commit 7943b7ad1c51ed894ec4e89f97a29c4d185c955b
author Shubham Chaudhary <36742242+shubhamvishu@users.noreply.github.com> Mon Oct 30 19:22:28 2023 +0530
committer GitHub <noreply@github.com> Mon Oct 30 09:52:28 2023 -0400
tree 13c10067f2b8b740dd3375420b3e25c587674aa2
parent 2a8d187a99e98897e1263ee2eca7912f51198cfb

Return the same input vector if its a unit vector in VectorUtil#l2normalize (#12726)

### Description

While going through [VectorUtil](https://github.com/apache/lucene/blob/main/lucene/core/src/java/org/apache/lucene/util/VectorUtil.java) class, I observed we don't have a check for unit vector in `VectorUtil#l2normalize` so passing a unit vector goes thorough the whole L2 normalization(which is totally not required and it should early exit?). I confirmed this by trying out a silly example of `VectorUtil.l2normalize(VectorUtil.l2normalize(nonUnitVector))` and it performed every calculation twice. We could also argue that user should not call for this for a unit vector but I believe there would be cases where user simply want to perform the L2 normalization without checking the vector or if there are some overflowing values.

TL;DR : We should early exit in `VectorUtil#l2normalize`, returning the same input vector if its a unit vector

This is easily avoidable if we introduce a light check to see if the L1 norm or squared sum of input vector is equal to 1.0 (or) maybe just check `Math.abs(l1norm - 1.0d) <= 1e-5` (as in this PR) because that unit vector dot product(`v x v`) are not exactly 1.0 but like example : `0.9999999403953552` etc. With `1e-5` delta here we would be assuming a vector v having `v x v` >= `0.99999` is a unit vector or say already L2 normalized which seems fine as the delta is really small? and also the check is not heavy one?.