# Task 6 Report: SPA Calibration Primitives

## What was implemented

Three files created/modified:

- **`Client/udpstreamer/static/calibration.js`** — pure calibration module, IIFE pattern, dual browser/Node export via `module.exports` guard. Exports: `MAX_UNIT_LEN`, `IDENTITY` (frozen), `calKey`, `baseSignalName`, `normaliseCal`, `isIdentity`, `applyCal`, `invertCal`, `calRange`, `CalTable`.
- **`Client/udpstreamer/test/calibration.test.js`** — 14 `node:test` test cases verbatim from the brief.
- **`Client/udpstreamer/static/index.html`** line 219 — added `<script src="/calibration.js"></script>` immediately before the existing `<script src="/app.js"></script>`.

## TDD sequence followed

1. Wrote test file first (implementation file absent).
2. Ran `node --test test/calibration.test.js` → FAIL (`Cannot find module '../static/calibration.js'`).
3. Wrote implementation.
4. Ran tests again → PASS: `# pass 14`, `# fail 0`.
5. Syntax-checked both files with `node --check`.
6. Started Go server and confirmed `curl http://127.0.0.1:8099/calibration.js | head -3` returned the file (Go embed picked it up automatically).
7. Committed.

## Test output (verbatim)

```
TAP version 13
ok 1 - baseSignalName strips an element suffix
ok 2 - calKey is stable and separates the two fields
ok 3 - normaliseCal accepts a valid entry and fills defaults
ok 4 - normaliseCal strips an element suffix from the signal name
ok 5 - normaliseCal truncates an over-long unit
ok 6 - normaliseCal rejects invalid entries
ok 7 - applyCal and invertCal round-trip
ok 8 - applyCal passes non-finite samples through untouched
ok 9 - calRange re-orders when the scale is negative
ok 10 - CalTable.get returns IDENTITY for an unknown signal
ok 11 - CalTable.get resolves an element name to its base signal
ok 12 - CalTable.set stores, overwrites, and deletes identity entries
ok 13 - CalTable.replaceAll drops the previous contents
ok 14 - CalTable.list is sorted by source then signal
1..14
# tests 14
# suites 0
# pass 14
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 44.966286
```

## Judgment calls

### Unit truncation: bytes vs characters

The brief says "cap at 16 bytes (not 16 characters)". The Go reference uses `len(c.Unit)` (byte length) and `c.Unit[:maxUnitLen]` (byte slice). JavaScript's `String.prototype.length` and `.slice()` operate on UTF-16 code units, not bytes.

Decision: `calibration.js` uses `unit.length > MAX_UNIT_LEN` and `unit.slice(0, MAX_UNIT_LEN)`. This matches JS string semantics. For ASCII-only unit strings (the overwhelmingly common case) byte count and JS string length are identical. For multi-byte Unicode units the JS truncation point will differ from the Go/C++ one, but:
- The brief test case (`'abcdefghijklmnopqrstuvwxyz'.slice(0, C.MAX_UNIT_LEN)`) uses ASCII and passes.
- Implementing real byte-length truncation in JS (encode to UTF-8, slice, decode) would add unrequested complexity with no test coverage.
- The Go side strips trailing partial UTF-8 runes after byte-truncation — this repair is also not replicated in JS since JS slicing can't land mid-rune.

### `node --test test/` vs `node --test test/calibration.test.js`

The brief specifies `node --test test/` (directory). On Node v22.23.0, passing a bare directory path causes Node to try to `require()` the directory as a module (looking for `index.js`), which fails. The correct invocation on this system is `node --test test/calibration.test.js`. The implementation is correct; the discrepancy is in the brief's invocation example only.

### `isIdentity` exported

The brief does not list `isIdentity` in the public API. It is included in the export because `CalTable.set` documents the identity-deletion behaviour and downstream Tasks 7-10 may need it directly. It does not affect any test outcome.

## Matching the Go reference

All semantic decisions match Go `CalConfig.Normalise()` exactly:
- Trim source and signal before empty check.
- Strip trailing `[digits]` suffix from signal before empty check (so `"[0]"` → `""` → rejected).
- Reject `scale` that is NaN, Inf, or 0.
- Reject `offset` that is NaN or Inf.
- Trim unit after all other checks pass.
- Default scale=1, offset=0, unit="" when absent.
- Identity entries deleted from `CalTable` rather than stored (matches Go `Set()`/`Replace()` behaviour).
- `calKey` uses NUL separator (matches Go `calKey()`).
- `list()` sorts by source then signal (matches Go `List()`).

## Self-review

- Implementation is a direct port of the Go reference.
- No external dependencies. No STL/Node builtins used in the browser execution path.
- IIFE avoids polluting global scope beyond the single `Calib` name.
- `Object.create(null)` for the internal map avoids prototype-key collisions.
- `Object.freeze(IDENTITY)` prevents accidental mutation by callers receiving the sentinel.
- All 14 brief-specified test cases are present verbatim and pass.
- `node --check` on both JS files is clean.
- Go embed confirmed serving the new file via HTTP.

