[SPARK-48284][SQL] Fix UTF8String indexOf behaviour for empty string search

[uros-db]

What changes were proposed in this pull request?

Currently, UTF8String.indexOf returns 0 when given an empty parameters string, and any integer start value. Examples:

"abc".indexOf("", 0);  // returns: 0
"abc".indexOf("", 2);  // returns: 0
"abc".indexOf("", 9);  // returns: 0
"abc".indexOf("", -3); // returns: 0

This is not correct, as "start" is not taken into consideration. Correct behaviour would be:

"abc".indexOf("", 0);  // returns: 0
"abc".indexOf("", 2);  // returns: 2
"abc".indexOf("", 9);  // returns: -1
"abc".indexOf("", -3); // returns: -1

Why are the changes needed?

Fix a bug in indexOf implementation.

Does this PR introduce any user-facing change?

Yes, indexOf behaves differently with empty patterns.

How was this patch tested?

New unit tests.

Was this patch authored or co-authored using generative AI tooling?

No.

[MaxGekk]

@uros-db Is the changes still valid?

[uros-db]

changes are valid, although some conflicts need to be resolved

@srielau @mkaravel do we want to revive this and proceed with the indexOf bug fix?

[MaxGekk]

@uros-db Is the test failure related to your PR?

[error] Test org.apache.spark.unsafe.types.CollationSupportSuite.testStringLocate failed: org.opentest4j.AssertionFailedError: expected: <1> but was: <-1>, took 0.006s
[error]     at org.apache.spark.unsafe.types.CollationSupportSuite.assertStringLocate(CollationSupportSuite.java:2123)
[error]     at org.apache.spark.unsafe.types.CollationSupportSuite.testStringLocate(CollationSupportSuite.java:2129)
[error]     at java.lang.reflect.Method.invoke(Method.java:569)
[error]     at java.util.ArrayList.forEach(ArrayList.java:1511)
[error]     at java.util.ArrayList.forEach(ArrayList.java:1511)

[uros-db]

yes, those tests likely need to be updated in order for this PR to pass

however, I'm not sure that we want to proceed with this at this time

[github-actions]

We're closing this PR because it hasn't been updated in a while. This isn't a judgement on the merit of the PR in any way. It's just a way of keeping the PR queue manageable.
If you'd like to revive this PR, please reopen it and ask a committer to remove the Stale tag!