gt:             return processNameEval(ec.getNameXEval((NameXPtg)ptg), ec);
context: /* ====================================================================
     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.Arrays;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.IdentityHashMap;
  import java.util.Map;
  import java.util.Stack;
  import java.util.TreeSet;
  
  import org.apache.poi.ss.SpreadsheetVersion;
  import org.apache.poi.ss.formula.CollaboratingWorkbooksEnvironment.WorkbookNotFoundException;
  import org.apache.poi.ss.formula.atp.AnalysisToolPak;
  import org.apache.poi.ss.formula.eval.*;
  import org.apache.poi.ss.formula.function.FunctionMetadataRegistry;
  import org.apache.poi.ss.formula.functions.*;
  import org.apache.poi.ss.formula.ptg.*;
  import org.apache.poi.ss.formula.udf.AggregatingUDFFinder;
  import org.apache.poi.ss.formula.udf.UDFFinder;
  import org.apache.poi.ss.usermodel.CellType;
  import org.apache.poi.ss.util.CellRangeAddressBase;
  import org.apache.poi.ss.util.CellReference;
  import org.apache.poi.util.Internal;
  import org.apache.poi.util.POILogFactory;
  import org.apache.poi.util.POILogger;
  /**
   * Evaluates formula cells.<p/>
   *
   * For performance reasons, this class keeps a cache of all previously calculated intermediate
   * cell values.  Be sure to call {@link #clearAllCachedResultValues()} if any workbook cells are changed between
   * calls to evaluate~ methods on this class.<br/>
   *
   * For POI internal use only
   *
   * @author Josh Micich
   * @author Thies Wellpott (debug output enhancements)
   */
  @Internal
  public final class WorkbookEvaluator {
      
      private static final POILogger LOG = POILogFactory.getLogger(WorkbookEvaluator.class);
  
      private final EvaluationWorkbook _workbook;
      private EvaluationCache _cache;
      /** part of cache entry key (useful when evaluating multiple workbooks) */
      private int _workbookIx;
  
      private final IEvaluationListener _evaluationListener;
      private final Map<EvaluationSheet, Integer> _sheetIndexesBySheet;
      private final Map<String, Integer> _sheetIndexesByName;
      private CollaboratingWorkbooksEnvironment _collaboratingWorkbookEnvironment;
      private final IStabilityClassifier _stabilityClassifier;
      private final AggregatingUDFFinder _udfFinder;
  
      private boolean _ignoreMissingWorkbooks;
  
      /**
       * whether print detailed messages about the next formula evaluation
       */
      private boolean dbgEvaluationOutputForNextEval;
  
      // special logger for formula evaluation output (because of possibly very large output)
      private final POILogger EVAL_LOG = POILogFactory.getLogger("POI.FormulaEval");
      // current indent level for evalution; negative value for no output
      private int dbgEvaluationOutputIndent = -1;
  
      /**
       * @param udfFinder pass <code>null</code> for default (AnalysisToolPak only)
       */
      public WorkbookEvaluator(EvaluationWorkbook workbook, IStabilityClassifier stabilityClassifier, UDFFinder udfFinder) {
          this (workbook, null, stabilityClassifier, udfFinder);
      }
      /* package */ WorkbookEvaluator(EvaluationWorkbook workbook, IEvaluationListener evaluationListener,
              IStabilityClassifier stabilityClassifier, UDFFinder udfFinder) {
          _workbook = workbook;
          _evaluationListener = evaluationListener;
          _cache = new EvaluationCache(evaluationListener);
          _sheetIndexesBySheet = new IdentityHashMap<>();
          _sheetIndexesByName = new IdentityHashMap<>();
          _collaboratingWorkbookEnvironment = CollaboratingWorkbooksEnvironment.EMPTY;
          _workbookIx = 0;
          _stabilityClassifier = stabilityClassifier;
  
