[TEXT-241] TextStringBuilder.lastIndexOf("") and StrBuilder.lastIndexOf("") return incorrect index for empty string (size - 1 instead of size)

src/main/java/org/apache/commons/text/StrBuilder.java

@@ -2275,7 +2275,7 @@ public int lastIndexOf(final char ch, int startIndex) {
      * @return The last index of the string, or -1 if not found.
      */
     public int lastIndexOf(final String str) {
-        return lastIndexOf(str, size - 1);
+        return lastIndexOf(str, size);
     }
 
     /**
@@ -2289,14 +2289,17 @@ public int lastIndexOf(final String str) {
      * @return The last index of the string, or -1 if not found.
      */
     public int lastIndexOf(final String str, int startIndex) {
-        startIndex = startIndex >= size ? size - 1 : startIndex;
+        startIndex = Math.min(startIndex, size);
         if (str == null || startIndex < 0) {
             return StringUtils.INDEX_NOT_FOUND;
         }
         final int strLen = str.length();
         if (strLen == 0) {
             return startIndex;
         }
+        if (startIndex >= size) {
+            startIndex = size - 1;
+        }
         if (strLen > size) {
             return StringUtils.INDEX_NOT_FOUND;
         }

src/main/java/org/apache/commons/text/TextStringBuilder.java

@@ -2388,7 +2388,7 @@ public int lastIndexOf(final char ch, int startIndex) {
      * @return The last index of the string, or -1 if not found.
      */
     public int lastIndexOf(final String str) {
-        return lastIndexOf(str, size - 1);
+        return lastIndexOf(str, size);
     }
 
     /**
@@ -2403,14 +2403,17 @@ public int lastIndexOf(final String str) {
      * @return The last index of the string, or -1 if not found.
      */
     public int lastIndexOf(final String str, int startIndex) {
-        startIndex = startIndex >= size ? size - 1 : startIndex;
+        startIndex = Math.min(startIndex, size);
         if (str == null || startIndex < 0) {
             return StringUtils.INDEX_NOT_FOUND;
         }
         final int strLen = str.length();
         if (strLen == 0) {
             return startIndex;
         }
+        if (startIndex >= size) {
+            startIndex = size - 1;
+        }
         if (strLen > size) {
             return StringUtils.INDEX_NOT_FOUND;
         }

src/test/java/org/apache/commons/text/StrBuilderTest.java

@@ -45,6 +45,7 @@
  *
  * @deprecated This class will be removed in 2.0.
  */
+@Deprecated
 class StrBuilderTest {
 
     private static final class MockReadable implements Readable {
@@ -1219,6 +1220,9 @@ void testLastIndexOf_String() {
         assertEquals(-1, sb.lastIndexOf("z"));
 
         assertEquals(-1, sb.lastIndexOf((String) null));
+
+        assertEquals(4, sb.lastIndexOf(""));
+        assertEquals("".lastIndexOf(""), new StringBuilder().lastIndexOf(""));
     }
 
     @Test

src/test/java/org/apache/commons/text/TextStringBuilderTest.java

@@ -1394,6 +1394,9 @@ void testLastIndexOf_String() {
         assertEquals(-1, sb.lastIndexOf("z"));
 
         assertEquals(-1, sb.lastIndexOf((String) null));
+
+        assertEquals(4, sb.lastIndexOf(""));
+        assertEquals("".lastIndexOf(""), new StringBuilder().lastIndexOf(""));
     }
 
     @Test