StringUtils.abbreviate(String, String, int) contract violations (#1572)

* add unit tests showing mishandling of null abbrevMarker

* fix handling of null abbrevMarker

- treat null marker as empty string
- ensure offset and maxWidth are applied as usual (simple 'substring' won't cut it)

* add unit tests showing 'abbreviate' contract violations

- Abbreviated strings should always retain the 'offset' character

* remove surplus blank lines from tests

* fix StringUtils.abbreviate for short strings

Adjust logic so it doesn't overwrite the offset value and get confused about which character to preserve. 
Instead, just return immediately if the offset is so close to the end that no ending abbrevMarker is possible.

Author: ThrawnCA

src/main/java/org/apache/commons/lang3/StringUtils.java

@@ -369,22 +369,16 @@ public static String abbreviate(final String str, String abbrevMarker, int offse
         if (strLen <= maxWidth) {
             return str;
         }
-        if (offset > strLen) {
-            offset = strLen;
-        }
-        if (strLen - offset < maxWidth - abbrevMarkerLength) {
-            offset = strLen - (maxWidth - abbrevMarkerLength);
+        if (strLen - offset <= maxWidth - abbrevMarkerLength) {
+            return abbrevMarker + str.substring(strLen - (maxWidth - abbrevMarkerLength));
         }
         if (offset <= abbrevMarkerLength + 1) {
             return str.substring(0, maxWidth - abbrevMarkerLength) + abbrevMarker;
         }
         if (maxWidth < minAbbrevWidthOffset) {
             throw new IllegalArgumentException(String.format("Minimum abbreviation width with offset is %d", minAbbrevWidthOffset));
         }
-        if (offset + maxWidth - abbrevMarkerLength < strLen) {
-            return abbrevMarker + abbreviate(str.substring(offset), abbrevMarker, maxWidth - abbrevMarkerLength);
-        }
-        return abbrevMarker + str.substring(strLen - (maxWidth - abbrevMarkerLength));
+        return abbrevMarker + abbreviate(str.substring(offset), abbrevMarker, maxWidth - abbrevMarkerLength);
     }
 
     /**

src/test/java/org/apache/commons/lang3/StringUtilsAbbreviateTest.java

@@ -101,6 +101,8 @@ void testAbbreviate_StringIntInt() {
         assertAbbreviateWithOffset("...ijklmno", 15, 10);
         assertAbbreviateWithOffset("...ijklmno", 16, 10);
         assertAbbreviateWithOffset("...ijklmno", Integer.MAX_VALUE, 10);
+        // abbreviating a shorter string allows maxWidth < 7
+        assertEquals("...efg", StringUtils.abbreviate("abcdefg", 5, 6));
     }
 
     @Test
@@ -163,6 +165,9 @@ void testAbbreviate_StringStringIntInt() {
         assertAbbreviateWithAbbrevMarkerAndOffset("999ijklmno", "999", 15, 10);
         assertAbbreviateWithAbbrevMarkerAndOffset("_ghijklmno", "_", 16, 10);
         assertAbbreviateWithAbbrevMarkerAndOffset("+ghijklmno", "+", Integer.MAX_VALUE, 10);
+        // abbreviating a shorter string allows maxWidth < abbrevMarker.length * 2 + 1
+        assertEquals("..de", StringUtils.abbreviate("abcde", "..", 4, 4));
+        assertEquals("....fg", StringUtils.abbreviate("abcdefg", "....", 5, 6));
     }
 
     // Fixed LANG-1463