mbeckerle commented on code in PR #1106:
URL: https://github.com/apache/daffodil/pull/1106#discussion_r1376685125


##########
daffodil-lib/src/test/scala/org/apache/daffodil/lib/util/TestFixedICUDecimalFormat.java:
##########
@@ -0,0 +1,159 @@
+/*
+ * 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.daffodil.lib.util;
+
+import com.ibm.icu.text.DecimalFormat;
+import com.ibm.icu.text.DecimalFormatSymbols;
+import org.junit.Test;
+import static junit.framework.TestCase.assertEquals;
+import static org.junit.Assert.*;
+
+import java.util.Locale;
+import java.util.Objects;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+/**
+ * Tests about ICU-22558
+ *
+ * https://unicode-org.atlassian.net/browse/ICU-22558
+ */
+public class TestFixedICUDecimalFormat {
+
+    // Create DecimalFormatSymbols for a Locale where the decimal separator is 
a dot
+    DecimalFormatSymbols symbols = new DecimalFormatSymbols(Locale.US);
+
+    /**
+     * Illustrates the bug in ICU.
+     */
+    @Test
+    public void testDecimalFormatWithHash() {
+        // Create DecimalFormat instance with the pattern "#.##"
+        DecimalFormat decimalFormat = new DecimalFormat("#.##", symbols);

Review Comment:
   I wrote the nested loop:
   ```
       @Test
       public void icuTesting() {
           List<String> stringList = Arrays.asList(".", "#.", ".#", "#.#");
           List<Double> numberList = Arrays.asList(0.0, 1.0, 0.1, 1.1);
           for (String str : stringList) {
               for (Double num : numberList) {
                   DecimalFormat df = new DecimalFormat(str);
                   String output = df.format(num);
                   System.out.println(String.format("For value %f and pattern 
'%s' output is '%s' (length %d)", num, str, output, output.length()));
               }
           }
       }
   ```
   
   Output is, plus my comments after the //. 
   ```
   For value 0.000000 and pattern '.' output is '0.' (length 2)  // so leaving 
off the # entirely means the fraction digit is optional, but not the decimal 
point. 
   For value 1.000000 and pattern '.' output is '1.' (length 2)
   For value 0.100000 and pattern '.' output is '0.' (length 2)
   For value 1.100000 and pattern '.' output is '1.' (length 2)   // So "." 
behaves exactly like "#."
   
   For value 0.000000 and pattern '#.' output is '0.' (length 2) // leaving off 
the # after "." =>  insisting on one in the output???
   For value 1.000000 and pattern '#.' output is '1.' (length 2)  
   For value 0.100000 and pattern '#.' output is '0.' (length 2)
   For value 1.100000 and pattern '#.' output is '1.' (length 2)
   
   For value 0.000000 and pattern '.#' output is '.0' (length 2) // notice not 
a leading integer digit here
   For value 1.000000 and pattern '.#' output is '1.0' (length 3)
   For value 0.100000 and pattern '.#' output is '.1' (length 2)
   For value 1.100000 and pattern '.#' output is '1.1' (length 3)
   
   For value 0.000000 and pattern '#.#' output is '0' (length 1) // suppresses 
trailing "."
   For value 1.000000 and pattern '#.#' output is '1' (length 1) // suppresses 
trailing "."
   For value 0.100000 and pattern '#.#' output is '0.1' (length 3)
   For value 1.100000 and pattern '#.#' output is '1.1' (length 3)
   ```
   I can come up with no rationale for this other than they have to preserve 
backward compatibility so this clearly ad-hoc behavior must stay. 
   



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