          AggregatingUDFFinder defaultToolkit = // workbook can be null in unit tests
                  workbook == null ? null : (AggregatingUDFFinder)workbook.getUDFFinder();
          if(defaultToolkit != null && udfFinder != null) {
              defaultToolkit.add(udfFinder);
          }
          _udfFinder = defaultToolkit;
      }
  
      /**
       * also for debug use. Used in toString methods
       */
      /* package */ String getSheetName(int sheetIndex) {
          return _workbook.getSheetName(sheetIndex);
      }
  
      /* package */ EvaluationSheet getSheet(int sheetIndex) {
          return _workbook.getSheet(sheetIndex);
      }
      
      /* package */ EvaluationWorkbook getWorkbook() {
          return _workbook;
      }
  
      /* package */ EvaluationName getName(String name, int sheetIndex) {
          return _workbook.getName(name, sheetIndex);
      }
  
      private static boolean isDebugLogEnabled() {
          return LOG.check(POILogger.DEBUG);
      }
      private static boolean isInfoLogEnabled() {
          return LOG.check(POILogger.INFO);
      }
      private static void logDebug(String s) {
          if (isDebugLogEnabled()) {
              LOG.log(POILogger.DEBUG, s);
          }
      }
      private static void logInfo(String s) {
          if (isInfoLogEnabled()) {
              LOG.log(POILogger.INFO, s);
          }
      }
      /* package */ void attachToEnvironment(CollaboratingWorkbooksEnvironment collaboratingWorkbooksEnvironment, EvaluationCache cache, int workbookIx) {
          _collaboratingWorkbookEnvironment = collaboratingWorkbooksEnvironment;
          _cache = cache;
          _workbookIx = workbookIx;
      }
      /* package */ CollaboratingWorkbooksEnvironment getEnvironment() {
          return _collaboratingWorkbookEnvironment;
      }
  
      /**
       * Discards the current workbook environment and attaches to the default 'empty' environment.
       * Also resets evaluation cache.
       */
      /* package */ void detachFromEnvironment() {
          _collaboratingWorkbookEnvironment = CollaboratingWorkbooksEnvironment.EMPTY;
          _cache = new EvaluationCache(_evaluationListener);
          _workbookIx = 0;
      }
      /**
       * @return the evaluator for another workbook which is part of the same {@link CollaboratingWorkbooksEnvironment}
       */
      /* package */ WorkbookEvaluator getOtherWorkbookEvaluator(String workbookName) throws WorkbookNotFoundException {
          return _collaboratingWorkbookEnvironment.getWorkbookEvaluator(workbookName);
      }
  
      /* 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
       * of the evaluate~ methods of this class
       */
      public void clearAllCachedResultValues() {
          _cache.clear();
          _sheetIndexesBySheet.clear();
          _workbook.clearAllCachedResultValues();
      }
  
      /**
       * Should be called to tell the cell value cache that the specified (value or formula) cell
       * has changed.
       */
      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 been
       * deleted.
       */
      public void notifyDeleteCell(EvaluationCell cell) {
          int sheetIndex = getSheetIndex(cell.getSheet());
          _cache.notifyDeleteCell(_workbookIx, sheetIndex, cell);
      }
      
      private int getSheetIndex(EvaluationSheet sheet) {
          Integer result = _sheetIndexesBySheet.get(sheet);
          if (result == null) {
              int sheetIndex = _workbook.getSheetIndex(sheet);
              if (sheetIndex < 0) {
                  throw new RuntimeException("Specified sheet from a different book");
              }
              result = Integer.valueOf(sheetIndex);
              _sheetIndexesBySheet.put(sheet, result);
          }
          return result.intValue();
      }
  
      public ValueEval evaluate(EvaluationCell srcCell) {
          int sheetIndex = getSheetIndex(srcCell.getSheet());
          return evaluateAny(srcCell, sheetIndex, srcCell.getRowIndex(), srcCell.getColumnIndex(), new EvaluationTracker(_cache));
      }
  
