Merged revisions 703063 via svnmerge from

https://svn.apache.org/repos/asf/poi/trunk

........
  r703063 | josh | 2008-10-08 23:07:36 -0700 (Wed, 08 Oct 2008) | 1 line
  
  Performance optimisation for EvaluationCache.  Changed to use HSSFCell as key into cache entry map.  Reduced dependency information storage for blank cells.  Introduced EvaluationSheet and EvaluationCell interfaces.  Modified HSSFFormulaEvaluator API to have simpler notify~ methods
........


git-svn-id: https://svn.apache.org/repos/asf/poi/branches/ooxml@703083 13f79535-47bb-0310-9956-ffa450edef68

18 files changed, 1096 insertions, 452 deletions

diff --git a/src/java/org/apache/poi/ss/formula/CellCacheEntry.java b/src/java/org/apache/poi/ss/formula/CellCacheEntry.java
index bd7500023c..1eb7298a50 100644
--- a/src/java/org/apache/poi/ss/formula/CellCacheEntry.java
+++ b/src/java/org/apache/poi/ss/formula/CellCacheEntry.java
@@ -17,42 +17,44 @@
 
 package org.apache.poi.ss.formula;
 
-import java.util.HashSet;
-import java.util.Set;
-
 import org.apache.poi.hssf.record.formula.eval.BlankEval;
 import org.apache.poi.hssf.record.formula.eval.BoolEval;
 import org.apache.poi.hssf.record.formula.eval.ErrorEval;
 import org.apache.poi.hssf.record.formula.eval.NumberEval;
 import org.apache.poi.hssf.record.formula.eval.StringEval;
 import org.apache.poi.hssf.record.formula.eval.ValueEval;
+import org.apache.poi.ss.formula.IEvaluationListener.ICacheEntry;
 
 /**
  * Stores the parameters that identify the evaluation of one cell.<br/>
  */
-final class CellCacheEntry {
+abstract class CellCacheEntry implements ICacheEntry {
+	public static final CellCacheEntry[] EMPTY_ARRAY = { };
+	
+	private final FormulaCellCacheEntrySet _consumingCells;
 	private ValueEval _value;
-	private final Set _consumingCells;
-	private CellLocation[] _usedCells;
-	private boolean _isPlainValueCell;
 	
-	public CellCacheEntry() {
-		_consumingCells = new HashSet();
+
+	protected CellCacheEntry() {
+		_consumingCells = new FormulaCellCacheEntrySet();
 	}
-	public boolean updatePlainValue(ValueEval value) {
-		boolean wasChanged = false;
-		if (!_isPlainValueCell) {
-			wasChanged = true;
-		}
-		if (!areValuesEqual(_value, value)) {
-			wasChanged = true;
+	protected final void clearValue() {
+		_value = null;
+	}
+
+	public final boolean updateValue(ValueEval value) {
+		if (value == null) {
+			throw new IllegalArgumentException("Did not expect to update to null");
 		}
-		_isPlainValueCell = true;
+		boolean result = !areValuesEqual(_value, value);
 		_value = value;
-		_usedCells = null;
-		return wasChanged;
+		return result;
+	}
+	public final ValueEval getValue() {
+		return _value;
 	}
-	private boolean areValuesEqual(ValueEval a, ValueEval b) {
+	
+	private static boolean areValuesEqual(ValueEval a, ValueEval b) {
 		if (a == null) {
 			return false;
 		}
@@ -78,41 +80,58 @@ final class CellCacheEntry {
 		}
 		throw new IllegalStateException("Unexpected value class (" + cls.getName() + ")");
 	}
-	public void setFormulaResult(ValueEval value, CellLocation[] usedCells) {
-		_isPlainValueCell = false;
-		_value = value;
-		_usedCells = usedCells;
-	}
-	public void addConsumingCell(CellLocation cellLoc) {
+
+	public final void addConsumingCell(FormulaCellCacheEntry cellLoc) {
 		_consumingCells.add(cellLoc);
 		
 	}
-	public CellLocation[] getConsumingCells() {
-		int nItems = _consumingCells.size();
-		if (nItems < 1) {
-			return CellLocation.EMPTY_ARRAY;
-		}
-		CellLocation[] result = new CellLocation[nItems];
-		_consumingCells.toArray(result);
-		return result;
+	public final FormulaCellCacheEntry[] getConsumingCells() {
+		return _consumingCells.toArray();
 	}
-	public CellLocation[] getUsedCells() {
-		return _usedCells;
+
+	public final void clearConsumingCell(FormulaCellCacheEntry cce) {
+		if(!_consumingCells.remove(cce)) {
+			throw new IllegalStateException("Specified formula cell is not consumed by this cell");
+		}
 	}
-	public void clearConsumingCell(CellLocation fc) {
-		if(!_consumingCells.remove(fc)) {
-			throw new IllegalStateException("Cell '" + fc.formatAsString() + "' does not use this cell");
+	public final void recurseClearCachedFormulaResults(IEvaluationListener listener) {
+		if (listener == null) {
+			recurseClearCachedFormulaResults();
+		} else {
+			listener.onClearCachedValue(this);
+			recurseClearCachedFormulaResults(listener, 1);
 		}
+	}
+	
+	/**
+	 * Calls formulaCell.setFormulaResult(null, null) recursively all the way up the tree of 
+	 * dependencies. Calls usedCell.clearConsumingCell(fc) for each child of a cell that is
+	 * cleared along the way.
+	 * @param formulaCells
+	 */
+	protected final void recurseClearCachedFormulaResults() {
+		FormulaCellCacheEntry[] formulaCells = getConsumingCells();
 		
+		for (int i = 0; i < formulaCells.length; i++) {
+			FormulaCellCacheEntry fc = formulaCells[i];
+			fc.clearFormulaEntry();
+			fc.recurseClearCachedFormulaResults();
+		}
 	}
-	public ValueEval getValue() {
-		ValueEval result = _value;
-		if (result == null) {
-			if (_isPlainValueCell) {
-				throw new IllegalStateException("Plain value cell should always have a value");
-				
-			}
+	
+	/**
+	 * Identical to {@link #recurseClearCachedFormulaResults()} except for the listener call-backs
+	 */
+	protected final void recurseClearCachedFormulaResults(IEvaluationListener listener, int depth) {
+		FormulaCellCacheEntry[] formulaCells = getConsumingCells();
+
+		listener.sortDependentCachedValues(formulaCells);
+		for (int i = 0; i < formulaCells.length; i++) {
+			FormulaCellCacheEntry fc = formulaCells[i];
+			listener.onClearDependentCachedValue(fc, depth);
+			fc.clearFormulaEntry();
+			fc.recurseClearCachedFormulaResults(listener, depth+1);
 		}
-		return result;
 	}
+	
 }
\ No newline at end of file
diff --git a/src/java/org/apache/poi/ss/formula/CellEvaluationFrame.java b/src/java/org/apache/poi/ss/formula/CellEvaluationFrame.java
index 1d438527ae..5dbd2339d5 100644
--- a/src/java/org/apache/poi/ss/formula/CellEvaluationFrame.java
+++ b/src/java/org/apache/poi/ss/formula/CellEvaluationFrame.java
@@ -20,44 +20,58 @@ package org.apache.poi.ss.formula;
 import java.util.HashSet;

 import java.util.Set;

 

+import org.apache.poi.hssf.record.formula.eval.ValueEval;

+

 /**

  * Stores details about the current evaluation of a cell.<br/>

  */

 final class CellEvaluationFrame {

 

-	private final CellLocation _cellLocation;

-	private final Set _usedCells;

+	private final FormulaCellCacheEntry _cce;

+	private final Set _sensitiveInputCells;

+	private FormulaUsedBlankCellSet _usedBlankCellGroup;

 

-	public CellEvaluationFrame(CellLocation cellLoc) {

-		_cellLocation = cellLoc;

-		_usedCells = new HashSet();

+	public CellEvaluationFrame(FormulaCellCacheEntry cce) {

+		_cce = cce;

+		_sensitiveInputCells = new HashSet();

 	}

-	public CellLocation getCoordinates() {

-		return _cellLocation;

+	public CellCacheEntry getCCE() {

+		return _cce;

 	}

 

 	public String toString() {

 		StringBuffer sb = new StringBuffer(64);

 		sb.append(getClass().getName()).append(" [");

-		sb.append(_cellLocation.formatAsString());

 		sb.append("]");

 		return sb.toString();

 	}

-	public void addUsedCell(CellLocation coordinates) {

-		_usedCells.add(coordinates);

+	/**

+	 * @param inputCell a cell directly used by the formula of this evaluation frame

+	 */

+	public void addSensitiveInputCell(CellCacheEntry inputCell) {

+		_sensitiveInputCells.add(inputCell);

 	}

 	/**

 	 * @return never <code>null</code>, (possibly empty) array of all cells directly used while 

-	 * evaluating the formula of this frame.  For non-formula cells this will always be an empty

-	 * array;

+	 * evaluating the formula of this frame.

