# AGENTS.md

This file provides guidance to Codex (Codex.ai/code) when working with code in this repository.

## What this is

A collection of Matter commissioning QR decoders (the `MT:` URI format used by
Matter/CHIP devices), plus a standalone bash decoder for Apple **HomeKit**
setup codes (the `X-HM://` payload — a different on-wire format, in
`homekit-decode.sh`). For Matter, a user runs `matter-decode.sh` with an `MT:`
URI (as a positional argument or on stdin); it decodes the 84-bit payload and
prints the fields — version, vendor/product IDs, commissioning flow, discovery
capabilities, discriminator, passcode, and the derived manual pairing code. Key
files:

- `matter-decode.sh` — the pure-bash CLI Matter decoder. Takes an `MT:` URI as an
  argument or on stdin and prints the decoded fields. It has no camera/QR-scanning
   (that was the only feature the former browser app added, and it is gone).
- `homekit-decode.sh` — a standalone pure-bash decoder for Apple **HomeKit**
  setup codes (the `X-HM://` payload an accessory shows on first setup — a
  *different* format from Matter's `MT:`). No companion TSV (the accessory-category
  table is inlined). See the HomeKit setup-code decode section below.
- `manufacturers.tsv` — `vendor ID → name` table (~900 rows), consumed by
   `matter-decode.sh`; optional (a missing file only drops the vendor-name lookup).
- `gen-manufacturers.py` — Python 3 stdlib script that regenerates
   `manufacturers.tsv`. Default source is the upstream `manufacturers.xml`
   **fetched over the network** (via `curl`; a GitHub `blob/` URL is auto-rewritten
   to its `raw.githubusercontent.com` form). A positional arg overrides with
   another URL or a local XML file. Reproduces the committed TSV byte-for-byte.
- `products.tsv` — 3-column `vendor ID → product ID → name` table, consumed by
   `matter-decode.sh`'s `get_product_name()`: when the decoded vendor ID (or an
   equivalent, see `vendor-equivalents.tsv`) is present, the product ID is
    resolved to a name and printed on the Product ID line. LIFX's rows are keyed
   under `4870`. Optional — a missing file only drops the product-name lookup.
- `vendor-equivalents.tsv` — 2-column `alias vendor ID → canonical vendor ID`
   table, hand-edited, recording companies that are the same after a
    merger/acquisition (e.g. Feit `5155` → LIFX `4870`). Used by
   `matter-decode.sh` so a product resolves under either ID and the vendor name
    is annotated `(a.k.a. ...)`. `#`-comment lines are ignored. Optional.
- `gen-lifx-products.py` — Python 3 stdlib script that regenerates LIFX's rows
   (vendor `4870`) in `products.tsv`. Default source is LIFX's upstream
   `products.json` **fetched over the network** (via `curl`, falling back to
   `urllib`); a positional arg overrides with another URL or a local `.json`
   file. It **merges** LIFX's rows into `products.tsv` (dropping LIFX's old
   rows, preserving every other vendor's), so re-running it is safe alongside
   other vendors' generators. Caveat: `products.json` carries only a `pid` per
   product (no separate Matter/CHIP product-id field), so the lookup assumes
   LIFX's Matter product ID equals their `pid`.
- `README.md` — user-facing docs; this file documents the internals.

## Commands

There is **no build, test, lint, or package step** — this is not a build project.
The two decoders are standalone bash scripts and the two `gen-*.py` are Python 3
stdlib regenerators; nothing is compiled or bundled.

- Run the Matter decoder: `./matter-decode.sh "MT:..."`, or pipe it:
   `echo "MT:..." | ./matter-decode.sh`.
- Run the HomeKit decoder: `./homekit-decode.sh [code]` (with no argument it
  decodes the demo payload `005KEOUCE`).
- Regenerate `manufacturers.tsv`: `python3 gen-manufacturers.py` pulls the
  pinned upstream `manufacturers.xml` over the network; pass a URL or a local
   `.xml` file to use a different source.
- Regenerate LIFX's `products.tsv` rows: `python3 gen-lifx-products.py` (network
  by default; a positional URL or local `.json` overrides).
- The decoders read their companion TSVs (`manufacturers.tsv`, `products.tsv`,
   `vendor-equivalents.tsv`) from their own directory; a missing TSV drops that
  extra lookup — it is never an error.
- Pure bash (3.x+); the `gen-*.py` scripts need only the Python 3 standard
  library.