      /**
       * Case-insensitive.
       * @return -1 if sheet with specified name does not exist
       */
      /* package */ int getSheetIndex(String sheetName) {
          Integer result = _sheetIndexesByName.get(sheetName);
          if (result == null) {
              int sheetIndex = _workbook.getSheetIndex(sheetName);
              if (sheetIndex < 0) {
                  return -1;
              }
              result = Integer.valueOf(sheetIndex);
              _sheetIndexesByName.put(sheetName, result);
          }
          return result.intValue();
      }
      
      /* package */ int getSheetIndexByExternIndex(int externSheetIndex) {
         return _workbook.convertFromExternSheetIndex(externSheetIndex);
      }
  
  
      /**
       * @return never <code>null</code>, never {@link BlankEval}
       */
      private ValueEval evaluateAny(EvaluationCell srcCell, int sheetIndex,
                  int rowIndex, int columnIndex, EvaluationTracker tracker) {
  
          // avoid tracking dependencies to cells that have constant definition
          boolean shouldCellDependencyBeRecorded = _stabilityClassifier == null ? true
                      : !_stabilityClassifier.isCellFinal(sheetIndex, rowIndex, columnIndex);
          if (srcCell == null || srcCell.getCellType() != CellType.FORMULA) {
              ValueEval result = getValueFromNonFormulaCell(srcCell);
              if (shouldCellDependencyBeRecorded) {
                  tracker.acceptPlainValueDependency(_workbook, _workbookIx, sheetIndex, rowIndex, columnIndex, result);
              }
              return result;
          }
  
          FormulaCellCacheEntry cce = _cache.getOrCreateFormulaCellEntry(srcCell);
          if (shouldCellDependencyBeRecorded || cce.isInputSensitive()) {
              tracker.acceptFormulaDependency(cce);
          }
          IEvaluationListener evalListener = _evaluationListener;
          ValueEval result;
          if (cce.getValue() == null) {
              if (!tracker.startEvaluate(cce)) {
                  return ErrorEval.CIRCULAR_REF_ERROR;
              }
  
              try {
  
                  Ptg[] ptgs = _workbook.getFormulaTokens(srcCell);
                  OperationEvaluationContext ec = new OperationEvaluationContext
                          (this, _workbook, sheetIndex, rowIndex, columnIndex, tracker);
                  if (evalListener == null) {
                      result = evaluateFormula(ec, ptgs);
                  } else {
                      evalListener.onStartEvaluate(srcCell, cce);
                      result = evaluateFormula(ec, ptgs);
                      evalListener.onEndEvaluate(cce, result);
                  }
  
                  tracker.updateCacheResult(result);
              }
               catch (NotImplementedException e) {
                  throw addExceptionInfo(e, sheetIndex, rowIndex, columnIndex);
               } catch (RuntimeException re) {
                   if (re.getCause() instanceof WorkbookNotFoundException && _ignoreMissingWorkbooks) {
                       logInfo(re.getCause().getMessage() + " - Continuing with cached value!");
                       switch(srcCell.getCachedFormulaResultType()) {
                           case NUMERIC:
                               result = new NumberEval(srcCell.getNumericCellValue());
                               break;
                           case STRING:
                               result =  new StringEval(srcCell.getStringCellValue());
                               break;
                           case BLANK:
                               result = BlankEval.instance;
                               break;
                           case BOOLEAN:
                               result =  BoolEval.valueOf(srcCell.getBooleanCellValue());
                               break;
                           case ERROR:
                              result =  ErrorEval.valueOf(srcCell.getErrorCellValue());
                              break;
                           case FORMULA:
                          default:
                              throw new RuntimeException("Unexpected cell type '" + srcCell.getCellType()+"' found!");
                       }
                   } else {
                       throw re;
                   }
               } finally {
                  tracker.endEvaluate(cce);
              }
          } else {
              if(evalListener != null) {
                  evalListener.onCacheHit(sheetIndex, rowIndex, columnIndex, cce.getValue());
              }
              return cce.getValue();
          }
          if (isDebugLogEnabled()) {
              String sheetName = getSheetName(sheetIndex);
              CellReference cr = new CellReference(rowIndex, columnIndex);
              logDebug("Evaluated " + sheetName + "!" + cr.formatAsString() + " to " + result);
          }
          // Usually (result === cce.getValue())
          // But sometimes: (result==ErrorEval.CIRCULAR_REF_ERROR, cce.getValue()==null)
          // When circular references are detected, the cache entry is only updated for
          // the top evaluation frame
          return result;
      }
  
