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+# Penrose — Standalone libraries: survey and structure
+
+Research note, 2026-07-04. Idea parked, not started. The question: extract the
+engine under `src/app/x/penrose/explore/lib` into standalone libraries for
+TypeScript, Python, and Go. This note records the ecosystem survey and the
+structure we would use, so the decision is already made when we pick it up.
+
+## What exists today (surveyed 2026-07-04)
+
+Every implementation we found answers "give me polygons to draw." None treats
+the tiling as an addressable mathematical object.
+
+**TypeScript / JavaScript.** No serious published library.
+
+- [patterncollider](https://github.com/aatishb/patterncollider) — the best of
+  them, a real de Bruijn multigrid, but a Vue app, not a package.
+- Demo repos: [apaleyes/penrose-tiling](https://github.com/apaleyes/penrose-tiling)
+  (triangle subdivision), [guinetik/penrose-js](https://github.com/guinetik/penrose-js),
+  [rictorlome/penrose](https://github.com/rictorlome/penrose) (p5.js),
+  [penrose-ii](https://fanyangxyz.github.io/penrose-ii/) (pentagrid demo).
+- Bare `penrose` on npm is the CMU diagramming language
+  ([@penrose/core](https://www.npmjs.com/package/@penrose/core)); a tiling
+  package must use a scope or another name.
+
+**Python.** The most mature ecosystem, still generation-only.
+
+- [pynrose](https://pypi.org/project/pynrose/) — published package, de Bruijn
+  method, P3, SVG out. Closest prior art; no addressing.
+- [xnx/penrose](https://github.com/xnx/penrose) — subdivision SVG generator.
+- [joshcol9232/tiling](https://github.com/joshcol9232/tiling) (`dualgrid`) —
+  de Bruijn in arbitrary dimensions, STL export.
+- [pywonderland](https://github.com/neozhaoliang/pywonderland) — reference
+  implementations, educational gallery, not a library.
+- Adjacent: [gglouser/cut-and-project-tiling](https://github.com/gglouser/cut-and-project-tiling)
+  (Rust/WASM, arbitrary cutting planes).
+
+**Go.** Green field. Nothing library-shaped found; toys and an esolang
+([Painrose](https://github.com/mousetail/Painrose)).
+
+## The gap our engine fills
+
+1. **Exact ℤ⁵ addressing.** Tile identity `[n; j, k]`, stable coordinates, a
+   wire codec. Nobody ships coordinates as API.
+2. **O(visible) viewport enumeration.** The fixed-gamma pentagrid enumerator,
+   verified key-for-key against the cut-and-project oracle, so unbounded
+   panning is a library feature.
+3. **The matching rule as computed, tested data.** Arc decoration with
+   shared-edge continuity tests; unique patch decoration by propagation.
+4. **The substitution ↔ pentagrid bridge.** The fold (D'Andrea Thm 5.16, see
+   `09-the-fold.md`) as working code. Not available anywhere as a library.
+
+## Structure, decided in advance
+
+One monorepo. **Go gets the repo root**, because Go is the only ecosystem
+where identity = repository layout (a module in a subdirectory carries a
+`go/vX.Y.Z` tag prefix forever; npm and PyPI publish from anywhere).
+
+```
+github.com/funcimp/penrose        (or n2p5/penrose; org undecided)
+├── go.mod        module github.com/funcimp/penrose
+├── *.go          the Go implementation
+├── spec/         spec.md + golden test vectors (JSON)
+├── ts/           npm: @funcimp/penrose (scope sidesteps the CMU collision)
+└── python/       PyPI: needs a distinct name (pynrose is taken)
+```
+
+**The spec vectors are the real product of the monorepo.** Golden JSON
+generated from the tested TS engine: viewport → expected face keys, codec
+round-trips, arc crossing points, oracle patches. Each port's test suite
+consumes the same files, so "port to Python" becomes make-the-vectors-pass,
+not re-derive-the-math. Version the vectors; lockstep the packages' minor
+versions to the vector version so parity is checkable.
+
+**Org and import-path stability.** If the repo ever moves, Go consumers break.
+Either pick the org to keep (funcimp leans right: the research and citations
+live here), or decouple with a vanity import path (`func.lol/penrose` via a
+`go-import` meta tag served from the site) at the cost of a little
+infrastructure.
+
+**API surface (instinct, to be designed twice).** Four seams and nothing else
+exported: enumerate (viewport → faces), address (face ↔ `[n; j, k]` ↔ wire
+codec), decorate (matching-rule arcs), deflate/inflate. The `explore/lib`
+modules are already pure functions with zero runtime dependencies, so the TS
+extraction is mostly moving files and writing the README.