# Reference implementation — OpenSCAD, revision 8.0.0

**This is not a live target.** It is kept so the acceptance oracle in
`fixtures/strap-beam-8.0.0/` can be regenerated, and so the port has something
to be checked against. The running application has no OpenSCAD dependency, and
none of this is installed on an instance host.

## What it produces

Eleven profiles across two generators. A profile is a cross-section swept along
a longitudinal axis; a "member" is the end view of one strap bundle, so its
length in the cross-section is the strap's **width**, never the beam's length.

| Generator | Profiles |
|---|---|
| `strap-beam-3x.scad` | Equilateral Triangle, General Triangle, A Frame, Y, T, Three-Fin |
| `strap-beam-4x.scad` | Square, Rectangle, Diamond, Cross, Four-Fin |

4x is the end of the N-strap family. Higher polygon counts compose from
triangles and squares rather than getting their own generators.

## The shared library

Everything reusable is in `lib/`, written for N members and used unchanged by
both generators:

| File | Contents |
|---|---|
| `sb-geom.scad` | GEO and MEMBER records, path construction, exact polyline distance, the monotone solver |
| `sb-join.scad` | butt joints, hull caps, concave-corner fillets, ring envelopes and bores, polygon fitting |
| `sb-report.scad` | value-based checks, section metrics, the `SB_KEY=value` report |
| `sb-profiles.scad` | three complete N-generic arrangements: ring, spokes, fins |
| `sb-core.scad` | umbrella include and the PROFILE record |

`strap-beam-4x.scad` is 210 lines and contains **no geometry** — five one-line
calls into the shared library. The same three calls with N=3 produce the
triangles, the Y and the Three-Fin. That is the evidence the split is real.

## Design rules the library enforces

1. Declared webs are cavity-to-cavity. A stated 1.2 mm web is 1.2 mm of plastic.
2. Placement that cannot be derived exactly is solved numerically against the
   measured quantity, not approximated with a closed form.
3. Junctions are structural before they are decorative: members butt through
   their neighbours, and fillets are applied on top of that overlap.
4. A member with no enclosed side takes the outside wall on both faces, so
   asymmetric wall settings never make a symmetric profile chiral.
5. Validation measures the finished section. Connectivity is necessary and
   never sufficient — revision 7 passed cross-sections joined by 0.13 mm.

## Running it

Requires OpenSCAD 2021.01 and BOSL2 at `92d697c2`. Use the pinned image:

```bash
docker run --rm -v "$PWD:/repo" mechcomp/reference-toolchain:8.0.0 \
    openscad -o /dev/null --export-format=asciistl \
    -Dprofile_type='"Three-Fin"' /repo/legacy/openscad/strap-beam-3x.scad
```

Every render emits a flat `SB_KEY=value` block on stderr, terminated by
`SB_END=1`. `render_mode="Section"` emits the 2D region instead of the swept
solid, which is what `--export-format=svg` consumes.

## Three BOSL2 behaviours worth knowing

Found the hard way; the library works around all three:

- Boolean functions return a bare `[]` rather than an empty region.
- `offset()` on a single-part region returns a bare path rather than a region.
- `round_corners()` raises rather than reporting when a radius will not fit.

`sb_as_region()`, `sb_area()`/`sb_nparts()` and `sb_path_max_round()` exist for
exactly these.