      /**
       * Adds the current cell reference to the exception for easier debugging.
       * Would be nice to get the formula text as well, but that seems to require
       * too much digging around and casting to get the FormulaRenderingWorkbook.
       */
      private NotImplementedException addExceptionInfo(NotImplementedException inner, int sheetIndex, int rowIndex, int columnIndex) {
  
          try {
              String sheetName = _workbook.getSheetName(sheetIndex);
              CellReference cr = new CellReference(sheetName, rowIndex, columnIndex, false, false);
              String msg =  "Error evaluating cell " + cr.formatAsString();
              return new NotImplementedException(msg, inner);
          } catch (Exception e) {
              // avoid bombing out during exception handling
              LOG.log(POILogger.ERROR, "Can't add exception info", e);
              return inner; // preserve original exception
          }
      }
      /**
       * 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, never <code>null</code>
       */
      /* package */ static ValueEval getValueFromNonFormulaCell(EvaluationCell cell) {
          if (cell == null) {
              return BlankEval.instance;
          }
          CellType cellType = cell.getCellType();
          switch (cellType) {
              case NUMERIC:
                  return new NumberEval(cell.getNumericCellValue());
              case STRING:
                  return new StringEval(cell.getStringCellValue());
              case BOOLEAN:
                  return BoolEval.valueOf(cell.getBooleanCellValue());
              case BLANK:
                  return BlankEval.instance;
              case ERROR:
                  return ErrorEval.valueOf(cell.getErrorCellValue());
              default:
                  throw new RuntimeException("Unexpected cell type (" + cellType + ")");
          }
          
      }
  
  
      // visibility raised for testing
      @Internal
      /* package */ ValueEval evaluateFormula(OperationEvaluationContext ec, Ptg[] ptgs) {
  
          String dbgIndentStr = "";        // always init. to non-null just for defensive avoiding NPE
          if (dbgEvaluationOutputForNextEval) {
              // first evaluation call when ouput is desired, so iit. this evaluator instance
              dbgEvaluationOutputIndent = 1;
              dbgEvaluationOutputForNextEval = false;
          }
          if (dbgEvaluationOutputIndent > 0) {
              // init. indent string to needed spaces (create as substring vom very long space-only string;
              // limit indendation for deep recursions)
              dbgIndentStr = "                                                                                                    ";
              dbgIndentStr = dbgIndentStr.substring(0, Math.min(dbgIndentStr.length(), dbgEvaluationOutputIndent*2));
              EVAL_LOG.log(POILogger.WARN, dbgIndentStr
                                 + "- evaluateFormula('" + ec.getRefEvaluatorForCurrentSheet().getSheetNameRange()
                                 + "'/" + new CellReference(ec.getRowIndex(), ec.getColumnIndex()).formatAsString()
                                 + "): " + Arrays.toString(ptgs).replaceAll("\\Qorg.apache.poi.ss.formula.ptg.\\E", ""));
              dbgEvaluationOutputIndent++;
          }
  
          EvaluationSheet evalSheet = ec.getWorkbook().getSheet(ec.getSheetIndex());
          EvaluationCell evalCell = evalSheet.getCell(ec.getRowIndex(), ec.getColumnIndex());
  
