[Mark Hildebrand (hildebrandmw)]

Turns out with the work done in #1469 and #1484, we can simplify the implementation of spherical quantization in diskann-inmem to also be behind Quantized. This does that and gets rid of the custom Spherical representation.

Since we now have a lot Quantizer monomorphizations, this also introduces quantized::Backed as kind of an intermediate type-erased quantizer to store in quantized::Config. The relationship goes like this:

• The Quantized trait has an into_backend method for making a Backend. This can make use of private constructors and so is largely unimplementable by user code (they could do something weird by swapping out the quantizer, but that wouldn't cause us any issues since we call into_backend before calling any methods.
• The IntoBackend trait is used to construct a backend. There is a blanket implementation for any Quantizer, delegating to Quantizer::into_backend. There is also an implementation for Backend which simply returns itself. Since Backend does not implement Quantizer, there is no coherence issue.
• Config can then be constructed from a concrete Quantizer or from a Backend. The latter allows quantizers to be type-erased before constructing a Config, which simplifies a lot of set up code.
  Now, the reason to go with an into_backend method on the Quantizer trait is to give quantizers defined within diskann-inmem an escape hatch for potentially specializing inner::Quantizer, keeping the door open for hybrid PQ pruning if absolutely necessary.

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 94.66667% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.07%. Comparing base (2a26c7b) to head (61b1bdd).

Files with missing lines	Patch %	Lines
diskann-inmem/src/repr/quantized/spherical.rs	97.33%	6 Missing ⚠️
diskann-benchmark/src/index/inmem2.rs	70.58%	5 Missing ⚠️
diskann-inmem/integration/index/runner.rs	64.28%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1485      +/-   ##
==========================================
- Coverage   92.08%   91.07%   -1.02%     
==========================================
  Files         591      591              
  Lines      122196   121845     -351     
==========================================
- Hits       112523   110969    -1554     
- Misses       9673    10876    +1203     

Flag	Coverage Δ
miri	91.07% <94.66%> (-1.02%)	⬇️
unittests	90.77% <94.66%> (-1.26%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-inmem/integration/index/object.rs	99.00% <ø> (ø)
diskann-inmem/src/repr/internal/intrusive.rs	100.00% <ø> (ø)
...ann-inmem/src/repr/internal/quantization/rerank.rs	98.75% <100.00%> (+1.22%)	⬆️
diskann-inmem/src/repr/mod.rs	93.75% <ø> (ø)
diskann-inmem/src/repr/quantized/mod.rs	98.42% <100.00%> (+0.04%)	⬆️
diskann-inmem/src/store/intrusive.rs	93.97% <100.00%> (-0.07%)	⬇️
diskann-quantization/src/spherical/quantizer.rs	98.29% <100.00%> (+0.01%)	⬆️
diskann-benchmark/src/index/inmem2.rs	62.10% <70.58%> (+0.97%)	⬆️
diskann-inmem/integration/index/runner.rs	91.78% <64.28%> (+3.21%)	⬆️
diskann-inmem/src/repr/quantized/spherical.rs	97.33% <97.33%> (ø)

... and 48 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Copilot]

🟢 Approval recommended

Static review identified no blocking issues in the migration or backend conversions; project tests were not run.

No open issues 🎉

What changed in this PR

Unifies spherical quantization with the shared Quantized representation, following the product and scalar quantization work.

Changes:

• Adds a spherical adapter and type-erased Backend configuration.
• Migrates integration and benchmark callers, removing the standalone representation.
• Handles zero-norm rescaling and adds adapter tests.

File	Description
diskann-quantization/​src/​spherical/​quantizer.rs	Guards zero-norm rescaling; exposes concrete quantizers.
diskann-inmem/​src/​store/​intrusive.rs	Removes guard access while retaining ownership.
diskann-inmem/​src/​repr/​spherical.rs	Deletes the standalone spherical representation.
diskann-inmem/​src/​repr/​quantized/​spherical.rs	Adds the spherical adapter and tests.
diskann-inmem/​src/​repr/​quantized/​mod.rs	Adds Backend, conversions, and spherical exports.
diskann-inmem/​src/​repr/​mod.rs	Removes standalone spherical exports.
diskann-inmem/​src/​repr/​internal/​quantization/​rerank.rs	Removes the unused disabled configuration variant.
diskann-inmem/​src/​repr/​internal/​intrusive.rs	Removes the unused guard accessor.
diskann-inmem/​integration/​index/​runner.rs	Migrates spherical and scalar setup to backends.
diskann-inmem/​integration/​index/​object.rs	Removes standalone spherical index registration.
diskann-benchmark/​src/​index/​inmem2.rs	Migrates benchmark setup to shared configuration.

_{Review effort: Balanced}