docs: add contour re-indexing design spec

2026-03-12 23:41:30 -04:00
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+# Contour Re-Indexing Design
+
+## Overview
+
+Add entity-splitting primitives and a `Shape.ReindexAt` method so that a closed contour can be reordered to start (and end) at an arbitrary point. Then wire this into `ContourCuttingStrategy.Apply()` to replace the `NotImplementedException` stubs.
+
+All geometry additions live on existing classes in `OpenNest.Geometry`. The strategy wiring is a change to the existing `ContourCuttingStrategy` in `OpenNest.CNC.CuttingStrategy`.
+
+## Entity Splitting Primitives
+
+### Line.SplitAt(Vector point)
+
+```csharp
+public (Line first, Line second)? SplitAt(Vector point)
+```
+
+- Returns two lines: `StartPoint → point` and `point → EndPoint`.
+- If the point is at `StartPoint` (within `Tolerance.Epsilon`), `first` is null.
+- If the point is at `EndPoint` (within `Tolerance.Epsilon`), `second` is null.
+- If both halves would be degenerate, returns null.
+- The point is assumed to lie on the line (caller is responsible — it comes from `ClosestPointTo`).
+
+### Arc.SplitAt(Vector point)
+
+```csharp
+public (Arc first, Arc second)? SplitAt(Vector point)
+```
+
+- Computes `splitAngle = Center.AngleTo(point)`, normalized via `Angle.NormalizeRad`.
+- First arc: same center, radius, direction — `StartAngle → splitAngle`.
+- Second arc: same center, radius, direction — `splitAngle → EndAngle`.
+- If `splitAngle` equals `StartAngle` (within tolerance), `first` is null.
+- If `splitAngle` equals `EndAngle` (within tolerance), `second` is null.
+- If both halves would be degenerate, returns null.
+
+### Circle.ToArcFrom(Vector point)
+
+```csharp
+public Arc ToArcFrom(Vector point)
+```
+
+- Computes angle: `Center.AngleTo(point)`.
+- Returns an `Arc` with same center, radius, `StartAngle = EndAngle = angle`, and `IsReversed` matching the circle's `Rotation` (CW → reversed, CCW → not reversed).
+- This produces a full-sweep arc that starts and ends at `point`.
+
+## Shape.ReindexAt
+
+```csharp
+public Shape ReindexAt(Vector point, Entity entity)
+```
+
+- `point`: the start/end point for the reindexed contour (from `ClosestPointTo`).
+- `entity`: the entity containing `point` (from `ClosestPointTo`'s `out` parameter).
+- Returns a **new** Shape (does not modify the original).
+
+### Algorithm
+
+1. If `entity` is a `Circle`:
+   - Return a new Shape with a single entity: `circle.ToArcFrom(point)`.
+
+2. Find the index `i` of `entity` in `Entities`.
+
+3. Split the entity at `point`:
+   - `Line` → `line.SplitAt(point)` → `(firstHalf, secondHalf)`
+   - `Arc` → `arc.SplitAt(point)` → `(firstHalf, secondHalf)`
+
+4. Build the new entity list (all non-null):
+   - `secondHalf` (if not null)
+   - `Entities[i+1]`, `Entities[i+2]`, ..., `Entities[count-1]` (after the split)
+   - `Entities[0]`, `Entities[1]`, ..., `Entities[i-1]` (before the split, wrapping around)
+   - `firstHalf` (if not null)
+
+5. Return a new Shape with this entity list.
+
+### Edge Cases
+
+- **Point lands on entity boundary** (start/end of an entity): one half of the split is null. The reordering still works — it just starts from the next full entity.
+- **Single-entity shape that is an Arc**: split produces two arcs, reorder is just `[secondHalf, firstHalf]`.
+- **Single-entity Circle**: handled by step 1 (converts to full-sweep arc).
+
+## Wiring into ContourCuttingStrategy
+
+### Entity-to-ICode Conversion
+
+Add a private method to `ContourCuttingStrategy`:
+
+```csharp
+private List<ICode> ConvertShapeToMoves(Shape shape)
+```
+
+Iterates `shape.Entities` and converts each to cutting moves:
+- `Line` → `LinearMove(line.EndPoint)`
+- `Arc` → `ArcMove(arc.EndPoint(), arc.Center, arc.IsReversed ? RotationType.CW : RotationType.CCW)`
+- `Circle` → should not appear (circles are converted to arcs by `ReindexAt`)
+
+No `RapidMove` between entities — they are contiguous in a reindexed shape. The lead-in already positions the head at the shape's start point.
+
+### Replace NotImplementedException
+
+In `ContourCuttingStrategy.Apply()`, replace both `throw new NotImplementedException(...)` blocks with:
+
+```
+var reindexed = shape.ReindexAt(closestPt, entity);
+result.Codes.AddRange(ConvertShapeToMoves(reindexed));
+```
+
+The full sequence for each contour becomes:
+1. Lead-in codes (rapid to pierce point, cutting moves to contour start)
+2. Contour body (reindexed entity moves from `ConvertShapeToMoves`)
+3. Lead-out codes (overcut moves away from contour)
+
+### MicrotabLeadOut Handling
+
+When the lead-out is `MicrotabLeadOut`, the last cutting move must be trimmed by `GapSize`. This is a separate concern from re-indexing — stub it with a TODO comment for now. The trimming logic will shorten the last `LinearMove` or `ArcMove` in the contour body.
+
+## Files Modified
+
+| File | Change |
+|------|--------|
+| `OpenNest.Core/Geometry/Line.cs` | Add `SplitAt(Vector)` method |
+| `OpenNest.Core/Geometry/Arc.cs` | Add `SplitAt(Vector)` method |
+| `OpenNest.Core/Geometry/Circle.cs` | Add `ToArcFrom(Vector)` method |
+| `OpenNest.Core/Geometry/Shape.cs` | Add `ReindexAt(Vector, Entity)` method |
+| `OpenNest.Core/CNC/CuttingStrategy/ContourCuttingStrategy.cs` | Add `ConvertShapeToMoves`, replace `NotImplementedException` blocks |
+
+## Out of Scope
+
+- **MicrotabLeadOut trimming** (trim last move by gap size — stubbed with TODO)
+- **Tab insertion** (inserting tab codes mid-contour — already stubbed)
+- **Lead-in editor UI** (interactive start point selection — separate feature)
+- **Contour re-indexing for open shapes** (only closed contours supported)