 	 */

-	public CellLocation[] getUsedCells() {

-		int nItems = _usedCells.size();

+	private CellCacheEntry[] getSensitiveInputCells() {

+		int nItems = _sensitiveInputCells.size();

 		if (nItems < 1) {

-			return CellLocation.EMPTY_ARRAY;

+			return CellCacheEntry.EMPTY_ARRAY;

 		}

-		CellLocation[] result = new CellLocation[nItems];

-		_usedCells.toArray(result);

+		CellCacheEntry[] result = new CellCacheEntry[nItems];

+		_sensitiveInputCells.toArray(result);

 		return result;

 	}

+	public void addUsedBlankCell(int bookIndex, int sheetIndex, int rowIndex, int columnIndex) {

+		if (_usedBlankCellGroup == null) {

+			_usedBlankCellGroup = new FormulaUsedBlankCellSet();

+		}

+		_usedBlankCellGroup.addCell(bookIndex, sheetIndex, rowIndex, columnIndex);

+	}

+	

+	public void updateFormulaResult(ValueEval result) {

+		_cce.updateFormulaResult(result, getSensitiveInputCells(), _usedBlankCellGroup);

+	}

 }
\ No newline at end of file
diff --git a/src/java/org/apache/poi/ss/formula/CellLocation.java b/src/java/org/apache/poi/ss/formula/CellLocation.java
deleted file mode 100644
index bb32e3dfa3..0000000000
--- a/src/java/org/apache/poi/ss/formula/CellLocation.java
+++ /dev/null
@@ -1,92 +0,0 @@
-/* ====================================================================
-   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.poi.ss.formula;
-
-import org.apache.poi.hssf.util.CellReference;
-
-/**
- * Stores the parameters that identify the evaluation of one cell.<br/>
- */
-final class CellLocation {
-	public static final CellLocation[] EMPTY_ARRAY = { };
-	
-	private final int _bookIx;
-	private final int _sheetIndex;
-	private final int _rowIndex;
-	private final int _columnIndex;
-	private final int _hashCode;
-
-	public CellLocation(int bookIx, int sheetIndex, int rowIndex, int columnIndex) {
-		if (sheetIndex < 0) {
-			throw new IllegalArgumentException("sheetIndex must not be negative");
-		}
-		_bookIx = bookIx;
-		_sheetIndex = sheetIndex;
-		_rowIndex = rowIndex;
-		_columnIndex = columnIndex;
-		_hashCode = _bookIx + 17 * (sheetIndex + 17 * (rowIndex + 17 * columnIndex));
-	}
-	public int getBookIndex() {
-		return _bookIx;
-	}
-	public int getSheetIndex() {
-		return _sheetIndex;
-	}
-	public int getRowIndex() {
-		return _rowIndex;
-	}
-	public int getColumnIndex() {
-		return _columnIndex;
-	}
-
-	public boolean equals(Object obj) {
-		CellLocation other = (CellLocation) obj;
-		if (getRowIndex() != other.getRowIndex()) {
-			return false;
-		}
-		if (getColumnIndex() != other.getColumnIndex()) {
-			return false;
-		}
-		if (getSheetIndex() != other.getSheetIndex()) {
-			return false;
-		}
-		if (getBookIndex() != other.getBookIndex()) {
-			return false;
-		}
-		return true;
-	}
-	public int hashCode() {
-		return _hashCode;
-	}
-
-	/**
-	 * @return human readable string for debug purposes
-	 */
-	public String formatAsString() {
-		CellReference cr = new CellReference(_rowIndex, _columnIndex, false, false);
-		return  "ShIx=" + getSheetIndex() + " " + cr.formatAsString();
-	}
-
-	public String toString() {
-		StringBuffer sb = new StringBuffer(64);
-		sb.append(getClass().getName()).append(" [");
-		sb.append(formatAsString());
-		sb.append("]");
-		return sb.toString();
-	}
-}
\ No newline at end of file
diff --git a/src/java/org/apache/poi/ss/formula/EvaluationCache.java b/src/java/org/apache/poi/ss/formula/EvaluationCache.java
index a1b06ad17e..81745825c7 100644
--- a/src/java/org/apache/poi/ss/formula/EvaluationCache.java
+++ b/src/java/org/apache/poi/ss/formula/EvaluationCache.java
@@ -17,16 +17,17 @@
 
 package org.apache.poi.ss.formula;
 
-import java.util.Arrays;
-import java.util.Collections;
-import java.util.Comparator;
-import java.util.HashMap;
-import java.util.HashSet;
-import java.util.Map;
-import java.util.Set;
-
+import org.apache.poi.hssf.record.formula.eval.BlankEval;
+import org.apache.poi.hssf.record.formula.eval.BoolEval;
+import org.apache.poi.hssf.record.formula.eval.ErrorEval;
+import org.apache.poi.hssf.record.formula.eval.NumberEval;
+import org.apache.poi.hssf.record.formula.eval.StringEval;
 import org.apache.poi.hssf.record.formula.eval.ValueEval;
+import org.apache.poi.hssf.usermodel.HSSFCell;
 import org.apache.poi.hssf.usermodel.HSSFFormulaEvaluator;
+import org.apache.poi.ss.formula.FormulaCellCache.IEntryOperation;
+import org.apache.poi.ss.formula.FormulaUsedBlankCellSet.BookSheetKey;
+import org.apache.poi.ss.formula.PlainCellCache.Loc;
 
 /**
  * Performance optimisation for {@link HSSFFormulaEvaluator}. This class stores previously
@@ -38,152 +39,138 @@ import org.apache.poi.hssf.usermodel.HSSFFormulaEvaluator;
  */
 final class EvaluationCache {
 
-	private final Map _entriesByLocation;
-	private final Map _consumingCellsByUsedCells;
+	private final PlainCellCache _plainCellCache;
+	private final FormulaCellCache _formulaCellCache;
 	/** only used for testing. <code>null</code> otherwise */
-	private final IEvaluationListener _evaluationListener;
+	final IEvaluationListener _evaluationListener;
 
 	/* package */EvaluationCache(IEvaluationListener evaluationListener) {
 		_evaluationListener = evaluationListener;
-		_entriesByLocation = new HashMap();
-		_consumingCellsByUsedCells = new HashMap();
-	}
-
-	/**
-	 * @param cellLoc never <code>null</code>
-	 * @return only ever <code>null</code> for formula cells that have had their cached value cleared
-	 */
-	public ValueEval getValue(CellLocation cellLoc) {
-		return getEntry(cellLoc).getValue();
+		_plainCellCache = new PlainCellCache();
+		_formulaCellCache = new FormulaCellCache();
 	}
 
-	/**
-	 * @param cellLoc
-	 * @param usedCells never <code>null</code>, (possibly zero length) array of all cells actually 
-	 * directly used when evaluating the formula
-	 * @param isPlainValue pass <code>true</code> if cellLoc refers to a plain value (non-formula) 
-	 * cell, <code>false</code> for a formula cell.
-	 * @param value the value of a non-formula cell or the result of evaluating the cell formula
-	 * Pass <code>null</code> to signify clearing the cached result of a formula cell) 
-	 */
-	public void setValue(CellLocation cellLoc, boolean isPlainValue, 
-			CellLocation[] usedCells, ValueEval value) {
+	public void notifyUpdateCell(int bookIndex, int sheetIndex, EvaluationCell cell) {
+		FormulaCellCacheEntry fcce = _formulaCellCache.get(cell);
+		
+		Loc loc = new Loc(bookIndex, sheetIndex, cell.getRowIndex(), cell.getColumnIndex());
+		PlainValueCellCacheEntry pcce = _plainCellCache.get(loc);
 
-		CellCacheEntry existingEntry = (CellCacheEntry) _entriesByLocation.get(cellLoc);
-		if (existingEntry != null && existingEntry.getValue() != null) {
-			if (isPlainValue) {
-				// can set a plain cell cached value any time
-			} else if (value == null) {
-				// or clear the cached value of a formula cell at any time
+		if (cell.getCellType() == HSSFCell.CELL_TYPE_FORMULA) {
+			if (fcce == null) {
+				if (pcce == null) {
+					updateAnyBlankReferencingFormulas(bookIndex, sheetIndex, cell.getRowIndex(), cell.getColumnIndex());
+				}
+				fcce = new FormulaCellCacheEntry();
+				_formulaCellCache.put(cell, fcce);
 			} else {
-				// but a formula cached value would only be getting updated if the cache didn't have a value to start with
-				throw new IllegalStateException("Already have cached value for this cell: "
-						+ cellLoc.formatAsString());
+				fcce.recurseClearCachedFormulaResults(_evaluationListener);
+				fcce.clearFormulaEntry();
 			}
-		}
-		sortCellLocationsForLogging(usedCells);
-		CellCacheEntry entry = getEntry(cellLoc);
-		CellLocation[] consumingFormulaCells = entry.getConsumingCells();
-		CellLocation[] prevUsedCells = entry.getUsedCells();
-		if (isPlainValue) {
-			
-			if(!entry.updatePlainValue(value)) {
-				return;
+			if (pcce == null) {
+				// was formula cell before - no change of type
+			} else {
+				// changing from plain cell to formula cell
+				pcce.recurseClearCachedFormulaResults(_evaluationListener);
+				_plainCellCache.remove(loc);
 			}
 		} else {
-			entry.setFormulaResult(value, usedCells);
-			for (int i = 0; i < usedCells.length; i++) {
-				getEntry(usedCells[i]).addConsumingCell(cellLoc);
+			ValueEval value = WorkbookEvaluator.getValueFromNonFormulaCell(cell);
+			if (pcce == null) {
+				if (fcce == null) {
+					updateAnyBlankReferencingFormulas(bookIndex, sheetIndex, cell.getRowIndex(), cell.getColumnIndex());
+				}
+				pcce = new PlainValueCellCacheEntry(value);
+				_plainCellCache.put(loc, pcce);
+				if (_evaluationListener != null) {
+					_evaluationListener.onReadPlainValue(sheetIndex, cell.getRowIndex(), cell.getColumnIndex(), pcce);
+				}
+			} else {
+				if(pcce.updateValue(value)) {
+					pcce.recurseClearCachedFormulaResults(_evaluationListener);
+				}
+			}
+			if (fcce == null) {
+				// was plain cell before - no change of type
+			} else {
+				// was formula cell before - now a plain value
+				_formulaCellCache.remove(cell);
+				fcce.setSensitiveInputCells(null);
+				fcce.recurseClearCachedFormulaResults(_evaluationListener);
 			}
 		}
-		// need to tell all cells that were previously used, but no longer are, 
-		// that they are not consumed by this cell any more
-		unlinkConsumingCells(prevUsedCells, usedCells, cellLoc);
-		
-		// clear all cells that directly or indirectly depend on this one
-		recurseClearCachedFormulaResults(consumingFormulaCells, 0);
 	}
 
