CDT.NET: Full ReadOnlySpan<T> input migration + round 2 performance optimizations by Copilot · Pull Request #8 · MichaCo/CDT.NET

Copilot · 2026-02-22T19:04:25Z

Part 1a-1e: Full ReadOnlySpan<T> input migration across Types.cs, CdtUtils.cs, KdTree.cs, Triangulation.cs
Part 2A-2D: Performance optimizations (span locals, FrozenSet, stack reuse, PeelLayer allocation elimination)
Update test files to use .ToArray() / CollectionsMarshal.AsSpan for span conversions
Fix KD-tree tests: materialize CollectionsMarshal.AsSpan(pts) once before loops
Fix FreezeFixedEdges doc/invalidation + remove IReadOnlySet<Edge> interface dispatch
Fix viz app build errors: CollectionsMarshal.AsSpan for List<T> → span at call sites
Fix benchmark build errors: BenchmarkInputReader.Read now returns (V2d<double>[], Edge[]) instead of (List<V2d<double>>, List<Edge>); field types in both benchmark classes changed to arrays; FloatVsDouble_Float uses Array.ConvertAll instead of LINQ .Select().ToList()
All 997 tests passing
CDT.Viz and CDT.Benchmarks build with 0 errors

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Convert List<T> arguments to T[] via .ToArray() for ReadOnlySpan<T> parameters, and convert IReadOnlyList<T> (cdt.Triangles) to T[] via .ToArray() as well. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

…kups Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot

Pull request overview

This PR migrates CDT.NET’s public and internal APIs away from IReadOnlyList<T> inputs to ReadOnlySpan<T> to reduce virtual/interface dispatch in hot paths, and applies additional allocation and lookup optimizations (notably around fixed-edge queries and peel-layer processing).

Changes:

Replace IReadOnlyList<T> inputs with ReadOnlySpan<T> across core geometry, utilities, KD-tree, and triangulation APIs.
Add frozen fixed-edge lookup support (FrozenSet<Edge>) and additional span-based local caching in hot loops.
Update tests to call the new span-based APIs (primarily via ToArray()).

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
test/CDT.Tests/TriangulationTests.cs	Updates utility calls to pass arrays/spans to match new span-based signatures.
test/CDT.Tests/ReadmeExamplesTests.cs	Updates README example tests to call the new span-based triangulation APIs.
test/CDT.Tests/GroundTruthTests.cs	Updates ground-truth + KD-tree tests for span-based APIs (introduces repeated `ToArray()` usage in loops).
src/CDT.Core/Types.cs	Changes `Box2d<T>` helpers to accept `ReadOnlySpan<V2d<T>>`.
src/CDT.Core/Triangulation.cs	Updates triangulation public API to spans; adds frozen fixed-edge lookup and multiple hot-path optimizations.
src/CDT.Core/KdTree.cs	Migrates KD-tree APIs to accept `ReadOnlySpan<V2d<T>>`.
src/CDT.Core/CdtUtils.cs	Migrates several utility APIs to spans and adds internal backing fields to `DuplicatesInfo` for faster internal flows.

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test/CDT.Tests/GroundTruthTests.cs

Copilot · 2026-02-22T19:51:43Z

src/CDT.Core/Triangulation.cs

+    /// Freezes the fixed-edge set for faster read-only Contains() queries.
+    /// Call after all InsertEdges/ConformToEdges calls are complete.
+    /// Invalidated automatically on any subsequent edge mutation.


FreezeFixedEdges() claims the frozen set is “invalidated automatically on any subsequent edge mutation”, but there are edge mutations that currently don't clear _frozenFixedEdges (e.g., _fixedEdges is remapped during finalization when removing the super-triangle). Consider ensuring every _fixedEdges mutation path also sets _frozenFixedEdges = null, or narrowing the XML doc to the cases that are actually invalidated.

Suggested change

/// Freezes the fixed-edge set for faster read-only Contains() queries.

/// Call after all InsertEdges/ConformToEdges calls are complete.

/// Invalidated automatically on any subsequent edge mutation.

/// Freezes (snapshots) the current fixed-edge set for faster read-only Contains() queries.

/// Call this only after all InsertEdges/ConformToEdges operations and any other edge

/// mutations that affect the fixed-edge set are complete, or call it again after such

/// mutations to refresh the frozen snapshot.

Fixed in 8c2b7a7. _frozenFixedEdges = null is now set in FinalizeTriangulation when the super-triangle edge keys are remapped (the one mutation path that wasn't covered). The XML doc has been updated to accurately describe that finalization does not auto-refresh the frozen snapshot — callers should call FreezeFixedEdges() again after finalization if needed.

Copilot · 2026-02-22T19:51:43Z

src/CDT.Core/Triangulation.cs

+    // Frozen set for fast read-only Contains() on fixed edges
+    private FrozenSet<Edge>? _frozenFixedEdges;
+
+    // Returns the frozen set when available, falling back to the live HashSet
+    private IReadOnlySet<Edge> FixedEdgeSet => _frozenFixedEdges ?? (IReadOnlySet<Edge>)_fixedEdges;
+


FixedEdgeSet is typed as IReadOnlySet<Edge>, so the hot-path Contains() calls become interface dispatch when _frozenFixedEdges is null (the common case unless users call FreezeFixedEdges). Consider branching on _frozenFixedEdges and calling _fixedEdges.Contains(...) / _frozenFixedEdges.Contains(...) directly (or a small helper) to avoid the interface call overhead in IsFlipNeeded, GrowToBoundary, and PeelLayer.