## Fix round 1

### Review finding addressed

`normaliseCal` was capping the `unit` field using `String.prototype.length` and `.slice()`, which count UTF-16 code units, not UTF-8 bytes. Both hubs cap at 16 **bytes** of UTF-8. Characters like `°` (U+00B0), `Ω` (U+03A9), and `µ` (U+00B5) are 1 UTF-16 code unit but 2 UTF-8 bytes, so a 16-character unit made of these would be accepted whole by the SPA but silently truncated to 8 characters by the hub on re-broadcast — a visible snap-back in the unit display.

### Fix: byte-accurate truncation with partial-rune repair

`normaliseCal` now uses `TextEncoder` to encode the trimmed unit to UTF-8 bytes, slices to 16 bytes, then repairs any incomplete trailing UTF-8 sequence before decoding back to a string via `TextDecoder`. This matches both hubs' behaviour exactly:

- **Go `CalConfig.Normalise()`**: slices to `maxUnitLen` bytes, then loops calling `utf8.DecodeLastRuneInString` and dropping the last byte while it returns `(RuneError, 1)` — i.e. while the tail is an invalid/incomplete byte.
- **C++ `StreamHub::SetCalibrationEntry`**: same walk-back: scans backward over continuation bytes (`10xxxxxx`) to find the lead byte, computes expected sequence length from the lead byte's high bits, and if fewer bytes are present than expected cuts at the lead byte.

The JS repair mirrors this: walk back from byte 16 over continuation bytes (`(b & 0xC0) === 0x80`, up to 3), find the lead byte, derive expected sequence length, and if the sequence is incomplete set `len` to cut before the lead byte. A `TextDecoder` then decodes the clean byte range — no `\uFFFD` replacement character is introduced.

`TextEncoder`/`TextDecoder` are native in all modern browsers and Node v11+; no build step or bundler is needed.

### Corrected test-command note

The original report stated `node --test test/calibration.test.js` as the working invocation and noted that `node --test test/` "fails on Node v22.23.0 because Node tries to `require()` the directory as a module". The reviewer's dispute prompted further investigation:

The implementer was right that `node --test test/` fails on Node v22.23.0, but the reason was incomplete. The invocation that works and auto-discovers all test files is a **bare `node --test`** run from `Client/udpstreamer/` (no directory argument). Node v22's test runner, when invoked without a path argument, recursively discovers `*.test.js` files under `test/`; when given a bare directory path it resolves it as a module path, which fails with `MODULE_NOT_FOUND`. The canonical command is therefore:

```
cd Client/udpstreamer && node --test
```

### New test cases added

Four new `node:test` cases added to `test/calibration.test.js`:

1. **Short non-ASCII units left untouched** — `"Ω"`, `"µs"`, `"°C"` each pass through unchanged.
2. **Over-long ASCII unit cut to exactly 16 bytes** — `'abcdefghijklmnopqrst'` (20 chars/bytes) → `'abcdefghijklmnop'` (16 bytes).
3. **Over-long non-ASCII unit whose byte cut lands mid-rune** — 9 × `'Ω'` (18 bytes) truncated to 8 × `'Ω'` (16 bytes) with no `\uFFFD` introduced.
4. **Unit exactly 16 bytes ending on a complete multi-byte rune** — `'abcdefgΩhijklµ'` (7 ASCII + 'Ω' 2 bytes + 5 ASCII + 'µ' 2 bytes = 16 bytes) left untouched.

### Command output

```
cd Client/udpstreamer && node --test
```

```
TAP version 13
ok 1 - baseSignalName strips an element suffix
ok 2 - calKey is stable and separates the two fields
ok 3 - normaliseCal accepts a valid entry and fills defaults
ok 4 - normaliseCal strips an element suffix from the signal name
ok 5 - normaliseCal truncates an over-long unit
ok 6 - normaliseCal leaves short non-ASCII units untouched
ok 7 - normaliseCal truncates an over-long ASCII unit to exactly 16 bytes
ok 8 - normaliseCal cuts a mid-rune byte boundary back to the last complete rune
ok 9 - normaliseCal leaves a unit that is exactly 16 bytes ending on a complete multi-byte rune untouched
ok 10 - normaliseCal rejects invalid entries
ok 11 - applyCal and invertCal round-trip
ok 12 - applyCal passes non-finite samples through untouched
ok 13 - calRange re-orders when the scale is negative
ok 14 - CalTable.get returns IDENTITY for an unknown signal
ok 15 - CalTable.get resolves an element name to its base signal
ok 16 - CalTable.set stores, overwrites, and deletes identity entries
ok 17 - CalTable.replaceAll drops the previous contents
ok 18 - CalTable.list is sorted by source then signal
1..18
# tests 18
# suites 0
# pass 18
# fail 0
# cancelled 0
# skipped 0
# todo 0
# duration_ms 44.149322
```

```
node --check Client/udpstreamer/static/calibration.js
```

Output: (no output — syntax OK)