# STOCK.md

**What the compiler is for, and what every catalogue entry has to declare.**

Written 2026-08-22, from CIVICVS's statement of the project's subject.

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## 1. The subject of this library

This is not a 3D-printing geometry library. BOSL2 is that, and it is vendored in
`legacy/` because the reference was built on it.

This library **describes commercial off-the-shelf hardware, and then generates
the printed part that encloses it, interfaces with it, or augments it.**

The pallet strap is not the subject. It is the **first stock entry**, and it was
described so thoroughly that the whole of rev 8.0.0 looks like a strap library.
Electrical conduit is the second. Stock can be anything you can buy or make on
site: pipe, sheet, net, poured, drilled, bent. Each gets described, one at a
time, and added to the catalogue.

The printed part is the *adapter*. The stock is what it adapts to.

### Why this framing changes the code

Three things in the current codebase are the same object seen three times, and
none of them knows about the others:

| Where | What it is | How it is expressed today |
|---|---|---|
| `Geo.cavity_w` / `cavity_t` | a strap's occupied void | strap width and thickness, plus `fit_clearance_mm`, times `bundle_count` |
| `three_fin_bore_side_mm` | a polygonal void through the centre | a bare side length in millimetres |
| `bore_from_members()` | the void left over between members | derived, not declared |

The first two are stock occupying space. The third is not stock at all — it is a
residual cavity, and the name collision between it and a declared bore is
already a source of confusion. **A declared stock void and a derived residual
void are different things and should stop sharing a word.**

## 2. What a stock entry declares

Every entry in the catalogue answers the same questions. An entry that cannot
answer one of them is not described well enough to generate a part.

**Designation.** What you ask for at the counter. `EMT 1/2"`, `PET strap
15.875 x 0.508`, `#3 rebar`. This is a label, never a dimension — see §5.

**Section.** The cross-section the stock presents where the printed part meets
it, as a closed path in millimetres: a rectangle for strap, a circle for
conduit, a hexagon for a bolt head, a deformed circle for rebar. Prismatic stock
has one section along its whole length; non-prismatic stock declares the section
at the interface and nothing else, because that is all this compiler can hold
(see `PRECISION.md`).

**Nominal versus actual.** These differ, and the difference is where parts
fail. Trade size is not outside diameter. Nominal lumber is not actual lumber.
**The catalogue stores actual, measured, with its source.** The designation is
looked up to reach it, never computed from it.

**Fit.** How much room the printed part must leave, and why. A strap that slides
through a channel, a conduit that is meant to be a press fit, and a bolt that
must clear a hole are three different numbers even at the same diameter. Fit is
a property of the *joint*, not of the stock, so an entry declares its default
and every placement may override it.

**Tolerance of the stock itself.** Extruded and rolled stock varies. A part
designed to the nominal section jams on the fat end of the run. Where a
manufacturing tolerance is known it is recorded; where it is not, that is
recorded too, and it is not silently assumed to be zero.

**Provenance.** Where the numbers came from — a standard, a spec sheet, or a
caliper. A measured value with a date beats a remembered one, and a remembered
one does not go in.

## 3. The three roles a printed part plays

Naming these keeps profiles honest about what they are doing.

**Enclose.** The part surrounds the stock and holds it. The strap channels are
this. Failure mode: the stock does not go in, or rattles once it is in.

**Interface.** The part mates two pieces of stock that were not made to meet, or
mates stock to a fastener. The Y's conduit core is this — the conduit carries
load or routes cable, and the printed body is what lets three straps meet it.
Failure mode: the mating surface is thinner than the load through it.

**Augment.** The part adds a feature the stock does not have: a mounting boss, a
cable exit, a label surface, a keyed orientation. Failure mode: the addition
compromises the enclosure or interface it is attached to.

A profile may do all three. Most useful ones do.

## 4. Adding an entry

The order matters, and it is the same order that produced the strap:

1. **Describe the stock**, per §2, with provenance. No geometry yet.
2. **State the fit**, and what happens at both ends of its tolerance.
3. **Generate the section**, and check it against a real sample if one exists.
4. **Only then** write the profile that uses it.

Steps 1 and 2 are where the errors are, and they cost nothing to correct.
Step 4 is where they become expensive.

**Every entry lands additively.** The 123-case oracle is frozen at rev 8.0.0 and
is the only evidence the geometry is right. A new stock entry, a new parameter,
or a new profile must leave all 123 cases building byte-identically, which means
new parameters default to *absent*. If adding a capability perturbs one recorded
value, the implementation is wrong — not the oracle. See `ACCEPTANCE.md`.

## 5. What stays out

**No standards tables in the geometry layer.** The map from `EMT 1/2"` to an
outside diameter is data about the world. It gets revised, it varies by region
and by decade, and a wrong entry in it is a wrong part. It belongs where it can
be corrected and cited without touching geometry, and where a person can see
which number was used. Geometry takes millimetres.

**No structural claims.** `PRECISION.md` §7 governs and is scope-locked. That a
part encloses a conduit says nothing about what the assembly carries. Measure and
attest; never adjudicate.

**No inferred stock — but never a blocked build.** A dimension nobody has
attributed is recorded as unattributed and used. What must not happen is a
number appearing from nowhere and looking authoritative.

This was originally written as a refusal — an entry without provenance does
not ship — and implemented as a constructor that raised. That was wrong, and
wrong against §1: a compiler whose subject is arbitrary COTS hardware cannot
require the hardware to be catalogued before it will run. **Every dimension is
an input at every run.** Type what your caliper reads, print, measure the
print, adjust, print again. That loop is the tool.

Provenance therefore travels with the output instead of guarding the entrance,
and a later measurement supersedes an earlier guess.

**No design decisions taken on the operator's behalf.** An interference fit is
a legitimate choice — a conduit core that must not rattle wants one, and PLA
deflects. The compiler computes the interference and reports it. Whether it is
acceptable is not the compiler's call. Measure and attest; never adjudicate.

## 6. Consequence for what exists

`Geo` currently derives its cavity from `strap_width_mm`, `strap_thickness_mm`,
`bundle_count` and `fit_clearance_mm` — a rectangular stock entry, inlined.
Expressing it *through* the stock descriptor rather than beside it is the first
migration, and it is the one that proves the abstraction: if all 123 cases stay
byte-identical with the strap expressed as a catalogue entry, the descriptor is
faithful. If they do not, it is not, and the second entry would have inherited
the flaw.

That migration comes before the conduit core, not after it.