# header

## Purpose

Encodes and parses the 100-byte metadata portion of a v1 chunk on its own (without sig or ciphertext). Every chunk on the wire begins with this exact byte layout; receivers parse the header first, validate structural invariants, and only then proceed to signature verification and AEAD decryption. The parser enforces every PACKER-CHUNK-* requirement that can be checked from the header alone — version pin, reserved-flags zero, payloadLen non-final/final invariants, in-range chunkId, sane chunkSize.

## Features

- `encodeHeader(meta)` writes a fresh 100-byte big-endian buffer at the locked field offsets.
- `parseHeader(buf)` reads the first 100 bytes and validates structural invariants.
- Returns metadata fields as zero-copy `Uint8Array.subarray` views into the input buffer.
- Throws `PackerError` with stable codes on every failure mode (`INVALID_FORMAT`, `VERSION_UNSUPPORTED`, `FLAGS_RESERVED_NONZERO`, `PAYLOAD_LEN_INVALID`).

## Usage: Basic Example

```ts
import { encodeHeader, parseHeader, CHUNK_VERSION_V1 } from "@suaveplan/packer/chunked";

const meta = {
    version: CHUNK_VERSION_V1,
    fileId: new Uint8Array(32),
    chunkId: 0,
    totalChunks: 4,
    fileSize: 4_000_000n,
    chunkSize: 1_048_576,
    payloadLen: 1_048_576 - 164,
    flags: 0,
    nonce: new Uint8Array(24),
    aeadTag: new Uint8Array(16),
};
const bytes = encodeHeader(meta);
const round = parseHeader(bytes);
```

## Usage: Advanced Example

```ts
import { parseHeader, PackerError, PACKER_CODES } from "@suaveplan/packer/chunked";

function safeParse(buf: Uint8Array) {
    try { return { ok: true as const, meta: parseHeader(buf) }; }
    catch (err) {
        if (err instanceof PackerError) return { ok: false as const, code: err.code };
        throw err;
    }
}

// Reject v2 chunks early without engaging the verify path:
const result = safeParse(receivedBytes);
if (!result.ok && result.code === PACKER_CODES.VERSION_UNSUPPORTED) {
    metrics.versionRejects.inc();
}
```

## API Reference

- **`encodeHeader(meta: ChunkV1Metadata): Uint8Array`** — writes 100 bytes at locked offsets. Throws `PackerError(PACKER.INVALID_FORMAT)` on `fileId.byteLength !== 32`, `nonce.byteLength !== 24`, or `aeadTag.byteLength !== 16`.
- **`parseHeader(buf: Uint8Array): ChunkV1Metadata`** — validates length >= 100, `version === 0x0001`, `flags === 0x0000`, `totalChunks >= 1`, `chunkId < totalChunks`, `chunkSize > 164`, and the non-final / final `payloadLen` invariants. Returns metadata; field byte ranges are zero-copy subarrays of `buf`.

## Implementation Notes

`DataView` is used for all multi-byte numeric reads/writes with `littleEndian: false` to enforce big-endian wire format. `Uint8Array.subarray` (not `slice`) is used for byte-range fields so the parser is zero-allocation in the happy path. The payloadLen invariant uses `BigInt` arithmetic for the final-chunk derivation because `fileSize` is `u64` and may exceed `Number.MAX_SAFE_INTEGER`. The non-final case stays in `number` arithmetic (chunkSize is `u32`).