[IVF] Remove unnecessary loop and some clean up

[iverase]

This commit removes an unnecessary loop when computing neighbours. In addition it removes the usage of lists and replace them with arrays as they are fixed length lists and some other clean up.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[Copilot]

Pull Request Overview

Refactors the local KMeans implementation by removing an unnecessary neighbor loop, switching from List to array for fixed-size collections, and tightening test assertions.

• Replaced List<NeighborHood> with NeighborHood[] and updated related methods.
• Streamlined computeNeighborhoods to return an array and changed assignSpilled to operate via side effects.
• Strengthened HierarchicalKMeansTests with range checks and improved assertion methods.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
server/src/test/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeansTests.java	Added explicit range checks on assignments and replaced bare assert with assertTrue/assertNotEquals.
server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/KMeansLocal.java	Converted List<NeighborHood> to NeighborHood[], refactored computeNeighborhoods, updated assignSpilled signature, and removed old loop over centroids.

Comments suppressed due to low confidence (3)

server/src/test/java/org/elasticsearch/index/codec/vectors/cluster/HierarchicalKMeansTests.java:43

• [nitpick] This assertion would benefit from a descriptive message (e.g., "assignment out of range: " + assignment) to make test failures easier to diagnose.

            assertTrue(assignment >= 0 && assignment < centroids.length);

server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/KMeansLocal.java:188

• [nitpick] The method signature was changed from returning int[] to void, but its name and any JavaDoc may not reflect that it now writes via side effects. Consider updating docs or renaming to clarify its behavior.

    private void assignSpilled(

server/src/main/java/org/elasticsearch/index/codec/vectors/cluster/KMeansLocal.java:311

• This assertion assumes soarAssignments starts at length 0, but its initial state may differ. It's safer to explicitly clear or reassign the array rather than relying on an assert about its prior size.

            assert kMeansIntermediate.soarAssignments().length == 0;

[john-wagster]

lgtm +1 to remove any List where possible