LOG10 function is not a cell func

Parser accepts cell references as functions, e.g. $A$1(45). When formula
is converted between A1 and R1C1, make sure the LOG10 is *not*
interpreted as a cell function. That would  cause all sorts of problems
with shifting or deleting areas.

Example: LOG10 would be turned to R1C1, the reference would shift
and then be turned back.

Author: jahav

src/ClosedXML.Parser.Tests/RefModVisitorTests.cs

@@ -71,6 +71,13 @@ public void Bang_references_is_modified(string formula, string reference, string
         AssertChangesA1(formula, factory, modifiedFormula);
     }
 
+    [Fact]
+    public void Log10_is_not_interpreted_as_cell_function()
+    {
+        var factory = new ShiftReferenceVisitor { ReferenceMap = { { "LOG10", "A1"} } };
+        AssertChangesA1("LOG10(LOG10)", factory, "LOG10(A1)");
+    }
+
     private static void AssertChangesA1(string formula, RefModVisitor visitor, string expected)
     {
         var ctx = new ModContext(formula, "Sheet", 1, 1, isA1: true);
@@ -99,5 +106,13 @@ private class ShiftReferenceVisitor : RefModVisitor
 
             return reference;
         }
+
+        internal override RowCol? ModifyCellFunction(ModContext ctx, RowCol cell)
+        {
+            if (ReferenceMap.TryGetValue(cell.GetDisplayStringA1(), out var replacement))
+                return replacement is not null ? ReferenceParser.ParseA1(replacement).First : null;
+
+            return cell;
+        }
     }
 }

src/ClosedXML.Parser/RefModVisitor.cs

@@ -193,6 +193,15 @@ public TransformedSymbol ExternalFunction(ModContext ctx, SymbolRange range, int
     /// <inheritdoc />
     public TransformedSymbol CellFunction(ModContext ctx, SymbolRange range, RowCol cell, IReadOnlyList<TransformedSymbol> arguments)
     {
+        // Parser doesn't detect LOG10 as a function (there is no list of functions),
+        // but as a cell function. Make sure not to transform it.
+        var functionName = ctx.Formula.AsSpan(range.Start, ctx.Formula.IndexOf('(', range.Start) - range.Start);
+        if (functionName.Equals("LOG10".AsSpan(), StringComparison.OrdinalIgnoreCase))
+        {
+            var modifiedName = ModifyFunction(ctx, functionName);
+            return s_copyVisitor.Function(ctx, range, modifiedName, arguments);
+        }
+
         var modifiedCell = ModifyCellFunction(ctx, cell);
         if (modifiedCell is null)
             return TransformedSymbol.ToText(ctx.Formula, range, REF_ERROR);