-	/**
-	 * This method sorts the supplied cellLocs so that the order of call-backs to the evaluation 
-	 * listener is more deterministic
-	 */
-	private void sortCellLocationsForLogging(CellLocation[] cellLocs) {
-		if (_evaluationListener == null) {
-			// optimisation - don't bother sorting if there is no listener.
-		} else {
-			Arrays.sort(cellLocs, CellLocationComparator);
-		}
-	}
+	private void updateAnyBlankReferencingFormulas(int bookIndex, int sheetIndex, 
+			final int rowIndex, final int columnIndex) {
+		final BookSheetKey bsk = new BookSheetKey(bookIndex, sheetIndex);
+		_formulaCellCache.applyOperation(new IEntryOperation() {
 
-	private void unlinkConsumingCells(CellLocation[] prevUsedCells, CellLocation[] usedCells,
-			CellLocation cellLoc) {
-		if (prevUsedCells == null) {
-			return;
-		}
-		int nPrevUsed = prevUsedCells.length;
-		if (nPrevUsed < 1) {
-			return;
-		}
-		int nUsed = usedCells.length;
-		Set usedSet;
-		if (nUsed < 1) {
-			usedSet = Collections.EMPTY_SET;
-		} else {
-			usedSet = new HashSet(nUsed * 3 / 2);
-			for (int i = 0; i < nUsed; i++) {
-				usedSet.add(usedCells[i]);
-			}
-		}
-		for (int i = 0; i < nPrevUsed; i++) {
-			CellLocation prevUsed = prevUsedCells[i];
-			if (!usedSet.contains(prevUsed)) {
-				// previously was used by cellLoc, but not anymore
-				getEntry(prevUsed).clearConsumingCell(cellLoc);
-				if (_evaluationListener != null) {
-					//TODO _evaluationListener.onUnconsume(prevUsed.getSheetIndex(), etc)
-				}
+			public void processEntry(FormulaCellCacheEntry entry) {
+				entry.notifyUpdatedBlankCell(bsk, rowIndex, columnIndex, _evaluationListener);
 			}
-		}
-		
+		});
 	}
 
-	/**
-	 * Calls formulaCell.setFormulaResult(null, null) recursively all the way up the tree of 
-	 * dependencies. Calls usedCell.clearConsumingCell(fc) for each child of a cell that is
-	 * cleared along the way.
-	 * @param formulaCells
-	 */
-	private void recurseClearCachedFormulaResults(CellLocation[] formulaCells, int depth) {
-		sortCellLocationsForLogging(formulaCells);
-		int nextDepth = depth+1;
-		for (int i = 0; i < formulaCells.length; i++) {
-			CellLocation fc = formulaCells[i];
-			CellCacheEntry formulaCell = getEntry(fc);
-			CellLocation[] usedCells = formulaCell.getUsedCells();
-			if (usedCells != null) {
-				for (int j = 0; j < usedCells.length; j++) {
-					CellCacheEntry usedCell = getEntry(usedCells[j]);
-					usedCell.clearConsumingCell(fc);
-				}
+	public PlainValueCellCacheEntry getPlainValueEntry(int bookIndex, int sheetIndex,
+			int rowIndex, int columnIndex, ValueEval value) {
+		
+		Loc loc = new Loc(bookIndex, sheetIndex, rowIndex, columnIndex);
+		PlainValueCellCacheEntry result = _plainCellCache.get(loc);
+		if (result == null) {
+			result = new PlainValueCellCacheEntry(value);
+			_plainCellCache.put(loc, result);
+			if (_evaluationListener != null) {
+				_evaluationListener.onReadPlainValue(sheetIndex, rowIndex, columnIndex, result);
+			}
+		} else {
+			// TODO - if we are confident that this sanity check is not required, we can remove 'value' from plain value cache entry  
+			if (!areValuesEqual(result.getValue(), value)) {
+				throw new IllegalStateException("value changed");
 			}
 			if (_evaluationListener != null) {
-				ValueEval value = formulaCell.getValue();
-				_evaluationListener.onClearDependentCachedValue(fc.getSheetIndex(), fc.getRowIndex(), fc.getColumnIndex(), value, nextDepth);
+				_evaluationListener.onCacheHit(sheetIndex, rowIndex, columnIndex, value);
 			}
-			formulaCell.setFormulaResult(null, null);
-			recurseClearCachedFormulaResults(formulaCell.getConsumingCells(), nextDepth);
 		}
+		return result;
 	}
-
-	private CellCacheEntry getEntry(CellLocation cellLoc) {
-		CellCacheEntry result = (CellCacheEntry)_entriesByLocation.get(cellLoc);
+	private boolean areValuesEqual(ValueEval a, ValueEval b) {
+		if (a == null) {
+			return false;
+		}
+		Class cls = a.getClass();
+		if (cls != b.getClass()) {
+			// value type is changing
+			return false;
+		}
+		if (a == BlankEval.INSTANCE) {
+			return b == a;
+		}
+		if (cls == NumberEval.class) {
+			return ((NumberEval)a).getNumberValue() == ((NumberEval)b).getNumberValue();
+		}
+		if (cls == StringEval.class) {
+			return ((StringEval)a).getStringValue().equals(((StringEval)b).getStringValue());
+		}
+		if (cls == BoolEval.class) {
+			return ((BoolEval)a).getBooleanValue() == ((BoolEval)b).getBooleanValue();
+		}
+		if (cls == ErrorEval.class) {
+			return ((ErrorEval)a).getErrorCode() == ((ErrorEval)b).getErrorCode();
+		}
+		throw new IllegalStateException("Unexpected value class (" + cls.getName() + ")");
+	}
+	
+	public FormulaCellCacheEntry getOrCreateFormulaCellEntry(EvaluationCell cell) {
+		FormulaCellCacheEntry result = _formulaCellCache.get(cell);
 		if (result == null) {
-			result = new CellCacheEntry();
-			_entriesByLocation.put(cellLoc, result);
+			
+			result = new FormulaCellCacheEntry();
+			_formulaCellCache.put(cell, result);
 		}
 		return result;
 	}
+
 	/**
 	 * Should be called whenever there are changes to input cells in the evaluated workbook.
 	 */
@@ -191,34 +178,28 @@ final class EvaluationCache {
 		if(_evaluationListener != null) {
 			_evaluationListener.onClearWholeCache();
 		}
-		_entriesByLocation.clear();
-		_consumingCellsByUsedCells.clear();
+		_plainCellCache.clear();
+		_formulaCellCache.clear();
 	}
-
-
-	private static final Comparator CellLocationComparator = new Comparator() {
-
-		public int compare(Object a, Object b) {
-			CellLocation clA = (CellLocation) a;
-			CellLocation clB = (CellLocation) b;
-			
-			int cmp;
-			cmp = clA.getBookIndex() - clB.getBookIndex();
-			if (cmp != 0) {
-				return cmp;
-			}
-			cmp = clA.getSheetIndex() - clB.getSheetIndex();
-			if (cmp != 0) {
-				return cmp;
-			}
-			cmp = clA.getRowIndex() - clB.getRowIndex();
-			if (cmp != 0) {
-				return cmp;
+	public void notifyDeleteCell(int bookIndex, int sheetIndex, EvaluationCell cell) {
+		
+		if (cell.getCellType() == HSSFCell.CELL_TYPE_FORMULA) {
+			FormulaCellCacheEntry fcce = _formulaCellCache.remove(cell);
+			if (fcce == null) {
+				// formula cell has not been evaluated yet 
+			} else {
+				fcce.setSensitiveInputCells(null);
+				fcce.recurseClearCachedFormulaResults(_evaluationListener);
 			}
-			cmp = clA.getColumnIndex() - clB.getColumnIndex();
+		} else {
+			Loc loc = new Loc(bookIndex, sheetIndex, cell.getRowIndex(), cell.getColumnIndex());
+			PlainValueCellCacheEntry pcce = _plainCellCache.get(loc);
 			