          Stack<ValueEval> stack = new Stack<>();
          for (int i = 0, iSize = ptgs.length; i < iSize; i++) {
              // since we don't know how to handle these yet :(
              Ptg ptg = ptgs[i];
              if (dbgEvaluationOutputIndent > 0) {
                  EVAL_LOG.log(POILogger.INFO, dbgIndentStr, "  * ptg ", i, ": ", ptg, ", stack: ", stack);
              }
              if (ptg instanceof AttrPtg) {
                  AttrPtg attrPtg = (AttrPtg) ptg;
                  if (attrPtg.isSum()) {
                      // Excel prefers to encode 'SUM()' as a tAttr token, but this evaluator
                      // expects the equivalent function token
                      ptg = FuncVarPtg.SUM;
                  }
                  if (attrPtg.isOptimizedChoose()) {
                      ValueEval arg0 = stack.pop();
                      int[] jumpTable = attrPtg.getJumpTable();
                      int dist;
                      int nChoices = jumpTable.length;
                      try {
                          int switchIndex = Choose.evaluateFirstArg(arg0, ec.getRowIndex(), ec.getColumnIndex());
                          if (switchIndex<1 || switchIndex > nChoices) {
                              stack.push(ErrorEval.VALUE_INVALID);
                              dist = attrPtg.getChooseFuncOffset() + 4; // +4 for tFuncFar(CHOOSE)
                          } else {
                              dist = jumpTable[switchIndex-1];
                          }
                      } catch (EvaluationException e) {
                          stack.push(e.getErrorEval());
                          dist = attrPtg.getChooseFuncOffset() + 4; // +4 for tFuncFar(CHOOSE)
                      }
                      // Encoded dist for tAttrChoose includes size of jump table, but
                      // countTokensToBeSkipped() does not (it counts whole tokens).
                      dist -= nChoices*2+2; // subtract jump table size
                      i+= countTokensToBeSkipped(ptgs, i, dist);
                      continue;
                  }
                  if (attrPtg.isOptimizedIf()) {
                      if(!evalCell.isPartOfArrayFormulaGroup()) {
                          ValueEval arg0 = stack.pop();
                          boolean evaluatedPredicate;
  
                          try {
                              evaluatedPredicate = IfFunc.evaluateFirstArg(arg0, ec.getRowIndex(), ec.getColumnIndex());
                          } catch (EvaluationException e) {
                              stack.push(e.getErrorEval());
                              int dist = attrPtg.getData();
                              i += countTokensToBeSkipped(ptgs, i, dist);
                              attrPtg = (AttrPtg) ptgs[i];
                              dist = attrPtg.getData() + 1;
                              i += countTokensToBeSkipped(ptgs, i, dist);
                              continue;
                          }
                          if (evaluatedPredicate) {
                              // nothing to skip - true param follows
                          } else {
                              int dist = attrPtg.getData();
                              i += countTokensToBeSkipped(ptgs, i, dist);
                              Ptg nextPtg = ptgs[i + 1];
                              if (ptgs[i] instanceof AttrPtg && nextPtg instanceof FuncVarPtg &&
                                      // in order to verify that there is no third param, we need to check
                                      // if we really have the IF next or some other FuncVarPtg as third param, e.g. ROW()/COLUMN()!
                                      ((FuncVarPtg) nextPtg).getFunctionIndex() == FunctionMetadataRegistry.FUNCTION_INDEX_IF) {
                                  // this is an if statement without a false param (as opposed to MissingArgPtg as the false param)
                                  //i++;
                                  stack.push(arg0);
                                  stack.push(BoolEval.FALSE);
                              }
                          }
                      }
                      continue;
                  }
                  if (attrPtg.isSkip() && !evalCell.isPartOfArrayFormulaGroup()) {
                      int dist = attrPtg.getData()+1;
                      i+= countTokensToBeSkipped(ptgs, i, dist);
                      if (stack.peek() == MissingArgEval.instance) {
                          stack.pop();
                          stack.push(BlankEval.instance);
                      }
                      continue;
                  }
              }
              if (ptg instanceof ControlPtg) {
                  // skip Parentheses, Attr, etc
                  continue;
              }
              if (ptg instanceof MemFuncPtg || ptg instanceof MemAreaPtg) {
                  // can ignore, rest of tokens for this expression are in OK RPN order
                  continue;
              }
              if (ptg instanceof MemErrPtg) {
                  continue;
              }
  
              if (ptg instanceof UnionPtg) {
                  ValueEval v2 = stack.pop();
                  ValueEval v1 = stack.pop();
                  stack.push(new RefListEval(v1, v2));
                  continue;
              }
  
