Move buffer position in ByteBufferIndexWriter#writeFloats

[michaeljmarshall]

Fixes #606

This PR fixes a bug where we put the vector into the buffer without moving the position forward.

When attempting to upgrade to 4.0.0-rc7, I hit the following exception:

Caused by: java.lang.IllegalStateException: Record size mismatch for ordinal 0: expected 2136 bytes, wrote 1736 bytes
	at io.github.jbellis.jvector.graph.disk.NodeRecordTask.call(NodeRecordTask.java:171)
	at io.github.jbellis.jvector.graph.disk.ParallelGraphWriter.lambda$writeL0Records$0(ParallelGraphWriter.java:217)
	at java.base/java.util.concurrent.FutureTask.run$$$capture(FutureTask.java:264)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java)
	at --- Async.Stack.Trace --- (captured by IntelliJ IDEA debugger)
	at java.base/java.util.concurrent.FutureTask.<init>(FutureTask.java:132)
	at java.base/java.util.concurrent.AbstractExecutorService.newTaskFor(AbstractExecutorService.java:108)
	at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:139)
	at io.github.jbellis.jvector.graph.disk.ParallelGraphWriter.writeL0Records(ParallelGraphWriter.java:200)
	at io.github.jbellis.jvector.graph.disk.OnDiskGraphIndexWriter.writeL0RecordsParallel(OnDiskGraphIndexWriter.java:269)
	at io.github.jbellis.jvector.graph.disk.OnDiskGraphIndexWriter.write(OnDiskGraphIndexWriter.java:210)
	at org.apache.cassandra.index.sai.disk.vector.CompactionGraph.flush(CompactionGraph.java:532)
	... 83 more

The issue seems to be in the ByteBufferIndexWriter#writeFloats. By calling asFloatBuffer(), we're creating a new object with its own position tracking, and as a result, putting the float[] moves the FloatBuffer's position, but not the source Buffer position.

This PR also adds a convenience method to simplify writing a vector to the IndexWriter to reduce the need for branching in the calling code.

[michaeljmarshall]

In working with this code, I noticed that it'd be very helpful to have custom implementations to write a VectorFloat into the IndexWriter. I added that in this PR since this hasn't been merged yet. Let me know if it needs to be a separate PR.

[Copilot]

Pull request overview

This PR fixes a critical bug in ByteBufferIndexWriter#writeFloats where the buffer position was not being advanced after writing float arrays, causing record size mismatches during graph serialization.

Changes:

• Fixed buffer position tracking in ByteBufferIndexWriter#writeFloats by explicitly advancing the position after writing
• Added writeTo(IndexWriter) method to VectorFloat interface and implementations to simplify vector serialization
• Added comprehensive test coverage for the writeFloats fix and the new writeTo methods

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
jvector-base/src/main/java/io/github/jbellis/jvector/disk/ByteBufferIndexWriter.java	Fixed bug by advancing buffer position after writing floats
jvector-base/src/main/java/io/github/jbellis/jvector/vector/types/VectorFloat.java	Added writeTo method to interface
jvector-base/src/main/java/io/github/jbellis/jvector/vector/ArrayVectorFloat.java	Implemented writeTo using writeFloats
jvector-native/src/main/java/io/github/jbellis/jvector/vector/MemorySegmentVectorFloat.java	Implemented writeTo with element-by-element writing
jvector-tests/src/test/java/io/github/jbellis/jvector/disk/TestByteBufferIndexWriter.java	Added comprehensive test suite for buffer position tracking

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[Copilot]

The writeTo implementation writes floats one at a time in a loop, which is less efficient than bulk writing. Consider converting to a float array first and using writeFloats for better performance, similar to the ArrayVectorFloat implementation.

Suggested change

	for (int i = 0; i < length(); i++) {
	writer.writeFloat(get(i));
	}
	int len = length();
	float[] data = (float[]) segment.heapBase().get();
	writer.writeFloats(data, 0, len);