alhudz opened a new pull request, #1759:
URL: https://github.com/apache/commons-lang/pull/1759
1. Cause: `lastIndexOf(String)` delegates to `lastIndexOf(String, int)` with
`size - 1`, but the last valid start position for an empty search string is
`size`.
2. Change: pass `size`. Non-empty searches are unaffected, because their
last possible match start is `size - str.length()`, which is at most `size - 1`.
Repro:
```java
new StrBuilder("abc").lastIndexOf("") // expected 3, actual 2
new StrBuilder("").lastIndexOf("") // expected 0, actual -1
new StrBuilder("abc").lastIndexOf("", 3) // 3, the two-arg overload is
already correct
new StrBuilder("abc").indexOf("") // 0
new StrBuilder("abc").contains("") // true
```
`String.lastIndexOf`, `StringBuilder.lastIndexOf` and
`StringUtils.lastIndexOf` all return the length for an empty search, so
`StrBuilder` disagreed with both the JDK and the rest of Lang, and with its own
`indexOf` and `contains`. Found by differential fuzzing of `StrBuilder` against
`StringBuilder`; over 400k random cases this was the only method that diverged.
The new assertions in `StrBuilderTest.testLastIndexOf_String` fail without the
runtime change (`expected: <4> but was: <3>`).
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`TextStringBuilder` in Commons Text (checked against `master`) has the same
`size - 1` in `lastIndexOf(String)`, and its hand-rolled two-arg overload
clamps `startIndex` to `size - 1` before the empty-search early return, so both
overloads diverge there. Glad to raise a matching PR if that is useful.
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