              ValueEval opResult;
              if (ptg instanceof OperationPtg) {
                  OperationPtg optg = (OperationPtg) ptg;
  
                  int numops = optg.getNumberOfOperands();
                  ValueEval[] ops = new ValueEval[numops];
  
                  // storing the ops in reverse order since they are popping
                  boolean areaArg = false; // whether one of the operands is an area
                  for (int j = numops - 1; j >= 0; j--) {
                      ValueEval p = stack.pop();
                      ops[j] = p;
                      if(p instanceof AreaEval){
                          areaArg = true;
                      }
                  }
  
                  boolean arrayMode = false;
                  if(areaArg) for (int ii = i; ii < iSize; ii++) {
                      if(ptgs[ii] instanceof FuncVarPtg){
                          FuncVarPtg f = (FuncVarPtg)ptgs[ii];
                          try {
                              Function func = FunctionEval.getBasicFunction(f.getFunctionIndex());
                              if (func != null && func instanceof ArrayMode) {
                                  arrayMode = true;
                              }
                          } catch (NotImplementedException ne){
                              //FunctionEval.getBasicFunction can throw NotImplementedException
                              // if the fucntion is not yet supported.
                          }
                          break;
                      }
                  }
                  ec.setArrayMode(arrayMode);
  
  //                logDebug("invoke " + operation + " (nAgs=" + numops + ")");
                  opResult = OperationEvaluatorFactory.evaluate(optg, ops, ec);
  
                  ec.setArrayMode(false);
  
              } else {
                  opResult = getEvalForPtg(ptg, ec);
              }
              if (opResult == null) {
                  throw new RuntimeException("Evaluation result must not be null");
              }
  //            logDebug("push " + opResult);
              stack.push(opResult);
              if (dbgEvaluationOutputIndent > 0) {
                  EVAL_LOG.log(POILogger.INFO, dbgIndentStr, "    = ", opResult);
              }
          }
  
          ValueEval value = stack.pop();
          if (!stack.isEmpty()) {
              throw new IllegalStateException("evaluation stack not empty");
          }
          
          ValueEval result;
          
          if (ec.isSingleValue()) {
              result = dereferenceResult(value, ec);
          }
          else {
              result = value;
          }
  
          if (dbgEvaluationOutputIndent > 0) {
              EVAL_LOG.log(POILogger.INFO, dbgIndentStr, "finished eval of ",
                              new CellReference(ec.getRowIndex(), ec.getColumnIndex()).formatAsString(),
                              ": ", result);
              dbgEvaluationOutputIndent--;
              if (dbgEvaluationOutputIndent == 1) {
                  // this evaluation is done, reset indent to stop logging
                  dbgEvaluationOutputIndent = -1;
              }
          } // if
          return result;
  
      }
  
      /**
       * Calculates the number of tokens that the evaluator should skip upon reaching a tAttrSkip.
       *
       * @return the number of tokens (starting from <tt>startIndex+1</tt>) that need to be skipped
       * to achieve the specified <tt>distInBytes</tt> skip distance.
       */
      private static int countTokensToBeSkipped(Ptg[] ptgs, int startIndex, int distInBytes) {
          int remBytes = distInBytes;
          int index = startIndex;
          while (remBytes != 0) {
              index++;
              remBytes -= ptgs[index].getSize();
              if (remBytes < 0) {
                  throw new RuntimeException("Bad skip distance (wrong token size calculation).");
              }
              if (index >= ptgs.length) {
                  throw new RuntimeException("Skip distance too far (ran out of formula tokens).");
              }
          }
          return index-startIndex;
      }
      