-			return cmp;
+			if (pcce == null) {
+				// cache entry doesn't exist. nothing to do
+			} else {
+				pcce.recurseClearCachedFormulaResults(_evaluationListener);
+			}
 		}
-		
-	};
+	}
 }
diff --git a/src/java/org/apache/poi/ss/formula/EvaluationCell.java b/src/java/org/apache/poi/ss/formula/EvaluationCell.java
new file mode 100644
index 0000000000..c15754b921
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/EvaluationCell.java
@@ -0,0 +1,42 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+/**
+ * Abstracts a cell for the purpose of formula evaluation.  This interface represents both formula
+ * and non-formula cells.<br/>
+ * 
+ * Implementors of this class must implement {@link #hashCode()} and {@link #equals(Object)}
+ * to provide an <em>identity</em> relationship based on the underlying HSSF or XSSF cell <p/>
+ * 
+ * For POI internal use only
+ * 
+ * @author Josh Micich
+ */
+public interface EvaluationCell {
+	// consider method Object getUnderlyingCell() to reduce memory consumption in formula cell cache
+	EvaluationSheet getSheet();
+	int getRowIndex();
+	int getColumnIndex();
+	int getCellType();
+
+	double getNumericCellValue();
+	String getStringCellValue();
+	boolean getBooleanCellValue();
+	int getErrorCellValue();
+}
diff --git a/src/java/org/apache/poi/ss/formula/EvaluationSheet.java b/src/java/org/apache/poi/ss/formula/EvaluationSheet.java
new file mode 100644
index 0000000000..5dec3aacda
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/EvaluationSheet.java
@@ -0,0 +1,33 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+/**
+ * Abstracts a sheet for the purpose of formula evaluation.<br/>
+ * 
+ * For POI internal use only
+ * 
+ * @author Josh Micich
+ */
+public interface EvaluationSheet {
+
+	/**
+	 * @return <code>null</code> if there is no cell at the specified coordinates
+	 */
+	EvaluationCell getCell(int rowIndex, int columnIndex);
+}
diff --git a/src/java/org/apache/poi/ss/formula/EvaluationTracker.java b/src/java/org/apache/poi/ss/formula/EvaluationTracker.java
index 834941e9be..eda62da8b8 100755
--- a/src/java/org/apache/poi/ss/formula/EvaluationTracker.java
+++ b/src/java/org/apache/poi/ss/formula/EvaluationTracker.java
@@ -22,7 +22,7 @@ import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
 
-import org.apache.poi.hssf.record.formula.eval.ErrorEval;
+import org.apache.poi.hssf.record.formula.eval.BlankEval;
 import org.apache.poi.hssf.record.formula.eval.ValueEval;
 import org.apache.poi.hssf.usermodel.HSSFCell;
 
@@ -32,11 +32,11 @@ import org.apache.poi.hssf.usermodel.HSSFCell;
  * The main purpose of this class is to detect an attempt to evaluate a cell
  * that is already being evaluated. In other words, it detects circular
  * references in spreadsheet formulas.
- * 
+ *
  * @author Josh Micich
  */
 final class EvaluationTracker {
-
+	// TODO - consider deleting this class and letting CellEvaluationFrame take care of itself
 	private final List _evaluationFrames;
 	private final Set _currentlyEvaluatingCells;
 	private final EvaluationCache _cache;
@@ -50,47 +50,51 @@ final class EvaluationTracker {
 	/**
 	 * Notifies this evaluation tracker that evaluation of the specified cell is
 	 * about to start.<br/>
-	 * 
+	 *
 	 * In the case of a <code>true</code> return code, the caller should
 	 * continue evaluation of the specified cell, and also be sure to call
 	 * <tt>endEvaluate()</tt> when complete.<br/>
-	 * 
-	 * In the case of a <code>false</code> return code, the caller should
+	 *
+	 * In the case of a <code>null</code> return code, the caller should
 	 * return an evaluation result of
-	 * <tt>ErrorEval.CIRCULAR_REF_ERROR<tt>, and not call <tt>endEvaluate()</tt>.  
+	 * <tt>ErrorEval.CIRCULAR_REF_ERROR<tt>, and not call <tt>endEvaluate()</tt>.
 	 * <br/>
-	 * @return <code>true</code> if the specified cell has not been visited yet in the current 
-	 * evaluation. <code>false</code> if the specified cell is already being evaluated.
+	 * @return <code>false</code> if the specified cell is already being evaluated
 	 */
-	public ValueEval startEvaluate(CellLocation cellLoc) {
-		if (cellLoc == null) {
+	public boolean startEvaluate(FormulaCellCacheEntry cce) {
+		if (cce == null) {
 			throw new IllegalArgumentException("cellLoc must not be null");
 		}
-		if (_currentlyEvaluatingCells.contains(cellLoc)) {
-			return ErrorEval.CIRCULAR_REF_ERROR;
+		if (_currentlyEvaluatingCells.contains(cce)) {
+			return false;
 		}
-		ValueEval result = _cache.getValue(cellLoc);
-		if (result == null) {
-			_currentlyEvaluatingCells.add(cellLoc);
-			_evaluationFrames.add(new CellEvaluationFrame(cellLoc));
+		_currentlyEvaluatingCells.add(cce);
+		_evaluationFrames.add(new CellEvaluationFrame(cce));
+		return true;
+	}
+
+	public void updateCacheResult(ValueEval result) {
+
+		int nFrames = _evaluationFrames.size();
+		if (nFrames < 1) {
+			throw new IllegalStateException("Call to endEvaluate without matching call to startEvaluate");
 		}
-		return result;
+		CellEvaluationFrame frame = (CellEvaluationFrame) _evaluationFrames.get(nFrames-1);
+
+		frame.updateFormulaResult(result);
 	}
 
 	/**
-	 * Notifies this evaluation tracker that the evaluation of the specified
-	 * cell is complete. <p/>
-	 * 
-	 * Every successful call to <tt>startEvaluate</tt> must be followed by a
-	 * call to <tt>endEvaluate</tt> (recommended in a finally block) to enable
+	 * Notifies this evaluation tracker that the evaluation of the specified cell is complete. <p/>
+	 *
+	 * Every successful call to <tt>startEvaluate</tt> must be followed by a call to <tt>endEvaluate</tt> (recommended in a finally block) to enable
 	 * proper tracking of which cells are being evaluated at any point in time.<p/>
-	 * 
+	 *
 	 * Assuming a well behaved client, parameters to this method would not be
 	 * required. However, they have been included to assert correct behaviour,
 	 * and form more meaningful error messages.
-	 * @param result 
 	 */
-	public void endEvaluate(CellLocation cellLoc, ValueEval result, boolean isPlainValueCell) {
+	public void endEvaluate(CellCacheEntry cce) {
 
 		int nFrames = _evaluationFrames.size();
 		if (nFrames < 1) {
@@ -99,32 +103,40 @@ final class EvaluationTracker {
 
 		nFrames--;
 		CellEvaluationFrame frame = (CellEvaluationFrame) _evaluationFrames.get(nFrames);
-		CellLocation coordinates = frame.getCoordinates();
-		if (!coordinates.equals(cellLoc)) {
-			throw new RuntimeException("Wrong cell specified. "
-					+ "Corresponding startEvaluate() call was for cell {"
-					+ coordinates.formatAsString() + "} this endEvaluate() call is for cell {"
-					+ cellLoc.formatAsString() + "}");
+		if (cce != frame.getCCE()) {
+			throw new IllegalStateException("Wrong cell specified. ");
 		}
-		// else - no problems so pop current frame 
+		// else - no problems so pop current frame
 		_evaluationFrames.remove(nFrames);
-		_currentlyEvaluatingCells.remove(coordinates);
-
-		// TODO - don't cache results of volatile formulas 
-		_cache.setValue(coordinates, isPlainValueCell, frame.getUsedCells(), result);
+		_currentlyEvaluatingCells.remove(cce);
 	}
-	/**
-	 * Tells the currently evaluating cell frame that it has a dependency on the specified 
-	 * <tt>usedCell<tt> 
-	 * @param usedCell location of cell which is referenced (and used) by the current cell.
-	 */
-	public void acceptDependency(CellLocation usedCell) {
+
+	public void acceptFormulaDependency(CellCacheEntry cce) {
+		// Tell the currently evaluating cell frame that it has a dependency on the specified
 		int prevFrameIndex = _evaluationFrames.size()-1;
 		if (prevFrameIndex < 0) {
-			throw new IllegalStateException("Call to acceptDependency without prior call to startEvaluate");
+			// Top level frame, there is no 'cell' above this frame that is using the current cell
+		} else {
+			CellEvaluationFrame consumingFrame = (CellEvaluationFrame) _evaluationFrames.get(prevFrameIndex);
+			consumingFrame.addSensitiveInputCell(cce);
 		}
-		CellEvaluationFrame consumingFrame = (CellEvaluationFrame) _evaluationFrames.get(prevFrameIndex);
-		consumingFrame.addUsedCell(usedCell);
 	}
 
+	public void acceptPlainValueDependency(int bookIndex, int sheetIndex,
+			int rowIndex, int columnIndex, ValueEval value) {
+		// Tell the currently evaluating cell frame that it has a dependency on the specified
+		int prevFrameIndex = _evaluationFrames.size() - 1;
+		if (prevFrameIndex < 0) {
+			// Top level frame, there is no 'cell' above this frame that is using the current cell
+		} else {
+			CellEvaluationFrame consumingFrame = (CellEvaluationFrame) _evaluationFrames.get(prevFrameIndex);
+			if (value == BlankEval.INSTANCE) {
+				consumingFrame.addUsedBlankCell(bookIndex, sheetIndex, rowIndex, columnIndex);
+			} else {
+				PlainValueCellCacheEntry cce = _cache.getPlainValueEntry(bookIndex, sheetIndex,
+						rowIndex, columnIndex, value);
+				consumingFrame.addSensitiveInputCell(cce);
+			}
+		}
+	}
 }
diff --git a/src/java/org/apache/poi/ss/formula/EvaluationWorkbook.java b/src/java/org/apache/poi/ss/formula/EvaluationWorkbook.java
index 6cf938fe9f..45b0c918f2 100644
--- a/src/java/org/apache/poi/ss/formula/EvaluationWorkbook.java
+++ b/src/java/org/apache/poi/ss/formula/EvaluationWorkbook.java
@@ -20,8 +20,7 @@ package org.apache.poi.ss.formula;
 import org.apache.poi.hssf.record.formula.NamePtg;

 import org.apache.poi.hssf.record.formula.NameXPtg;

 import org.apache.poi.hssf.record.formula.Ptg;

-import org.apache.poi.ss.usermodel.Cell;

-import org.apache.poi.ss.usermodel.Sheet;