Fixed in 8c2b7a7. Removed the IReadOnlySet<Edge> FixedEdgeSet property. All three hot-path call sites (IsFlipNeeded, GrowToBoundary, PeelLayer) now capture var frozen = _frozenFixedEdges; once and use frozen != null ? frozen.Contains(e) : _fixedEdges.Contains(e) — direct concrete-type dispatch in both branches with no interface cast overhead.

src/CDT.Core/CdtUtils.cs

Copilot · 2026-02-22T19:51:43Z

test/CDT.Tests/GroundTruthTests.cs

+        for (int i = 0; i < pts.Count; i++) kd.Insert(i, pts.ToArray());

        // Querying the exact location of each grid point returns that point
        for (int i = 0; i < pts.Count; i++)
        {
-            int nearest = kd.Nearest(pts[i].X, pts[i].Y, pts);
+            int nearest = kd.Nearest(pts[i].X, pts[i].Y, pts.ToArray());
            Assert.Equal(i, nearest);


These KD-tree tests call pts.ToArray() inside tight loops, which allocates a new array every iteration (O(n) allocations) and can significantly slow the test suite. Materialize the array once (or use CollectionsMarshal.AsSpan(pts) if you want to avoid allocating) and reuse it for all Insert/Nearest calls.

@copilot apply changes based on this feedback

Fixed in 8c2b7a7. All KD-tree tests now use CollectionsMarshal.AsSpan(pts) materialized once before the loops — no per-iteration heap allocations. The stress test especially benefits since it previously allocated 100 + 1000 arrays.

MichaConrad · 2026-02-22T20:06:29Z

@copilot fix viz app build errors

…FixedEdgeSet dispatch Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

…pan conversions Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot · 2026-02-22T20:11:08Z

@copilot fix viz app build errors

Fixed in 150f3fb. Added using System.Runtime.InteropServices and updated the three call sites in MainWindow.xaml.cs to use CollectionsMarshal.AsSpan(workPts) and CollectionsMarshal.AsSpan(workEdges) for InsertVertices, InsertEdges, and ConformToEdges. CDT.Viz now builds with 0 errors.

MichaConrad · 2026-02-22T20:15:54Z

@copilot Fix the benchmarks, make BenchmarkInputReader.Read return 2 arrays

… field types Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot · 2026-02-22T20:17:55Z

@copilot Fix the benchmarks, make BenchmarkInputReader.Read return 2 arrays

Fixed in 6fc4132. BenchmarkInputReader.Read now returns (V2d<double>[] Vertices, Edge[] Edges) — arrays allocate exactly once and implicitly convert to ReadOnlySpan<T> at every call site with no overhead. Both benchmark class fields updated to V2d<double>[] / Edge[], and FloatVsDouble_Float uses Array.ConvertAll instead of LINQ. CDT.Benchmarks builds with 0 errors.

MichaConrad · 2026-02-22T20:36:29Z

This literarily has no effect on performance or memory, at all.

Initial plan

d9dadcb

Copilot AI assigned Copilot and MichaConrad Feb 22, 2026

Copilot started work on behalf of MichaConrad February 22, 2026 19:04 View session

Copilot AI and others added 2 commits February 22, 2026 19:21

Fix test compilation errors after ReadOnlySpan<T> API changes

4296801

Convert List<T> arguments to T[] via .ToArray() for ReadOnlySpan<T> parameters, and convert IReadOnlyList<T> (cdt.Triangles) to T[] via .ToArray() as well. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Extract FixedEdgeSet property to reduce duplication in frozen set loo…

d9da68a

…kups Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Migrate IReadOnlyList<T> to ReadOnlySpan<T> for performance~~ CDT.NET: Full ReadOnlySpan<T> input migration + round 2 performance optimizations Feb 22, 2026

Copilot AI requested a review from MichaConrad February 22, 2026 19:27

Copilot finished work on behalf of MichaConrad February 22, 2026 19:27

MichaConrad requested a review from Copilot February 22, 2026 19:46

Copilot started reviewing on behalf of MichaConrad February 22, 2026 19:47 View session

Copilot AI reviewed Feb 22, 2026

View reviewed changes

Copilot started work on behalf of MichaConrad February 22, 2026 20:05 View session

Fix KD-tree test allocations, FreezeFixedEdges doc/invalidation, and …

8c2b7a7

…FixedEdgeSet dispatch Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot finished work on behalf of MichaConrad February 22, 2026 20:09

Copilot started work on behalf of MichaConrad February 22, 2026 20:09 View session

Fix viz app build errors: use CollectionsMarshal.AsSpan for List-to-s…

150f3fb

…pan conversions Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot finished work on behalf of MichaConrad February 22, 2026 20:11

Copilot started work on behalf of MichaConrad February 22, 2026 20:16 View session

Fix benchmark build: BenchmarkInputReader.Read returns arrays, update…

6fc4132

… field types Co-authored-by: MichaCo <5837539+MichaCo@users.noreply.github.com>

Copilot stopped work on behalf of MichaConrad due to an error February 22, 2026 20:18
Copilot has encountered an error. See logs for additional details.

MichaConrad closed this Feb 22, 2026

MichaConrad deleted the copilot/replace-ireadonlylist-with-readonlyspan branch February 22, 2026 20:36

-    /// Freezes the fixed-edge set for faster read-only Contains() queries.
-    /// Call after all InsertEdges/ConformToEdges calls are complete.
-    /// Invalidated automatically on any subsequent edge mutation.
+    /// Freezes (snapshots) the current fixed-edge set for faster read-only Contains() queries.
+    /// Call this only after all InsertEdges/ConformToEdges operations and any other edge
+    /// mutations that affect the fixed-edge set are complete, or call it again after such
+    /// mutations to refresh the frozen snapshot.

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