId.getRowNumber();
    
    // loop until we find the next valid row or run out of pages
    while(true) {

      currentRowNumber = handler.getAnotherRowNumber(currentRowNumber);
      curRowId = new RowIdImpl(curRowId.getPageNumber(), currentRowNumber);
      TableImpl.positionAtRowHeader(rowState, curRowId);
        
      if(!rowState.isValid()) {
          
        // load next page
        curRowId = new RowIdImpl(handler.getAnotherPageNumber(),
                                 RowIdImpl.INVALID_ROW_NUMBER);
        TableImpl.positionAtRowHeader(rowState, curRowId);
          
        if(!rowState.isHeaderPageNumberValid()) {
          //No more owned pages.  No more rows.
          return handler.getEndPosition();
        }

        // update row count and initial row number
        currentRowNumber = handler.getInitialRowNumber(
            rowState.getRowsOnHeaderPage());

      } else if(!rowState.isDeleted()) {
          
        // we found a valid, non-deleted row, return it
        return new ScanPosition(curRowId);
      }
        
    }
  }

  /**
   * Handles moving the table scan cursor in a given direction.  Separates
   * cursor logic from value storage.
   */
  private abstract class ScanDirHandler extends DirHandler {
    public abstract int getAnotherRowNumber(int curRowNumber);
    public abstract int getAnotherPageNumber();
    public abstract int getInitialRowNumber(int rowsOnPage);
  }
    
  /**
   * Handles moving the table scan cursor forward.
   */
  private final class ForwardScanDirHandler extends ScanDirHandler {
    @Override
    public PositionImpl getBeginningPosition() {
      return getFirstPosition();
    }
    @Override
    public PositionImpl getEndPosition() {
      return getLastPosition();
    }
    @Override
    public int getAnotherRowNumber(int curRowNumber) {
      return curRowNumber + 1;
    }
    @Override
    public int getAnotherPageNumber() {
      return _ownedPagesCursor.getNextPage();
    }
    @Override
    public int getInitialRowNumber(int rowsOnPage) {
      return -1;
    }
  }
    
  /**
   * Handles moving the table scan cursor backward.
   */
  private final class ReverseScanDirHandler extends ScanDirHandler {
    @Override
    public PositionImpl getBeginningPosition() {
      return getLastPosition();
    }
    @Override
    public PositionImpl getEndPosition() {
      return getFirstPosition();
    }
    @Override
    public int getAnotherRowNumber(int curRowNumber) {
      return curRowNumber - 1;
    }
    @Override
    public int getAnotherPageNumber() {
      return _ownedPagesCursor.getPreviousPage();
    }
    @Override
    public int getInitialRowNumber(int rowsOnPage) {
      return rowsOnPage;
    }
  }    

  /**
   * Value object which maintains the current position of a TableScanCursor.
   */
  private static final class ScanPosition extends PositionImpl
  {
    private final RowIdImpl _rowId;

    private ScanPosition(RowIdImpl rowId) {
      _rowId = rowId;
    }

    @Override
    public RowIdImpl getRowId() {
      return _rowId;
    }

    @Override
    protected boolean equalsImpl(Object o) {
      return getRowId().equals(((ScanPosition)o).getRowId());
    }
    
    @Override
    public String toString() {
      return "RowId = " + getRowId();
    }
  }
}