Optimize PossibleIntersection and StablePriorityQueue

[JimBobSquarePants]

A rework of #12 and #13 with added unsafe code!

Note: I've discovered that Clipper2 does not return the correct result for the test data so pay attention to our own results only.

CC @stefannikolei

BenchmarkDotNet v0.14.0, Windows 11 (10.0.26100.4652)
11th Gen Intel Core i7-11370H 3.30GHz, 1 CPU, 8 logical and 4 physical cores
.NET SDK 9.0.302
  [Host]     : .NET 9.0.7 (9.0.725.31616), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI
  DefaultJob : .NET 9.0.7 (9.0.725.31616), X64 RyuJIT AVX-512F+CD+BW+DQ+VL+VBMI

Main

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
Clipper	48.65 us	0.501 us	0.444 us	2.90	0.03	7.8125	0.9155	47.87 KB	1.50
Clipper2	16.78 us	0.151 us	0.118 us	1.00	0.01	5.1880	0.3357	31.84 KB	1.00

PR (PossibleIntersection)

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
PolygonClipper	47.07 us	0.467 us	0.414 us	2.73	0.03	7.6904	0.6714	47.23 KB	1.48
Clipper2	17.25 us	0.160 us	0.150 us	1.00	0.01	5.1880	0.3357	31.84 KB	1.00

PR (StablePriorityQueue)

Method	Mean	Error	StdDev	Ratio	RatioSD	Gen0	Gen1	Allocated	Alloc Ratio
PolygonClipper	44.23 us	0.246 us	0.218 us	2.60	0.02	6.7139	0.4883	41.34 KB	1.30
Clipper2	16.99 us	0.157 us	0.139 us	1.00	0.01	5.1880	0.3357	31.84 KB	1.00

[stefannikolei]

LGTM.

But whoever designed that Unsafe API... .Add to retrieve a value 🙈🙈

[JimBobSquarePants]

LGTM.

But whoever designed that Unsafe API... .Add to retrieve a value 🙈🙈

Haha yeah, it can be clunky!

I just pushed an update for the StablePriorityQueue also. This takes a few percent off the time.

[Copilot]

Pull Request Overview

This PR applies performance optimizations by replacing temporary allocations in the sweep‐line intersection logic and the priority queue with a reusable buffer and unsafe memory access, and updates supporting tests and benchmarks to use the new APIs.

• Introduce a reusable Span<SweepEvent> workspace in PossibleIntersection and switch from list allocations to Unsafe/MemoryMarshal operations.
• Add a capacity-aware constructor to StablePriorityQueue and refactor its Up/Down methods to use CollectionsMarshal and Unsafe.Add for faster heap adjustments.
• Update tests, utilities, and benchmarks to call the new overloads and utilize C# 12 collection expressions.

Reviewed Changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/PolygonClipper.Tests/TestPolygonUtilitiesTests.cs	New unit tests for ConvertToPolygon and union operations
tests/PolygonClipper.Tests/TestPolygonUtilities.cs	Conversion helpers updated to C# 12 collection expressions
tests/PolygonClipper.Tests/TestData.cs	Added null-forgiving operators on deserialization
tests/PolygonClipper.Tests/PolygonClipper.Tests.csproj	Added Clipper2 package reference for tests
tests/PolygonClipper.Tests/GenericTestCases.cs	Replaced .ToList() with a collection expression for features
tests/PolygonClipper.Benchmarks/PolygonClipper.Benchmarks.csproj	Linked updated test utilities and bumped Clipper2 version
tests/PolygonClipper.Benchmarks/ClippingLibraryComparison.cs	Benchmarks updated to use TestPolygonUtilities and renamed method
src/PolygonClipper/StablePriorityQueue{T,TComparer}.cs	Added capacity constructor; refactored Up/Down to use unsafe spans
src/PolygonClipper/PolygonClipper.cs	Added workspace span to PossibleIntersection and updated calls

Comments suppressed due to low confidence (1)

tests/PolygonClipper.Tests/TestPolygonUtilities.cs:89

• This comment appears in ConvertToClipper2Polygon but the method returns a Clipper2 PathsD, not your internal Polygon type. Update the comment to reflect the actual return type, for example: // Convert GeoJSON Polygon to Clipper2 PathsD.

            // Convert GeoJSON Polygon to our Polygon type

[stefannikolei]

Typo sift --> shift

[JimBobSquarePants]

Actually, that's intentional, sift is standard terminology in heap operations