+

 /**

  * Abstracts a workbook for the purpose of formula evaluation.<br/>

  * 

@@ -34,10 +33,10 @@ public interface EvaluationWorkbook {
 	/**

 	 * @return -1 if the specified sheet is from a different book

 	 */

-	int getSheetIndex(Sheet sheet);

+	int getSheetIndex(EvaluationSheet sheet);

 	int getSheetIndex(String sheetName);

 

-	Sheet getSheet(int sheetIndex);

+	EvaluationSheet getSheet(int sheetIndex);

 

 	/**

 	 * @return <code>null</code> if externSheetIndex refers to a sheet inside the current workbook

@@ -46,7 +45,7 @@ public interface EvaluationWorkbook {
 	int convertFromExternSheetIndex(int externSheetIndex);

 	EvaluationName getName(NamePtg namePtg);

 	String resolveNameXText(NameXPtg ptg);

-	Ptg[] getFormulaTokens(Cell cell);

+	Ptg[] getFormulaTokens(EvaluationCell cell);

 	

 	class ExternalSheet {

 		private final String _workbookName;

diff --git a/src/java/org/apache/poi/ss/formula/FormulaCellCache.java b/src/java/org/apache/poi/ss/formula/FormulaCellCache.java
new file mode 100644
index 0000000000..4a1fce6b80
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/FormulaCellCache.java
@@ -0,0 +1,73 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+import java.util.HashMap;
+import java.util.Iterator;
+import java.util.Map;
+
+/**
+ * 
+ * @author Josh Micich
+ */
+final class FormulaCellCache {
+
+	static interface IEntryOperation {
+		void processEntry(FormulaCellCacheEntry entry);
+	}
+
+	private Map _formulaEntriesByCell;
+
+	public FormulaCellCache() {
+		// assumes HSSFCell does not override hashCode or equals, otherwise we need IdentityHashMap
+		_formulaEntriesByCell = new HashMap(); 
+	}
+
+	public CellCacheEntry[] getCacheEntries() {
+
+		FormulaCellCacheEntry[] result = new FormulaCellCacheEntry[_formulaEntriesByCell.size()];
+		_formulaEntriesByCell.values().toArray(result);
+		return result;
+	}
+
+	public void clear() {
+		_formulaEntriesByCell.clear();
+	}
+
+	/**
+	 * @return <code>null</code> if not found
+	 */
+	public FormulaCellCacheEntry get(EvaluationCell cell) {
+		return (FormulaCellCacheEntry) _formulaEntriesByCell.get(cell);
+	}
+
+	public void put(EvaluationCell cell, FormulaCellCacheEntry entry) {
+		_formulaEntriesByCell.put(cell, entry);
+	}
+
+	public FormulaCellCacheEntry remove(EvaluationCell cell) {
+		return (FormulaCellCacheEntry) _formulaEntriesByCell.remove(cell);
+	}
+
+	public void applyOperation(IEntryOperation operation) {
+		Iterator i = _formulaEntriesByCell.values().iterator();
+		while (i.hasNext()) {
+			operation.processEntry((FormulaCellCacheEntry) i.next());
+		}
+	}
+}
diff --git a/src/java/org/apache/poi/ss/formula/FormulaCellCacheEntry.java b/src/java/org/apache/poi/ss/formula/FormulaCellCacheEntry.java
new file mode 100644
index 0000000000..f0fd5c3a76
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/FormulaCellCacheEntry.java
@@ -0,0 +1,113 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+
+import org.apache.poi.hssf.record.formula.eval.ValueEval;
+import org.apache.poi.ss.formula.FormulaUsedBlankCellSet.BookSheetKey;
+
+
+/**
+ * Stores the cached result of a formula evaluation, along with the set of sensititive input cells
+ * 
+ * @author Josh Micich
+ */
+final class FormulaCellCacheEntry extends CellCacheEntry {
+	public static final FormulaCellCacheEntry[] EMPTY_ARRAY = { };
+	
+	/**
+	 * Cells 'used' in the current evaluation of the formula corresponding to this cache entry
+	 *
+	 * If any of the following cells change, this cache entry needs to be cleared
+	 */
+	private CellCacheEntry[] _sensitiveInputCells;
+
+	private FormulaUsedBlankCellSet _usedBlankCellGroup;
+
+	public FormulaCellCacheEntry() {
+		
+	}
+
+	public void setSensitiveInputCells(CellCacheEntry[] sensitiveInputCells) {
+		// need to tell all cells that were previously used, but no longer are, 
+		// that they are not consumed by this cell any more
+		changeConsumingCells(sensitiveInputCells == null ? CellCacheEntry.EMPTY_ARRAY : sensitiveInputCells);
+		_sensitiveInputCells = sensitiveInputCells;
+	}
+
+	public void clearFormulaEntry() {
+		CellCacheEntry[] usedCells = _sensitiveInputCells;
+		if (usedCells != null) {
+			for (int i = usedCells.length-1; i>=0; i--) {
+				usedCells[i].clearConsumingCell(this);
+			}
+		}
+		_sensitiveInputCells = null;
+		clearValue();
+	}
+	
+	private void changeConsumingCells(CellCacheEntry[] usedCells) {
+
+		CellCacheEntry[] prevUsedCells = _sensitiveInputCells;
+		int nUsed = usedCells.length;
+		for (int i = 0; i < nUsed; i++) {
+			usedCells[i].addConsumingCell(this);
+		}
+		if (prevUsedCells == null) {
+			return;
+		}
+		int nPrevUsed = prevUsedCells.length;
+		if (nPrevUsed < 1) {
+			return;
+		}
+		Set usedSet;
+		if (nUsed < 1) {
+			usedSet = Collections.EMPTY_SET;
+		} else {
+			usedSet = new HashSet(nUsed * 3 / 2);
+			for (int i = 0; i < nUsed; i++) {
+				usedSet.add(usedCells[i]);
+			}
+		}
+		for (int i = 0; i < nPrevUsed; i++) {
+			CellCacheEntry prevUsed = prevUsedCells[i];
+			if (!usedSet.contains(prevUsed)) {
+				// previously was used by cellLoc, but not anymore
+				prevUsed.clearConsumingCell(this);
+			}
+		}
+	}
+
+	public void updateFormulaResult(ValueEval result, CellCacheEntry[] sensitiveInputCells, FormulaUsedBlankCellSet usedBlankAreas) {
+		updateValue(result);
+		setSensitiveInputCells(sensitiveInputCells);
+		_usedBlankCellGroup = usedBlankAreas;
+	}
+
+	public void notifyUpdatedBlankCell(BookSheetKey bsk, int rowIndex, int columnIndex, IEvaluationListener evaluationListener) {
+		if (_usedBlankCellGroup != null) {
+			if (_usedBlankCellGroup.containsCell(bsk, rowIndex, columnIndex)) {
+				clearFormulaEntry();
+				recurseClearCachedFormulaResults(evaluationListener);
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/src/java/org/apache/poi/ss/formula/FormulaCellCacheEntrySet.java b/src/java/org/apache/poi/ss/formula/FormulaCellCacheEntrySet.java
new file mode 100644
index 0000000000..780d2661e2
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/FormulaCellCacheEntrySet.java
@@ -0,0 +1,156 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+/**
+ * A custom implementation of {@link java.util.HashSet} in order to reduce memory consumption.
+ *
+ * Profiling tests (Oct 2008) have shown that each element {@link FormulaCellCacheEntry} takes
+ * around 32 bytes to store in a HashSet, but around 6 bytes to store here.  For Spreadsheets with
+ * thousands of formula cells with multiple interdependencies, the savings can be very significant.
+ *
+ * @author Josh Micich
+ */
+final class FormulaCellCacheEntrySet {
+
+	private int _size;
+	private FormulaCellCacheEntry[] _arr;
+
+	public FormulaCellCacheEntrySet() {
+		_arr = FormulaCellCacheEntry.EMPTY_ARRAY;
+	}
+
+	public FormulaCellCacheEntry[] toArray() {
+		int nItems = _size;
+		if (nItems < 1) {
+			return FormulaCellCacheEntry.EMPTY_ARRAY;
+		}
+		FormulaCellCacheEntry[] result = new FormulaCellCacheEntry[nItems];
+		int j=0;
+		for(int i=0; i<_arr.length; i++) {
+			FormulaCellCacheEntry cce = _arr[i];
+			if (cce != null) {
+				result[j++] = cce;
+			}
+		}
+		if (j!= nItems) {
+			throw new IllegalStateException("size mismatch");
+		}
+		return result;
+	}
+
+
+	public void add(CellCacheEntry cce) {
+		if (_size * 3 >= _arr.length * 2) {
+			// re-hash
+			FormulaCellCacheEntry[] prevArr = _arr;
+			FormulaCellCacheEntry[] newArr = new FormulaCellCacheEntry[4 + _arr.length * 3 / 2]; // grow 50%
+			for(int i=0; i<prevArr.length; i++) {
+				FormulaCellCacheEntry prevCce = _arr[i];
+				if (prevCce != null) {
+					addInternal(newArr, prevCce);
+				}
+			}
+			_arr = newArr;
+		}
+		if (addInternal(_arr, cce)) {
+			_size++;
+		}
+	}
+
+
+	private static boolean addInternal(CellCacheEntry[] arr, CellCacheEntry cce) {
+
+		int startIx = cce.hashCode() % arr.length;