## Architecture

The decode logic lives directly in the bash scripts — there is no shared library,
no embedded data map, and no build output. There are two decoders, and they
decode **two different on-wire formats**, so they are independent and should not
be unified.

### matter-decode.sh — the Matter `MT:` pipeline

`main()` decodes a URI given as an argument or read from stdin, in order:

1. Validate the `MT:` prefix, then take the base38 payload after it.
2. Base38-decode it to bytes. The charset is `0-9A-Z-.` (38 chars). Chunking is
   **greedy**: 5 chars→3 bytes, 4→2, 2→1; a length that doesn't fit those
    groupings errors. Bytes are extracted **little-endian**
    (`value & 0xFF`, then `>>= 8`).
3. Read the 84-bit field layout **LSB-first**. Fixed field order/widths (see the
   "Data Encoding Format" table in `README.md` for the source of truth):
   version(3), vendor ID(16), product ID(16), custom flow(2), discovery caps(8),
   discriminator(12), passcode(27).
4. Display helpers translate the raw numbers: a custom-flow name, discovery
   capabilities (bit flags: SoftAP / BLE / On IP Network), and decimal + `0x`-hex
    (padded to 4) for the vendor/product IDs.

### Vendor / product / equivalence lookups (matter-decode.sh only)

Three optional companion TSVs add names on top of the raw decode; each degrades
silently (a missing file drops its extra, never an error):

- **`manufacturers.tsv`** (`vendor ID \t name`) — resolves the decoded vendor ID
  to a name, printed on the Vendor ID line.
- **`products.tsv`** (`vendor ID \t product ID \t name`) — resolved via
   `get_product_name VENDOR PRODUCT`, which checks the exact vendor first, then
   every equivalent; first match wins.
- **`vendor-equivalents.tsv`** (`alias \t canonical`, `#` lines ignored,
   canonical-form: the canonical id never appears in col 1) — resolved via
   `equivalent_vendor_ids VENDOR`, which returns the whole equivalence class
   (the vendor plus every alias sharing its canonical id, sorted). The *other*
   class members' names are joined and appended to the Vendor ID line as
   `(a.k.a. X, ...)`, so a Feit (`5155`) device reads `Feit Electric Company,
   Inc. (a.k.a. Lifi Labs INC. dba LIFX)` and a LIFX (`4870`) device reads the
   mirror image.

`get_product_name` must `return 0` on a miss: under `set -e`, a non-zero status
in the `pname=$(...)` substitution would abort the script before anything prints.

`gen-lifx-products.py` regenerates LIFX's rows into `products.tsv` by merging
(dropping only vendor `4870`'s rows, preserving others). To add another vendor,
write a sibling `gen-<vendor>-products.py` that parses that vendor's catalog and
merges its rows — keyed under its own vendor ID — into the same `products.tsv`;
then add that vendor's equivalences (if any) to `vendor-equivalents.tsv`.

### Manual pairing code (matter-decode.sh only)

`matter-decode.sh` also prints a **Pairing Code:** line: the Matter *manual
pairing code* — the PIN a commissioning app (Apple Home, Google Home, the Matter
app) asks you to type. It is **not** a raw payload field; it is *derived* from
the passcode and discriminator and, for non-standard commissioning flows, the
vendor/product IDs, then guarded by a Verhoeff check digit. This is a port of
`generateManualPairingCode()` from the standalone "Matter QR Code Tool"
(`chill-uk/matter-qr-app`, `web/app.js`).