      /**
       * Dereferences a single value from any AreaEval or RefEval evaluation
       * result. If the supplied evaluationResult is just a plain value, it is
       * returned as-is.
       *
       * @return a {@link NumberEval}, {@link StringEval}, {@link BoolEval}, or
       *         {@link ErrorEval}. Never <code>null</code>. {@link BlankEval} is
       *         converted to {@link NumberEval#ZERO}
       */
      private static ValueEval dereferenceResult(ValueEval evaluationResult, OperationEvaluationContext ec) {
          ValueEval value;
  
          if (ec == null) {
              throw new IllegalArgumentException("OperationEvaluationContext ec is null");
          }
          if (ec.getWorkbook() == null) {
              throw new IllegalArgumentException("OperationEvaluationContext ec.getWorkbook() is null");
          }
  
          EvaluationSheet evalSheet = ec.getWorkbook().getSheet(ec.getSheetIndex());
          EvaluationCell evalCell = evalSheet.getCell(ec.getRowIndex(), ec.getColumnIndex());
   
          if (evalCell != null && evalCell.isPartOfArrayFormulaGroup() && evaluationResult instanceof AreaEval) {
              value = OperandResolver.getElementFromArray((AreaEval) evaluationResult, evalCell);
          }
          else {
              value = dereferenceResult(evaluationResult, ec.getRowIndex(), ec.getColumnIndex());
          }
          if (value == BlankEval.instance) {
              // Note Excel behaviour here. A blank final final value is converted to zero.
              return NumberEval.ZERO;
              // Formulas _never_ evaluate to blank.  If a formula appears to have evaluated to
              // blank, the actual value is empty string. This can be verified with ISBLANK().
          }
          
          return value;
      }
  
      /**
       * Dereferences a single value from any AreaEval or RefEval evaluation
       * result. If the supplied evaluationResult is just a plain value, it is
       * returned as-is.
       *
       * @return a {@link NumberEval}, {@link StringEval}, {@link BoolEval}, or
       *         {@link ErrorEval}. Never <code>null</code>. {@link BlankEval} is
       *         converted to {@link NumberEval#ZERO}
       */
      public static ValueEval dereferenceResult(ValueEval evaluationResult, int srcRowNum, int srcColNum) {
          ValueEval value;
          try {
              value = OperandResolver.getSingleValue(evaluationResult, srcRowNum, srcColNum);
          } catch (EvaluationException e) {
              return e.getErrorEval();
          }
          if (value == BlankEval.instance) {
              // Note Excel behaviour here. A blank final final value is converted to zero.
              return NumberEval.ZERO;
              // Formulas _never_ evaluate to blank.  If a formula appears to have evaluated to
              // blank, the actual value is empty string. This can be verified with ISBLANK().
          }
          return value;
      }
  
  
      /**
       * returns an appropriate Eval impl instance for the Ptg. The Ptg must be
       * one of: Area3DPtg, AreaPtg, ReferencePtg, Ref3DPtg, IntPtg, NumberPtg,
       * StringPtg, BoolPtg <br/>special Note: OperationPtg subtypes cannot be
       * passed here!
       */
      private ValueEval getEvalForPtg(Ptg ptg, OperationEvaluationContext ec) {
          //  consider converting all these (ptg instanceof XxxPtg) expressions to (ptg.getClass() == XxxPtg.class)
  
          if (ptg instanceof NamePtg) {
              // Named ranges, macro functions
              NamePtg namePtg = (NamePtg) ptg;
              EvaluationName nameRecord = _workbook.getName(namePtg);
              return getEvalForNameRecord(nameRecord, ec);
          }
          if (ptg instanceof NameXPtg) {
              // Externally defined named ranges or macro functions
  