+
+		for(int i=startIx; i<arr.length; i++) {
+			CellCacheEntry item = arr[i];
+			if (item == cce) {
+				// already present
+				return false;
+			}
+			if (item == null) {
+				arr[i] = cce;
+				return true;
+			}
+		}
+		for(int i=0; i<startIx; i++) {
+			CellCacheEntry item = arr[i];
+			if (item == cce) {
+				// already present
+				return false;
+			}
+			if (item == null) {
+				arr[i] = cce;
+				return true;
+			}
+		}
+		throw new IllegalStateException("No empty space found");
+	}
+
+	public boolean remove(CellCacheEntry cce) {
+		FormulaCellCacheEntry[] arr = _arr;
+
+		if (_size * 3 < _arr.length && _arr.length > 8) {
+			// re-hash
+			boolean found = false;
+			FormulaCellCacheEntry[] prevArr = _arr;
+			FormulaCellCacheEntry[] newArr = new FormulaCellCacheEntry[_arr.length / 2]; // shrink 50%
+			for(int i=0; i<prevArr.length; i++) {
+				FormulaCellCacheEntry prevCce = _arr[i];
+				if (prevCce != null) {
+					if (prevCce == cce) {
+						found=true;
+						_size--;
+						// skip it
+						continue;
+					}
+					addInternal(newArr, prevCce);
+				}
+			}
+			_arr = newArr;
+			return found;
+		}
+		// else - usual case
+		// delete single element (without re-hashing)
+
+		int startIx = cce.hashCode() % arr.length;
+
+		// note - can't exit loops upon finding null because of potential previous deletes
+		for(int i=startIx; i<arr.length; i++) {
+			FormulaCellCacheEntry item = arr[i];
+			if (item == cce) {
+				// found it
+				arr[i] = null;
+				_size--;
+				return true;
+			}
+		}
+		for(int i=0; i<startIx; i++) {
+			FormulaCellCacheEntry item = arr[i];
+			if (item == cce) {
+				// found it
+				arr[i] = null;
+				_size--;
+				return true;
+			}
+		}
+		return false;
+	}
+
+}
diff --git a/src/java/org/apache/poi/ss/formula/FormulaUsedBlankCellSet.java b/src/java/org/apache/poi/ss/formula/FormulaUsedBlankCellSet.java
new file mode 100644
index 0000000000..7447edca0f
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/FormulaUsedBlankCellSet.java
@@ -0,0 +1,189 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.apache.poi.hssf.util.CellReference;
+
+/**
+ * Optimisation - compacts many blank cell references used by a single formula. 
+ * 
+ * @author Josh Micich
+ */
+final class FormulaUsedBlankCellSet {
+	public static final class BookSheetKey {
+
+		private final int _bookIndex;
+		private final int _sheetIndex;
+
+		public BookSheetKey(int bookIndex, int sheetIndex) {
+			_bookIndex = bookIndex;
+			_sheetIndex = sheetIndex;
+		}
+		public int hashCode() {
+			return _bookIndex * 17 + _sheetIndex;
+		}
+		public boolean equals(Object obj) {
+			BookSheetKey other = (BookSheetKey) obj;
+			return _bookIndex == other._bookIndex && _sheetIndex == other._sheetIndex;
+		}
+	}
+
+	private static final class BlankCellSheetGroup {
+		private final List _rectangleGroups;
+		private int _currentRowIndex;
+		private int _firstColumnIndex;
+		private int _lastColumnIndex;
+		private BlankCellRectangleGroup _currentRectangleGroup;
+
+		public BlankCellSheetGroup() {
+			_rectangleGroups = new ArrayList();
+			_currentRowIndex = -1;
+		}
+
+		public void addCell(int rowIndex, int columnIndex) {
+			if (_currentRowIndex == -1) {
+				_currentRowIndex = rowIndex;
+				_firstColumnIndex = columnIndex;
+				_lastColumnIndex = columnIndex;
+			} else {
+				if (_currentRowIndex == rowIndex && _lastColumnIndex+1 == columnIndex) {
+					_lastColumnIndex = columnIndex;
+				} else {
+					// cell does not fit on end of current row
+					if (_currentRectangleGroup == null) {
+						_currentRectangleGroup = new BlankCellRectangleGroup(_currentRowIndex, _firstColumnIndex, _lastColumnIndex);
+					} else {
+						if (!_currentRectangleGroup.acceptRow(_currentRowIndex, _firstColumnIndex, _lastColumnIndex)) {
+							_rectangleGroups.add(_currentRectangleGroup);
+							_currentRectangleGroup = new BlankCellRectangleGroup(_currentRowIndex, _firstColumnIndex, _lastColumnIndex);
+						}
+					}
+					_currentRowIndex = rowIndex;
+					_firstColumnIndex = columnIndex;
+					_lastColumnIndex = columnIndex;
+				}
+			}
+		}
+
+		public boolean containsCell(int rowIndex, int columnIndex) {
+			for (int i=_rectangleGroups.size()-1; i>=0; i--) {
+				BlankCellRectangleGroup bcrg = (BlankCellRectangleGroup) _rectangleGroups.get(i);
+				if (bcrg.containsCell(rowIndex, columnIndex)) {
+					return true;
+				}
+			}
+			if(_currentRectangleGroup != null && _currentRectangleGroup.containsCell(rowIndex, columnIndex)) {
+				return true;
+			}
+			if (_currentRowIndex != -1 && _currentRowIndex == rowIndex) {
+				if (_firstColumnIndex <=  columnIndex && columnIndex <= _lastColumnIndex) {
+					return true;
+				}
+			}
+			return false;
+		}
+	}
+
+	private static final class BlankCellRectangleGroup {
+
+		private final int _firstRowIndex;
+		private final int _firstColumnIndex;
+		private final int _lastColumnIndex;
+		private int _lastRowIndex;
+
+		public BlankCellRectangleGroup(int firstRowIndex, int firstColumnIndex, int lastColumnIndex) {
+			_firstRowIndex = firstRowIndex;
+			_firstColumnIndex = firstColumnIndex;
+			_lastColumnIndex = lastColumnIndex;
+			_lastRowIndex = firstRowIndex;
+		}
+
+		public boolean containsCell(int rowIndex, int columnIndex) {
+			if (columnIndex < _firstColumnIndex) {
+				return false;
+			}
+			if (columnIndex > _lastColumnIndex) {
+				return false;
+			}
+			if (rowIndex < _firstRowIndex) {
+				return false;
+			}
+			if (rowIndex > _lastRowIndex) {
+				return false;
+			}
+			return true;
+		}
+
+		public boolean acceptRow(int rowIndex, int firstColumnIndex, int lastColumnIndex) {
+			if (firstColumnIndex != _firstColumnIndex) {
+				return false;
+			}
+			if (lastColumnIndex != _lastColumnIndex) {
+				return false;
+			}
+			if (rowIndex != _lastRowIndex+1) {
+				return false;
+			}
+			_lastRowIndex = rowIndex;
+			return true;
+		}
+		public String toString() {
+			StringBuffer sb = new StringBuffer(64);
+			CellReference crA = new CellReference(_firstRowIndex, _firstColumnIndex, false, false);
+			CellReference crB = new CellReference(_lastRowIndex, _lastColumnIndex, false, false);
+			sb.append(getClass().getName());
+			sb.append(" [").append(crA.formatAsString()).append(':').append(crB.formatAsString()).append("]");
+			return sb.toString();
+		}
+	}
+
+	private final Map _sheetGroupsByBookSheet;
+
+	public FormulaUsedBlankCellSet() {
+		_sheetGroupsByBookSheet = new HashMap();
+	}
+
+	public void addCell(int bookIndex, int sheetIndex, int rowIndex, int columnIndex) {
+		BlankCellSheetGroup sbcg = getSheetGroup(bookIndex, sheetIndex);
+		sbcg.addCell(rowIndex, columnIndex);
+	}
+
+	private BlankCellSheetGroup getSheetGroup(int bookIndex, int sheetIndex) {
+		BookSheetKey key = new BookSheetKey(bookIndex, sheetIndex);
+
+		BlankCellSheetGroup result = (BlankCellSheetGroup) _sheetGroupsByBookSheet.get(key);
+		if (result == null) {
+			result = new BlankCellSheetGroup();
+			_sheetGroupsByBookSheet.put(key, result);
+		}
+		return result;
+	}
+
+	public boolean containsCell(BookSheetKey key, int rowIndex, int columnIndex) {
+		BlankCellSheetGroup bcsg = (BlankCellSheetGroup) _sheetGroupsByBookSheet.get(key);
+		if (bcsg == null) {
+			return false;
+		}
+		return bcsg.containsCell(rowIndex, columnIndex);
+	}
+}
diff --git a/src/java/org/apache/poi/ss/formula/IEvaluationListener.java b/src/java/org/apache/poi/ss/formula/IEvaluationListener.java
index deba4d59b0..caf254edff 100644
--- a/src/java/org/apache/poi/ss/formula/IEvaluationListener.java
+++ b/src/java/org/apache/poi/ss/formula/IEvaluationListener.java
@@ -19,6 +19,7 @@ package org.apache.poi.ss.formula;
 
 import org.apache.poi.hssf.record.formula.Ptg;
 import org.apache.poi.hssf.record.formula.eval.ValueEval;
+import org.apache.poi.hssf.usermodel.HSSFCell;
 
