# Nesting engines

Whole-job engines implement `INestingEngine.Solve(NestJob)` and are selected by name through
`NestingEngineRegistry`. Every automatic nesting front end validates engine output the same way;
see [automatic nesting and validation](automatic-nesting.md).

## Built-in engines

Engines are named for the jobs they suit, not for how or by whom they were built. Engine code lives
in `OpenNest.Engine/NestingEngines/<Name>/`, its tests in `OpenNest.Engine.Tests/NestingEngines/`.

| Engine | Best for | Method |
|---|---|---|
| Rectangles | Plain and near-rectangular plates | Each part packed as the box of its material at its minimum-area rotation, using a maximal-rectangles free list; stock chosen sheet by sheet by salvage-credited look-ahead cost |
| Irregular | Irregular profiles | No-fit-polygon frontier packing with gap filling, six whole-job strategy variants and a tail re-plan |
| StockLadder | Caller-supplied stock ladders | Constrained-first fill with equivalent-demand area repacking |
| Default, Strip, Vertical Remnant, Horizontal Remnant | Single-strategy fills | The fixed placement strategies behind interactive fill |

Rectangles places irregular parts validly, but only as their bounding boxes; it never nests into a
notch or hole. Box sides account for how the layout check flattens arcs, so round-edged parts stay
valid at box contact.

## Renamed engines

Earlier releases shipped these as plug-ins under other names. The registry maps the old names so
saved desktop selections, scripts and API requests keep working:

| Old name | Now |
|---|---|
| `Opus55NestingEngine` | Irregular |
| `RectanglesNestingEngine` | Rectangles |

Gpt6Astra and Qwen38FlashNext are no longer shipped and have no alias. A saved selection of either
falls back to Default with the usual status-bar warning.

## Changing an engine

- A change lands only when it beats the engine's current result on `OpenNest.Benchmark` for the
  jobs that engine targets, with every layout valid. Report cost, validity, unplaced parts and time.
- Placement must be deterministic: no clocks, unseeded randomness or environment variables. Budget
  work by counting it; wall time may stop work only through the cancellation token.
- Keep each engine's tests passing, including `EngineContractTests<TEngine>` (quadrants, overflow,
  priority, cancellation, stock and plate limits, determinism). Every layout in those tests is
  checked with `NestLayoutCheck`, the benchmark's validator.
- Engines may share code. Move a helper into shared Engine code when a second engine needs it,
  rather than copying it.

## Plug-ins

External engines still load from an `Engines/` folder beside the desktop, console, MCP or benchmark
executable. A plug-in implements `INestingEngine` with a public parameterless constructor and
registers under its CLR type name. A plug-in whose name matches a built-in engine, or a renamed
engine's old name, is skipped: a leftover `OpenNest.Engine.Opus55.dll` cannot shadow Irregular.
Leftover Gpt6Astra or Qwen38FlashNext DLLs still load as ordinary plug-ins until deleted.