[diskann-garnet] Allow configurable metric types

[tiagonapoli]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies?
• Does this PR modify any existing APIs?
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones?

What does this implement/fix? Briefly explain your changes.

Allow Garnet to configure the Metric type to be used by DiskANN

Any other comments?

[Copilot]

Pull request overview

This PR makes the Garnet DiskANN FFI/index creation support selectable distance metrics (instead of always using cosine), and updates tests/docs accordingly.

Changes:

• Add a metric_type parameter to the create_index FFI and thread it through GarnetProvider so distance/query computers use the requested metric.
• Fix test store multi-read parsing (pos advancement) and update affected expectations.
• Expand FFI test coverage for valid/invalid metric values and add a grid search test using variable-length (string) external IDs; update FFI design docs.

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
diskann-garnet/src/test_utils.rs	Fixes multi-read buffer parsing and adjusts tests to match correct behavior; updates provider construction for new metric parameter.
diskann-garnet/src/provider.rs	Stores selected metric on GarnetProvider and uses it when building distance/query computers.
diskann-garnet/src/lib.rs	Extends create_index FFI to accept a raw metric_type and passes it into provider construction.
diskann-garnet/src/ffi_tests.rs	Adds tests for metric validation and adds a string-ID grid search test; updates existing calls for new FFI signature.
diskann-garnet/docs/ffi-design.rs	Documents the new metric_type argument and adds the repository license header.

Comments suppressed due to low confidence (2)

diskann-garnet/src/lib.rs:223

• create_index now accepts metric_type, but the config is still built with config::PruneKind::TriangleInequality. For Metric::InnerProduct, DiskANN expects occluding-style pruning; using the triangle-inequality prune kind can produce incorrect/low-quality graphs for that metric. Consider using config::PruneKind::from_metric(metric_type) (or equivalent match) when building the config.

        target_degree,
        config::MaxDegree::Value(max_degree as usize),
        l_build as usize,
        config::PruneKind::TriangleInequality,
    )

diskann-garnet/docs/ffi-design.rs:37

• The docs signature for create_index is still missing the rmw_callback parameter that exists in the actual exported function (diskann-garnet/src/lib.rs). Since this doc block was updated for metric_type, it should be brought back in sync with the real FFI signature.

extern "C" fn create_index(
    context: u64,
    dimensions: u32,
    reduce_dims: u32,
    quant_type: SomeCStyleEnumeration,
    metric_type: i32,
    build_exploration_factor: u32,
    num_links: u32,
    read_callback: unsafe extern "C" fn(u64, *const u8, usize, *mut u8, usize) -> i32,
    write_callback: unsafe extern "C" fn(u64, *const u8, usize, *const u8, usize) -> bool,
    delete_callback: unsafe extern "C" fn(u64, *const u8, usize) -> bool,
) -> *mut c_void;

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Codecov Report

❌ Patch coverage is 91.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.11%. Comparing base (5e0e49d) to head (ab3e479).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
diskann-vector/src/distance/metric.rs	89.28%	3 Missing ⚠️
diskann-garnet/src/lib.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #824      +/-   ##
==========================================
+ Coverage   88.96%   89.11%   +0.14%     
==========================================
  Files         442      443       +1     
  Lines       81906    83354    +1448     
==========================================
+ Hits        72868    74281    +1413     
- Misses       9038     9073      +35     

Flag	Coverage Δ
miri	89.11% <91.66%> (+0.14%)	⬆️
unittests	88.95% <91.66%> (+0.13%)	⬆️

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

Files with missing lines	Coverage Δ
diskann-garnet/src/provider.rs	80.83% <100.00%> (+0.17%)	⬆️
diskann-garnet/src/test_utils.rs	99.31% <100.00%> (+<0.01%)	⬆️
diskann-garnet/src/lib.rs	67.13% <87.50%> (+0.30%)	⬆️
diskann-vector/src/distance/metric.rs	90.16% <89.28%> (-0.75%)	⬇️

... and 37 files with indirect coverage changes

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