 /**
  * Tests can implement this class to track the internal working of the {@link WorkbookEvaluator}.<br/>
@@ -28,12 +29,26 @@ import org.apache.poi.hssf.record.formula.eval.ValueEval;
  * @author Josh Micich
  */
 interface IEvaluationListener {
+	/**
+	 * A (mostly) opaque interface to allow test clients to trace cache values
+	 * Each spreadsheet cell gets one unique cache entry instance.  These objects
+	 * are safe to use as keys in {@link java.util.HashMap}s 
+	 */
+	interface ICacheEntry {
+		ValueEval getValue();
+	}
 
 	void onCacheHit(int sheetIndex, int rowIndex, int columnIndex, ValueEval result);
-	void onReadPlainValue(int sheetIndex, int srcRowNum, int srcColNum, ValueEval value);
-	void onStartEvaluate(int sheetIndex, int rowIndex, int columnIndex, Ptg[] ptgs);
-	void onEndEvaluate(int sheetIndex, int rowIndex, int columnIndex, ValueEval result);
+	void onReadPlainValue(int sheetIndex, int rowIndex, int columnIndex, ICacheEntry entry);
+	void onStartEvaluate(EvaluationCell cell, ICacheEntry entry, Ptg[] ptgs);
+	void onEndEvaluate(ICacheEntry entry, ValueEval result);
 	void onClearWholeCache();
-	void onClearCachedValue(int sheetIndex, int rowIndex, int columnIndex, ValueEval value);
-	void onClearDependentCachedValue(int sheetIndex, int rowIndex, int columnIndex, ValueEval value, int depth);
+	void onClearCachedValue(ICacheEntry entry);
+	/**
+	 * Internally, formula {@link ICacheEntry}s are stored in sets which may change ordering due 
+	 * to seemingly trivial changes.  This method is provided to make the order of call-backs to 
+	 * {@link #onClearDependentCachedValue(ICacheEntry, int)} more deterministic.
+	 */
+	void sortDependentCachedValues(ICacheEntry[] formulaCells);
+	void onClearDependentCachedValue(ICacheEntry formulaCell, int depth);
 }
diff --git a/src/java/org/apache/poi/ss/formula/PlainCellCache.java b/src/java/org/apache/poi/ss/formula/PlainCellCache.java
new file mode 100644
index 0000000000..2e6476f17d
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/PlainCellCache.java
@@ -0,0 +1,84 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ *
+ * @author Josh Micich
+ */
+final class PlainCellCache {
+
+	public static final class Loc {
+
+		private final int _bookSheetColumn;
+
+		private final int _rowIndex;
+
+		public Loc(int bookIndex, int sheetIndex, int rowIndex, int columnIndex) {
+			_bookSheetColumn = toBookSheetColumn(bookIndex, sheetIndex, columnIndex);
+			_rowIndex = rowIndex;
+		}
+
+		public static int toBookSheetColumn(int bookIndex, int sheetIndex, int columnIndex) {
+			return ((bookIndex & 0x00FF) << 24) + ((sheetIndex & 0x00FF) << 16)
+					+ ((columnIndex & 0xFFFF) << 0);
+		}
+
+		public Loc(int bookSheetColumn, int rowIndex) {
+			_bookSheetColumn = bookSheetColumn;
+			_rowIndex = rowIndex;
+		}
+
+		public int hashCode() {
+			return _bookSheetColumn + 17 * _rowIndex;
+		}
+
+		public boolean equals(Object obj) {
+			Loc other = (Loc) obj;
+			return _bookSheetColumn == other._bookSheetColumn && _rowIndex == other._rowIndex;
+		}
+
+		public int getRowIndex() {
+			return _rowIndex;
+		}
+		public int getColumnIndex() {
+			return _bookSheetColumn & 0x000FFFF;
+		}
+	}
+
+	private Map _plainValueEntriesByLoc;
+
+	public PlainCellCache() {
+		_plainValueEntriesByLoc = new HashMap();
+	}
+	public void put(Loc key, PlainValueCellCacheEntry cce) {
+		_plainValueEntriesByLoc.put(key, cce);
+	}
+	public void clear() {
+		_plainValueEntriesByLoc.clear();
+	}
+	public PlainValueCellCacheEntry get(Loc key) {
+		return (PlainValueCellCacheEntry) _plainValueEntriesByLoc.get(key);
+	}
+	public void remove(Loc key) {
+		_plainValueEntriesByLoc.remove(key);
+	}
+}
diff --git a/src/java/org/apache/poi/ss/formula/PlainValueCellCacheEntry.java b/src/java/org/apache/poi/ss/formula/PlainValueCellCacheEntry.java
new file mode 100644
index 0000000000..d1ffb51fa3
--- /dev/null
+++ b/src/java/org/apache/poi/ss/formula/PlainValueCellCacheEntry.java
@@ -0,0 +1,31 @@
+/* ====================================================================
+   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.poi.ss.formula;
+
+import org.apache.poi.hssf.record.formula.eval.ValueEval;
+
+/**
+ * Used for non-formula cells, primarily to keep track of the referencing (formula) cells.
+ * 
+ * @author Josh Micich
+ */
+final class PlainValueCellCacheEntry extends CellCacheEntry {
+	public PlainValueCellCacheEntry(ValueEval value) {
+		updateValue(value);
+	}
+}
\ No newline at end of file
diff --git a/src/java/org/apache/poi/ss/formula/SheetRefEvaluator.java b/src/java/org/apache/poi/ss/formula/SheetRefEvaluator.java
index 4f46e31a21..bf636a4da3 100644
--- a/src/java/org/apache/poi/ss/formula/SheetRefEvaluator.java
+++ b/src/java/org/apache/poi/ss/formula/SheetRefEvaluator.java
@@ -18,7 +18,6 @@
 package org.apache.poi.ss.formula;

 

 import org.apache.poi.hssf.record.formula.eval.ValueEval;

-import org.apache.poi.ss.usermodel.Sheet;

 /**

  * 

  * 

@@ -28,7 +27,7 @@ final class SheetRefEvaluator {
 

 	private final WorkbookEvaluator _bookEvaluator;

 	private final EvaluationTracker _tracker;

-	private final Sheet _sheet;

+	private final EvaluationSheet _sheet;

 	private final int _sheetIndex;

 

 	public SheetRefEvaluator(WorkbookEvaluator bookEvaluator, EvaluationTracker tracker,

diff --git a/src/java/org/apache/poi/ss/formula/WorkbookEvaluator.java b/src/java/org/apache/poi/ss/formula/WorkbookEvaluator.java
index 41a7c72fae..ef5975d3e6 100644
--- a/src/java/org/apache/poi/ss/formula/WorkbookEvaluator.java
+++ b/src/java/org/apache/poi/ss/formula/WorkbookEvaluator.java
@@ -63,8 +63,6 @@ import org.apache.poi.hssf.record.formula.eval.ValueEval;
 import org.apache.poi.hssf.util.CellReference;
 import org.apache.poi.ss.formula.EvaluationWorkbook.ExternalSheet;
 import org.apache.poi.ss.usermodel.Cell;
-import org.apache.poi.ss.usermodel.Row;
-import org.apache.poi.ss.usermodel.Sheet;
 
 /**
  * Evaluates formula cells.<p/>
@@ -131,7 +129,7 @@ public final class WorkbookEvaluator {
 	/* package */ IEvaluationListener getEvaluationListener() {
 		return _evaluationListener;
 	}
-	 
+
 	/**
 	 * Should be called whenever there are changes to input cells in the evaluated workbook.
 	 * Failure to call this method after changing cell values will cause incorrect behaviour
@@ -143,29 +141,23 @@ public final class WorkbookEvaluator {
 	}
 
 	/**
-	 * Sets the cached value for a plain (non-formula) cell.
-	 * @param never <code>null</code>. Use {@link BlankEval#INSTANCE} when the cell is being 
-	 * cleared. Otherwise an instance of {@link NumberEval}, {@link StringEval}, {@link BoolEval}
-	 * or {@link ErrorEval} to represent a plain cell value.
+	 * Should be called to tell the cell value cache that the specified (value or formula) cell 
+	 * has changed.
 	 */
-	public void setCachedPlainValue(Sheet sheet, int rowIndex, int columnIndex, ValueEval value) {
-		if (value == null) {
-			throw new IllegalArgumentException("value must not be null");
-		}
-		int sheetIndex = getSheetIndex(sheet);
-		_cache.setValue(getCellLoc(sheetIndex, rowIndex, columnIndex), true, CellLocation.EMPTY_ARRAY, value);
-
+	public void notifyUpdateCell(EvaluationCell cell) {
+		int sheetIndex = getSheetIndex(cell.getSheet());
+		_cache.notifyUpdateCell(_workbookIx, sheetIndex, cell);
 	}
 	/**
-	 * Should be called to tell the cell value cache that the specified cell has just become a
-	 * formula cell, or the formula text has changed 
+	 * Should be called to tell the cell value cache that the specified cell has just been
+	 * deleted. 
 	 */
-	public void notifySetFormula(Sheet sheet, int rowIndex, int columnIndex) {
-		int sheetIndex = getSheetIndex(sheet);
-		_cache.setValue(getCellLoc(sheetIndex, rowIndex, columnIndex), false, CellLocation.EMPTY_ARRAY, null);
-
+	public void notifyDeleteCell(EvaluationCell cell) {
+		int sheetIndex = getSheetIndex(cell.getSheet());
+		_cache.notifyDeleteCell(_workbookIx, sheetIndex, cell);
 	}
-	private int getSheetIndex(Sheet sheet) {
+
+	private int getSheetIndex(EvaluationSheet sheet) {
 		Integer result = (Integer) _sheetIndexesBySheet.get(sheet);
 		if (result == null) {
 			int sheetIndex = _workbook.getSheetIndex(sheet);
@@ -178,78 +170,76 @@ public final class WorkbookEvaluator {
 		return result.intValue();
 	}
 
-	public ValueEval evaluate(Cell srcCell) {
+	public ValueEval evaluate(EvaluationCell srcCell) {
 		int sheetIndex = getSheetIndex(srcCell.getSheet());
-		CellLocation cellLoc = getCellLoc(sheetIndex, srcCell.getRowIndex(), srcCell.getCellNum());
-		return internalEvaluate(srcCell, cellLoc, new EvaluationTracker(_cache));
+		return evaluateAny(srcCell, sheetIndex, srcCell.getRowIndex(), srcCell.getColumnIndex(), new EvaluationTracker(_cache));
 	}
 
