Adding unit test for #39410. Implementing fix by switching from using Math.log to Integer.toString to figure out the number of digits.

git-svn-id: https://svn.apache.org/repos/asf/jakarta/commons/proper/lang/trunk@398530 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/commons/lang/time/FastDateFormat.java

@@ -31,6 +31,8 @@
 import java.util.Map;
 import java.util.TimeZone;
 
+import org.apache.commons.lang.Validate;
+
 /**
  * <p>FastDateFormat is a fast and thread-safe version of
  * {@link java.text.SimpleDateFormat}.</p>
@@ -93,9 +95,6 @@ public class FastDateFormat extends Format {
      */
     public static final int SHORT = DateFormat.SHORT;
     
-    // package scoped as used by inner class
-    static final double LOG_10 = Math.log(10);
-
     private static String cDefaultPattern;
 
     private static Map cInstanceCache = new HashMap(7);
@@ -1283,7 +1282,8 @@ public final void appendTo(StringBuffer buffer, int value) {
                 if (value < 1000) {
                     digits = 3;
                 } else {
-                    digits = (int)(Math.log(value) / LOG_10) + 1;
+                    Validate.isTrue(value > -1, "Negative values should not be possible", value);
+                    digits = Integer.toString(value).length();
                 }
                 for (int i = mSize; --i >= digits; ) {
                     buffer.append('0');

src/test/org/apache/commons/lang/time/FastDateFormatTest.java

@@ -250,6 +250,17 @@ public void testLowYearPadding() {
         cal.set(999,0,1);
         assertEquals("0999/01/01", format.format(cal));
     }
+    /**
+     * Show Bug #39410 is solved
+     */
+    public void testMilleniumBug() {
+        Calendar cal = Calendar.getInstance();
+        FastDateFormat format = FastDateFormat.getInstance("dd.MM.yyyy");
+
+        cal.set(1000,0,1);
+        assertEquals("01.01.1000", format.format(cal));
+    }
+
     /**
      * testLowYearPadding showed that the date was buggy
      * This test confirms it, getting 366 back as a date