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

[kzhunmax]

StrBuilder.lastIndexOf(String) and extStringBuilder.lastIndexOf(String)
do not match java.lang.String / StringBuilder for the empty string.

For a builder of length n, lastIndexOf("") should return n (empty match
at the end). Both implementations currently return n - 1.

Cause:
lastIndexOf(String, int) clamps startIndex with:
startIndex = startIndex >= size ? size - 1 : startIndex
before the empty-string case, which returns that clamped index. So
lastIndexOf("") can never return size.

Example:

new StrBuilder("abab").lastIndexOf("") // actual 3, expected 4
"abab".lastIndexOf("") // 4
new StringBuilder("abab").lastIndexOf("") // 4
Same issue in TextStringBuilder.

Fix:
In lastIndexOf(String, int), for an empty search string allow the start
index up to size and return that, keep the existing size - 1 clamp for non-empty strings.

Affects:

• org.apache.commons.text.StrBuilder
• org.apache.commons.text.TextStringBuilder

[kzhunmax]

Jira ticket TEXT-241

[kzhunmax]

Additionally, I have question on what code is preferred startIndex = startIndex >= size ? size : startIndex; or Math.min(startIndex, size)?

[garydgregory]

@kzhunmax
I prefer Math.min(startIndex, size).

[Copilot]

Pull request overview

Fixes lastIndexOf("") behavior in StrBuilder and TextStringBuilder to match java.lang.String / StringBuilder semantics (for a builder of length n, lastIndexOf("") returns n).

Changes:

• Update lastIndexOf(String) to delegate using startIndex = size so empty-search can return size.
• Adjust lastIndexOf(String, int) clamping so empty search strings allow startIndex up to size, while non-empty searches still clamp to size - 1.
• Add regression tests for lastIndexOf("") on a non-empty builder.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/main/java/org/apache/commons/text/TextStringBuilder.java	Fixes empty-string lastIndexOf clamping to allow returning size.
src/main/java/org/apache/commons/text/StrBuilder.java	Same fix as TextStringBuilder for parity with JDK behavior.
src/test/java/org/apache/commons/text/TextStringBuilderTest.java	Adds regression assertion for lastIndexOf("") returning the builder length.
src/test/java/org/apache/commons/text/StrBuilderTest.java	Adds regression assertion for lastIndexOf("") and adds @Deprecated annotation to match the existing Javadoc deprecation tag.

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[garydgregory]

Merged 🚀 , thank you @kzhunmax