+
 	/**
 	 * @return never <code>null</code>, never {@link BlankEval}
 	 */
-	private ValueEval internalEvaluate(Cell srcCell, CellLocation cellLoc, EvaluationTracker tracker) {
-		int sheetIndex = cellLoc.getSheetIndex();
-		int rowIndex = cellLoc.getRowIndex();
-		int columnIndex = cellLoc.getColumnIndex();
+	private ValueEval evaluateAny(EvaluationCell srcCell, int sheetIndex,
+				int rowIndex, int columnIndex, EvaluationTracker tracker) {
 
-		ValueEval result;
+		if (srcCell == null || srcCell.getCellType() != Cell.CELL_TYPE_FORMULA) {
+			ValueEval result = getValueFromNonFormulaCell(srcCell);
+			tracker.acceptPlainValueDependency(_workbookIx, sheetIndex, rowIndex, columnIndex, result);
+			return result;
+		}
 
-		result = tracker.startEvaluate(cellLoc);
+		FormulaCellCacheEntry cce = _cache.getOrCreateFormulaCellEntry(srcCell);
+		tracker.acceptFormulaDependency(cce);
 		IEvaluationListener evalListener = _evaluationListener;
-		if (result != null) {
-			if(evalListener != null) {
-				evalListener.onCacheHit(sheetIndex, rowIndex, columnIndex, result);
+		if (cce.getValue() == null) {
+			if (!tracker.startEvaluate(cce)) {
+				return ErrorEval.CIRCULAR_REF_ERROR;
 			}
-			return result;
-		}
 
-		boolean isPlainFormulaCell = false;
-		try {
-			result = getValueFromNonFormulaCell(srcCell);
-			if (result != null) {
-				isPlainFormulaCell = true;
-				if(evalListener != null) {
-					evalListener.onReadPlainValue(sheetIndex, rowIndex, columnIndex, result);
-				}
-			} else {
-				isPlainFormulaCell = false;
+			try {
+				ValueEval result;
+
 				Ptg[] ptgs = _workbook.getFormulaTokens(srcCell);
-				if(evalListener == null) {
-					result = evaluateFormula(sheetIndex, rowIndex, (short)columnIndex, ptgs, tracker);
+				if (evalListener == null) {
+					result = evaluateFormula(sheetIndex, rowIndex, columnIndex, ptgs, tracker);
 				} else {
-					evalListener.onStartEvaluate(sheetIndex, rowIndex, columnIndex, ptgs);
-					result = evaluateFormula(sheetIndex, rowIndex, (short)columnIndex, ptgs, tracker);
-					evalListener.onEndEvaluate(sheetIndex, rowIndex, columnIndex, result);
+					evalListener.onStartEvaluate(srcCell, cce, ptgs);
+					result = evaluateFormula(sheetIndex, rowIndex, columnIndex, ptgs, tracker);
+					evalListener.onEndEvaluate(cce, result);
 				}
+
+				tracker.updateCacheResult(result);
+			} finally {
+				tracker.endEvaluate(cce);
+			}
+		} else {
+			if(evalListener != null) {
+				evalListener.onCacheHit(sheetIndex, rowIndex, columnIndex, cce.getValue());
 			}
-		} finally {
-			tracker.endEvaluate(cellLoc, result, isPlainFormulaCell);
+			return cce.getValue();
 		}
 		if (isDebugLogEnabled()) {
 			String sheetName = getSheetName(sheetIndex);
 			CellReference cr = new CellReference(rowIndex, columnIndex);
-			logDebug("Evaluated " + sheetName + "!" + cr.formatAsString() + " to " + result.toString());
+			logDebug("Evaluated " + sheetName + "!" + cr.formatAsString() + " to " + cce.getValue().toString());
 		}
-		return result;
+		return cce.getValue();
 	}
 	/**
 	 * Gets the value from a non-formula cell.
 	 * @param cell may be <code>null</code>
-	 * @return {@link BlankEval} if cell is <code>null</code> or blank, <code>null</code> if cell 
-	 * is a formula cell.
+	 * @return {@link BlankEval} if cell is <code>null</code> or blank, never <code>null</code>
 	 */
-	private static ValueEval getValueFromNonFormulaCell(Cell cell) {
+	/* package */ static ValueEval getValueFromNonFormulaCell(EvaluationCell cell) {
 		if (cell == null) {
 			return BlankEval.INSTANCE;
 		}
 		int cellType = cell.getCellType();
 		switch (cellType) {
-			case Cell.CELL_TYPE_FORMULA:
-				return null;
 			case Cell.CELL_TYPE_NUMERIC:
 				return new NumberEval(cell.getNumericCellValue());
 			case Cell.CELL_TYPE_STRING:
-				return new StringEval(cell.getRichStringCellValue().getString());
+				return new StringEval(cell.getStringCellValue());
 			case Cell.CELL_TYPE_BOOLEAN:
 				return BoolEval.valueOf(cell.getBooleanCellValue());
 			case Cell.CELL_TYPE_BLANK:
@@ -273,7 +263,7 @@ public final class WorkbookEvaluator {
 					// Excel prefers to encode 'SUM()' as a tAttr token, but this evaluator
 					// expects the equivalent function token
 					byte nArgs = 1;  // tAttrSum always has 1 parameter
-					ptg = new FuncVarPtg("SUM", nArgs); 
+					ptg = new FuncVarPtg("SUM", nArgs);
 				}
 			}
 			if (ptg instanceof ControlPtg) {
@@ -381,7 +371,7 @@ public final class WorkbookEvaluator {
 		}
 		int otherSheetIndex = _workbook.convertFromExternSheetIndex(externSheetIndex);
 		return new SheetRefEvaluator(this, tracker, _workbook, otherSheetIndex);
-		
+
 	}
 
 	/**
@@ -428,7 +418,7 @@ public final class WorkbookEvaluator {
 		if (ptg instanceof MissingArgPtg) {
 			return MissingArgEval.instance;
 		}
-		if (ptg instanceof AreaErrPtg ||ptg instanceof RefErrorPtg 
+		if (ptg instanceof AreaErrPtg ||ptg instanceof RefErrorPtg
 				|| ptg instanceof DeletedArea3DPtg || ptg instanceof DeletedRef3DPtg) {
 				return ErrorEval.REF_INVALID;
 		}
@@ -465,25 +455,14 @@ public final class WorkbookEvaluator {
 		}
 		return getEvalForPtg(ptgs[0], sheetIndex, tracker);
 	}
-	
+
 	/**
 	 * Used by the lazy ref evals whenever they need to get the value of a contained cell.
 	 */
-	/* package */ ValueEval evaluateReference(Sheet sheet, int sheetIndex, int rowIndex,
+	/* package */ ValueEval evaluateReference(EvaluationSheet sheet, int sheetIndex, int rowIndex,
 			int columnIndex, EvaluationTracker tracker) {
-		
-		Row row = sheet.getRow(rowIndex);
-		Cell cell;
-		if (row == null) {
-			cell = null;
-		} else {
-			cell = row.getCell(columnIndex);
- 		}
-		CellLocation cellLoc = getCellLoc(sheetIndex, rowIndex, columnIndex);
-		tracker.acceptDependency(cellLoc);
-		return internalEvaluate(cell, cellLoc, tracker);
-	}
-	private CellLocation getCellLoc(int sheetIndex, int rowIndex, int columnIndex) {
-		return new CellLocation(_workbookIx, sheetIndex, rowIndex, columnIndex);
+
+		EvaluationCell cell = sheet.getCell(rowIndex, columnIndex);
+		return evaluateAny(cell, sheetIndex, rowIndex, columnIndex, tracker);
 	}
 }
diff --git a/src/java/org/apache/poi/ss/usermodel/FormulaEvaluator.java b/src/java/org/apache/poi/ss/usermodel/FormulaEvaluator.java
index 629d4f5a12..15711195b1 100644
--- a/src/java/org/apache/poi/ss/usermodel/FormulaEvaluator.java
+++ b/src/java/org/apache/poi/ss/usermodel/FormulaEvaluator.java
@@ -45,20 +45,17 @@ public interface FormulaEvaluator {
      */
     void clearAllCachedResultValues();
 	/**
-	 * Sets the cached value for a plain (non-formula) cell.
-	 * Should be called whenever there are changes to individual input cells in the evaluated workbook.
+	 * Should be called to tell the cell value cache that the specified (value or formula) cell 
+	 * has changed.
 	 * Failure to call this method after changing cell values will cause incorrect behaviour
 	 * of the evaluate~ methods of this class
-	 * @param never <code>null</code>. Use {@link BlankEval#INSTANCE} when the cell is being 
-	 * cleared. Otherwise an instance of {@link NumberEval}, {@link StringEval}, {@link BoolEval}
-	 * or {@link ErrorEval} to represent a plain cell value.
 	 */
-    void setCachedPlainValue(Sheet sheet, int rowIndex, int columnIndex, ValueEval value);
+    void notifySetFormula(Cell cell);
 	/**
 	 * Should be called to tell the cell value cache that the specified cell has just become a
 	 * formula cell, or the formula text has changed 
 	 */
-	void notifySetFormula(HSSFSheet sheet, int rowIndex, int columnIndex);
+    void notifyDeleteCell(Cell cell);
     /**
      * If cell contains a formula, the formula is evaluated and returned,
      * else the CellValue simply copies the appropriate cell value from