OpenNest/docs/superpowers/specs/2026-03-20-nfp-bestfit-strategy-design.md

NFP-Based Best-Fit Strategy

Problem

The current best-fit pair generation uses RotationSlideStrategy, which samples Part2 positions by sliding it toward Part1 from 4 directions at discrete step sizes. This is brute-force: more precision requires more samples, it can miss optimal interlocking positions between steps, and it generates hundreds of candidates per rotation angle.

Solution

Replace the slide-based sampling with NFP (No-Fit Polygon) computation. The NFP of two polygons gives the exact mathematical boundary of all valid positions where Part2 can touch Part1 without overlapping. Every point on that boundary is a guaranteed-valid candidate offset.

Approach

Implement NfpSlideStrategy : IBestFitStrategy that plugs into the existing BestFitFinder pipeline. No changes to PairEvaluator, BestFitFilter, BestFitResult, tiling, or caching.

Design

New class: NfpSlideStrategy

Location: OpenNest.Engine/BestFit/NfpSlideStrategy.cs

Implements: IBestFitStrategy

Constructor parameters:

• double part2Rotation — rotation angle for Part2 (same as RotationSlideStrategy)
• int type — strategy type id (same as RotationSlideStrategy)
• string description — human-readable description

GenerateCandidates(Drawing drawing, double spacing, double stepSize):

1. Extract perimeter polygon from the drawing inflated by spacing / 2 using PolygonHelper.ExtractPerimeterPolygon (shared helper, extracted from AutoNester)
2. Create a rotated copy of the polygon at part2Rotation using PolygonHelper.RotatePolygon (also extracted)
3. Compute NoFitPolygon.Compute(stationaryPoly, orbitingPoly) — single call
4. If the NFP is null or has fewer than 3 vertices, return empty list
5. Walk the NFP boundary:
   • Each vertex becomes a PairCandidate with that vertex as Part2Offset
   • For edges longer than stepSize, add intermediate sample points at stepSize intervals along the edge (catches optimal positions on long straight NFP edges)
6. Return the candidates list

Shared helper: PolygonHelper

Location: OpenNest.Engine/BestFit/PolygonHelper.cs

Static methods extracted from AutoNester:

• ExtractPerimeterPolygon(Drawing drawing, double halfSpacing) — extracts and inflates the perimeter polygon
• RotatePolygon(Polygon polygon, double angle) — creates a rotated copy normalized to origin

After extraction, AutoNester delegates to these methods to avoid duplication.

Changes to BestFitFinder.BuildStrategies

Replace RotationSlideStrategy instances with NfpSlideStrategy instances. Same rotation angles from GetRotationAngles(drawing), different strategy class. No ISlideComputer dependency needed.

private List<IBestFitStrategy> BuildStrategies(Drawing drawing)
{
    var angles = GetRotationAngles(drawing);
    var strategies = new List<IBestFitStrategy>();
    var type = 1;

    foreach (var angle in angles)
    {
        var desc = $"{Angle.ToDegrees(angle):F1} deg NFP";
        strategies.Add(new NfpSlideStrategy(angle, type++, desc));
    }

    return strategies;
}

No changes required

• PairEvaluator — still evaluates candidates (overlap check becomes redundant but harmless and fast)
• BestFitFilter — still filters results by aspect ratio, plate fit, etc.
• BestFitResult — unchanged
• BestFitCache — unchanged
• Tiling pipeline — unchanged
• PairsFillStrategy — unchanged

Edge Sampling

NFP vertices alone may miss optimal positions along long straight edges. For each edge of the NFP polygon where edgeLength > stepSize, interpolate additional points at stepSize intervals. This reuses the existing stepSize parameter meaningfully — it controls resolution along NFP edges rather than grid spacing.

Files Changed

File	Change
OpenNest.Engine/BestFit/NfpSlideStrategy.cs	New — IBestFitStrategy implementation
OpenNest.Engine/BestFit/PolygonHelper.cs	New — shared polygon extraction/rotation
OpenNest.Engine/Nfp/AutoNester.cs	Delegate to PolygonHelper methods
OpenNest.Engine/BestFit/BestFitFinder.cs	Swap RotationSlideStrategy for NfpSlideStrategy in BuildStrategies

What This Does NOT Change

• The RotationSlideStrategy class stays in the codebase (not deleted) in case GPU slide computation is still wanted
• The ISlideComputer / GPU pipeline remains available
• BestFitFinder constructor still accepts ISlideComputer but it won't be passed to NFP strategies (they don't need it)