Bug: Replacing initial entry affects visibility of other entries

[jaysenmarais]

Describe the bug

Removing items from the index then re-adding them generally works, except when removing the first item added to the index then re-adding it. In that case, other items (all but the first-added) no longer appear in search results (though count still hints at their existence in the index).

Steps to reproduce

Here's an adaptation of a Swift test that illustrates the issue

let index = USearchIndex.make(
    metric: USearchMetric.l2sq,
    dimensions: 1,
    connectivity: 8,
    quantization: USearchScalar.F32
)
index.reserve(3)

// add 3 entries then ensure all 3 entries are returned
index.add(key: 1, vector: [1.1])
index.add(key: 2, vector: [2.1])
index.add(key: 3, vector: [3.1])
assert(index.count == 3)
assert(index.search(vector: [1.0], count: 3).0 == [1, 2, 3]) // v2.11.7 works 😎

// replace second-added entry then ensure all 3 entries are still returned
index.remove(key: 2)
index.add(key: 2, vector: [2.2])
assert(index.count == 3)
assert(index.search(vector: [1.0], count: 3).0 == [1, 2, 3]) // v2.11.7 works 😎

// replace first-added entry then ensure all 3 entries are still returned
index.remove(key: 1)
index.add(key: 1, vector: [1.2])
let afterReplacingInitial = index.search(vector: [1.0], count: 3).0
assert(index.count == 3)
assert(afterReplacingInitial == [1, 2, 3]) // v2.11.7 fails with "[1] != [1, 2, 3]" 😨

Expected behavior

In the above example the final assertion should succeed, but in fact it fails as the second and third items added to the index no longer appear in search results after the first-added item is replaced.

USearch version

v2.11.7

Operating System

macOS 14.4.1 (building for iOS 17.4)

Hardware architecture

Arm

Which interface are you using?

Other bindings

Are you open to being tagged as a contributor?

• I am open to being mentioned in the project .git history as a contributor

Is there an existing issue for this?

• I have searched the existing issues

Code of Conduct

• I agree to follow this project's Code of Conduct

[ashvardanian]

Great catch! Can you please open a PR with those tests, I'll look into it 🤗

[jaysenmarais]

I've now created PR #400 which adds the above snippet as a runnable test.