# ax25.js — AX.25 + HDLC utilities (TypeScript)

This repository provides low-level utilities for AX.25 frame construction/parsing and HDLC framing used in amateur radio applications. It's intentionally small and focused on framing, FCS and control-field helpers.

## Highlights

- `Address` – create and parse callsigns, produce 7-byte AX.25 address fields.
- `Frame` – `InformationFrame`, `SupervisoryFrame`, `UnnumberedFrame` and `Frame.fromBytes()` parsing.
- Control builders: `buildIControl`, `buildSControl`, `buildUControl` (and extended 2‑byte builders).
- Encoders: `encodeFrame`, convenience `encodeAx25` / alias `encodeAX25`.
- HDLC: `encodeHDLC`, `escapeBuffer`, `unescapeBuffer`, `computeFcs` / `crc16Ccitt`, `verifyAndStripFcs`.
- Convenience stream helpers: `toHDLC`, `toWire`, `toWireStream`.
- `FX25` class: detection heuristics; full FEC decoding (Reed–Solomon) is a TODO.

## Install & tests

```bash
npm install
npm test
```

## Examples

Note: during development you can import directly from `src/*`. When published, import from the package entrypoint.

### Address

```ts
import { Address } from './src/address';

const a = Address.fromString('N0CALL-0');
console.log(a.toString()); // N0CALL-0
```

### Build an AX.25 UI payload and HDLC-wrap it

```ts
import { Address } from './src/address';
import { UnnumberedFrame, encodeFrame, encodeAX25, toHDLC } from './src/frame';

const src = Address.fromString('SRC-1');
const dst = Address.fromString('DST-0');
const info = new TextEncoder().encode('payload');

// quick builder
const ax = encodeAX25(dst, src, info, { ui: true });

// or construct Frame and HDLC-wrap
const f = new UnnumberedFrame(src, dst, 0x03, 'UI', 0, info);
const hdlc = toHDLC(f); // includes FCS, escapes and flags
```

### Parsing

```ts
import { Frame } from './src/frame';

const frame = Frame.fromBytes(ax);
console.log(frame.toString());
```

## HDLC & FCS

- `computeFcs(payload)` — CRC-16-CCITT with AX.25/X.25 inversion semantics.
- `encodeHDLC(payload, { includeFcs: true })` — wraps payload with 0x7E flags, appends FCS and escapes special bytes.
- `verifyAndStripFcs(buf)` — validates and strips a little-endian 2-byte FCS from a buffer.

## Testing and style

- Tests live in `test/` and are run with Vitest. They follow `describe('Class.method', ...)` naming.
- Keep tests targeted and add coverage for new behavior.