Fix Strings.CI.indexOf returning out-of-range index for empty search (#1737)

* fix Strings.CI.indexOf returning out-of-range index for empty search

* Refactor test, add assertions.

---------

Co-authored-by: Gary Gregory <garydgregory@users.noreply.github.com>

Author: alhudz

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

@@ -152,7 +152,7 @@ public int indexOf(final CharSequence str, final CharSequence searchStr, int sta
                 startPos = 0;
             }
             final int endLimit = str.length() - searchStr.length() + 1;
-            if (startPos > endLimit) {
+            if (startPos >= endLimit) {
                 return INDEX_NOT_FOUND;
             }
             if (searchStr.length() == 0) {

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

@@ -59,6 +59,26 @@ void testCaseInsensitiveConstant() {
         assertFalse(Strings.CI.isCaseSensitive());
     }
 
+    /**
+     * For an empty search the case-insensitive {@code indexOf} returned {@code startPos} unchanged once it reached
+     * {@code str.length() + 1}, so a start position one past the end yielded an index beyond the string instead of
+     * {@code -1}.
+     */
+    @Test
+    void testCaseInsensitiveIndexOfEmptyOutOfRange() {
+        // repro: returned 4 (past the end of a length-3 string) before the fix
+        final String emptySearch = StringUtils.EMPTY;
+        assertEquals(-1, Strings.CI.indexOf("abc", emptySearch, 4));
+        // documented out-of-range example, also -1
+        assertEquals(-1, Strings.CI.indexOf("abc", emptySearch, 9));
+        // the end position is still a valid empty match
+        assertEquals(3, Strings.CI.indexOf("abc", emptySearch, 3));
+        assertEquals(2, Strings.CI.indexOf("aabaabaa", emptySearch, 2));
+        assertEquals(0, Strings.CI.indexOf(emptySearch, emptySearch, 0));
+        assertEquals(0, Strings.CI.indexOf(emptySearch, emptySearch, -1));
+        assertEquals(0, Strings.CI.indexOf("a", emptySearch, -1));
+    }
+
     /**
      * {@code U+0130} lower-cases to the two-char sequence {@code "i̇"} outside Turkish locales, so pre-lower-casing the
      * search argument made the case-insensitive replace look for a two-char needle that no longer matches the single source