Use cached value for __xludf.DUMMYFUNCTION when recalculating cell

poi/src/main/java/org/apache/poi/ss/formula/atp/AnalysisToolPak.java

@@ -68,6 +68,10 @@ public FreeRefFunction findFunction(String name) {
         // case-sensitive
         if(name.startsWith(prefix)) name = name.substring(prefix.length());
 
+        // Google Sheets wraps non-Excel functions with the __xludf. prefix
+        final String xludfPrefix = "__xludf.";
+        if(name.startsWith(xludfPrefix)) name = name.substring(xludfPrefix.length());
+
         // FIXME: inconsistent case-sensitivity
         return _functionsByName.get(name.toUpperCase(Locale.ROOT));
     }
@@ -118,6 +122,7 @@ private Map<String, FreeRefFunction> createFunctionsMap() {
         r(m, "DEC2OCT", null);
         r(m, "DELTA", Delta.instance);
         r(m, "DISC", null);
+        r(m, "DUMMYFUNCTION", DummyFunction.instance);
         r(m, "DOLLARDE", DollarDe.instance);
         r(m, "DOLLARFR", DollarFr.instance);
         r(m, "DURATION", null);

poi/src/main/java/org/apache/poi/ss/formula/functions/DummyFunction.java

@@ -0,0 +1,39 @@
+/* ====================================================================
+   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.functions;
+
+import org.apache.poi.ss.formula.OperationEvaluationContext;
+import org.apache.poi.ss.formula.eval.ErrorEval;
+import org.apache.poi.ss.formula.eval.ValueEval;
+
+/**
+ * Implementation for the IFERROR __xludf.DUMMYFUNCTION
+ * <p>
+ * https://support.microsoft.com/en-au/office/iferror-function-c526fd07-caeb-47b8-8bb6-63f3e417f611
+ */
+public class DummyFunction implements FreeRefFunction {
+    public static final DummyFunction instance = new DummyFunction();
+
+    @Override
+    public ValueEval evaluate(ValueEval[] args, OperationEvaluationContext ec) {
+        if (args.length < 2) {
+            return ErrorEval.VALUE_INVALID;
+        }
+        return args[1];
+    }
+}

poi/src/test/java/org/apache/poi/ss/formula/functions/TestDummyFunction.java

@@ -0,0 +1,75 @@
+/* ====================================================================
+   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.functions;
+
+import org.apache.poi.ss.formula.OperationEvaluationContext;
+import org.apache.poi.ss.formula.eval.ErrorEval;
+import org.apache.poi.ss.formula.eval.NumberEval;
+import org.apache.poi.ss.formula.eval.StringEval;
+import org.apache.poi.ss.formula.eval.ValueEval;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * Tests for {@link DummyFunction}
+ */
+final class TestDummyFunction {
+
+    private static final OperationEvaluationContext ec = new OperationEvaluationContext(null, null, 0, 0, 0, null);
+
+    @Test
+    void testReturnsCachedNumericValue() {
+        ValueEval[] args = {
+            new StringEval("QUERY(A1:D10,\"SELECT A\",1)"),
+            new NumberEval(42.0)
+        };
+        ValueEval result = DummyFunction.instance.evaluate(args, ec);
+        assertEquals(NumberEval.class, result.getClass());
+        assertEquals(42.0, ((NumberEval) result).getNumberValue(), 0.0);
+    }
+
+    @Test
+    void testReturnsCachedStringValue() {
+        ValueEval[] args = {
+            new StringEval("GOOGLEFINANCE(\"GOOG\",\"price\")"),
+            new StringEval("hello")
+        };
+        ValueEval result = DummyFunction.instance.evaluate(args, ec);
+        assertEquals(StringEval.class, result.getClass());
+        assertEquals("hello", ((StringEval) result).getStringValue());
+    }
+
+    @Test
+    void testNoArgsReturnsValueInvalid() {
+        ValueEval[] args = {};
+        ValueEval result = DummyFunction.instance.evaluate(args, ec);
+        assertEquals(ErrorEval.class, result.getClass());
+        assertEquals(ErrorEval.VALUE_INVALID, result);
+    }
+
+    @Test
+    void testSingleArgReturnsValueInvalid() {
+        ValueEval[] args = {
+            new StringEval("IMPORTRANGE(\"url\",\"Sheet1!A1\")")
+        };
+        ValueEval result = DummyFunction.instance.evaluate(args, ec);
+        assertEquals(ErrorEval.class, result.getClass());
+        assertEquals(ErrorEval.VALUE_INVALID, result);
+    }
+}