Two helpers, defined just above `main()`:

- `verhoeff_check_digit DIGITS` — the Verhoeff check digit over a decimal string
  (the three standard Verhoeff tables are inlined; digits are processed
  right-to-left, as the reference does).
- `compute_pairing_code VENDOR PRODUCT FLOW DISCRIMINATOR PASSCODE` — assembles
  the code. The 12-bit discriminator contributes its top 4 bits; the 27-bit
  passcode is split into its low 14 and high 13 bits; non-standard flows
   (user-intent `1`, custom `2`) append 5-digit vendor and product groups.
  Result: 11 digits for the standard flow, 21 otherwise, plus the check digit.

### HomeKit setup-code decode (homekit-decode.sh)

`homekit-decode.sh` is a *second, independent* decoder — for Apple **HomeKit**
setup codes, the `X-HM://` payload an accessory (or its packaging) shows on first
setup. That is a different on-wire format from Matter's `MT:`, so this is
**not** a port of `matter-decode.sh`'s pipeline; it is standalone, with no
companion TSV (the accessory-category table is inlined). It sits alongside
`matter-decode.sh` as the repo's other CLI decoder.

Input is an optional positional argument, defaulting to the demo payload
`005KEOUCE`. A leading `X-HM://` is stripped if present. The first 9 characters
are the fixed parameter block; anything after them is a variable-length **setup
id**, carried through verbatim (it is not part of the numeric block).

Unlike Matter's **base38** payload (charset `0-9 A-Z - .`, 38 symbols), the
HomeKit block is **base36** (`0-9 A-Z`, 36 symbols): each char does
`val = val*36 + digit`, and an out-of-range character is a hard error (to
stderr, exit 1). `val` is then sliced **LSB-first** into the HomeKit layout:

    pin(27) | flag(4) | categoryId(8) | reserved(4) | version(3)

- `flag` is a 4-bit sub-flag set — `1`=NFC, `2`=IP, `4`=BLE, `8`=WAC/MFi —
  printed as True/False lines.
- `categoryId` is resolved to its HAP `homekit_accessory_category_*` name (e.g.
   `5` → "Lightbulb", `8` → "Switch") via an inlined `case`; an id not in the
  table prints "unknown" (id `25` is reserved by HomeKit, so it is unknown by
  design, and "Heater" corrects a "Header" typo in the source table). A `case`
  rather than an associative array is used on purpose so the script runs on the
  stock macOS bash 3.2 that `#!/usr/bin/env bash` resolves to.
- The decoded fields are packed back into a 9-char block (mirroring
   `composeSetupUri`'s stepwise packing) and printed as a reconstructed
   `X-HM://…` payload, so the output is a complete record the barcode can be
   rebuilt from.

This is the **corrected** bit layout — the one `composeSetupUri` in the
`homekit-code` package actually encodes — rather than a misaligned earlier one.
The header comments in `homekit-decode.sh` record each fix (the categoryId / flag
/ version positions, and the dropped "Thread" misread of categoryId bits).

## Gotchas

- `matter-decode.sh` (Matter `MT:`, **base38**) and `homekit-decode.sh`
   (HomeKit `X-HM://`, **base36**) decode *different* on-wire formats. Don't try
  to unify the two decoders; when editing `homekit-decode.sh`, keep the
  corrected bit layout (`pin / flag / categoryId / reserved / version`) as the
  source of truth.
- `manufacturers.tsv` is the *only* copy of the vendor map now. Regenerate it with
   `gen-manufacturers.py`: the default source is the **upstream `manufacturers.xml`
   fetched over the network** (via `curl`; a GitHub `blob/` URL is auto-rewritten
   to its `raw.githubusercontent.com` form), pinned to the same commit the
   committed TSV was generated from, so it reproduces the TSV byte-for-byte. A
   positional arg overrides with another URL or a local XML file.
- The product-name lookup and vendor equivalences live **only in
   `matter-decode.sh`** — `products.tsv` and `vendor-equivalents.tsv` are
   optional and degrade silently (a missing file means no name / no equivalent,
   never an error). `vendor-equivalents.tsv` is canonical-form, one hop: the
   canonical id must not itself appear in col 1. `get_product_name` must
   `return 0` on a miss or `set -e` aborts the decode.