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packages/foundation/data/packer/src/chunked/codec/chunk.md

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+# chunk
+
+## Purpose
+
+Encodes and parses the wire-format v1 chunk in its entirety: `metadata(100) || sig(64) || ciphertext(payloadLen)`. Composes `encodeHeader` / `parseHeader` with byte-level concatenation, length validation, and the canonical signing-payload helper. Every receive path in the packer (chunked unpacker, chunk verifier, mesh transport) calls `parseChunk` before any cryptographic work; every send path calls `encodeChunk` once metadata, signature, and ciphertext are ready.
+
+## Features
+
+- `encodeChunk({ metadata, sig, ciphertext })` returns a single `Uint8Array` of `164 + payloadLen` bytes.
+- `parseChunk(buf)` returns `{ metadata, sig, ciphertext }` with all three fields as zero-copy subarrays of `buf`.
+- `signingPayload(metadataBytes, ciphertext)` returns the canonical bytes the server signs / receivers verify.
+- Length and structural validation lifted from `parseHeader`; chunk-level validation adds the total-length check.
+
+## Usage: Basic Example
+
+```ts
+import { encodeChunk, parseChunk } from "@suaveplan/packer/chunked";
+
+const wire = encodeChunk({
+    metadata,                           // ChunkV1Metadata
+    sig: new Uint8Array(64),            // ed25519 signature
+    ciphertext: new Uint8Array(payloadLen),
+});
+
+const back = parseChunk(wire);
+// back.metadata, back.sig, back.ciphertext
+```
+
+## Usage: Advanced Example
+
+```ts
+import { encodeChunk, parseChunk, signingPayload, encodeHeader } from "@suaveplan/packer/chunked";
+import { ed25519 } from "@noble/curves/ed25519";
+
+// Producer:
+const headerBytes = encodeHeader(metadata);
+const signed = signingPayload(headerBytes, ciphertext);
+const sig = ed25519.sign(signed, serverPrivKey);
+const wire = encodeChunk({ metadata, sig, ciphertext });
+
+// Receiver:
+const { metadata: m, sig: s, ciphertext: c } = parseChunk(wire);
+const signedAgain = signingPayload(encodeHeader(m), c);
+if (!ed25519.verify(s, signedAgain, serverPubKey)) {
+    throw new Error("bad sig");
+}
+```
+
+## API Reference
+
+- **`interface EncodeChunkInput { metadata: ChunkV1Metadata; sig: Uint8Array; ciphertext: Uint8Array }`**
+- **`encodeChunk(input: EncodeChunkInput): Uint8Array`** — total length `164 + payloadLen`. Throws `PackerError(PACKER.INVALID_FORMAT)` on `sig.byteLength !== 64` and `PackerError(PACKER.PAYLOAD_LEN_INVALID)` on ciphertext-length mismatch.
+- **`parseChunk(buf: Uint8Array): ParsedChunkV1`** — validates total length matches `164 + metadata.payloadLen`. Throws on header-parse failures and on length mismatch.
+- **`signingPayload(metadataBytes: Uint8Array, ciphertext: Uint8Array): Uint8Array`** — concatenates the two for ed25519 sign/verify.
+
+## Implementation Notes
+
+`encodeChunk` performs `metadata` field-length validation indirectly via `encodeHeader`, then writes a single fresh `Uint8Array(164 + payloadLen)` with three `set` calls — no intermediate copy of the metadata or ciphertext beyond the final placement. `parseChunk` re-uses `parseHeader`'s zero-copy subarray pattern; the returned struct shares its underlying buffer with the input, so callers must clone if they intend to mutate either side independently. `signingPayload` allocates a fresh buffer because the AEAD-side callers must not mutate the wire bytes after sig generation.