// Sync identity — long-term X25519 keypair tied to the vault, not the
// device installation.
//
//   sync_identity_keypair = X25519-keygen(HKDF(master_key,
//                              salt="peach-sync-identity-v1",
//                              info=device_id))
//
//   vault_id = first 8 bytes of BLAKE3(master_key || "peach-vault-id"), hex
//
// Determinism is the point: losing and reinstalling Peach against the same
// vault recovers the same sync identity, so paired peers do not need to
// re-pair after a reinstall. The master_key must be the same byte buffer
// that the vault layer holds in memory after unlock — not the user-typed
// password, not the password-derived key.
//
// device_id is a per-installation UUID v4. Two devices that share the
// same vault always derive distinct identities; the device_id is the
// salt that breaks the symmetry.

import { HKDF } from "@stablelib/hkdf";
import { SHA256 } from "@stablelib/sha256";
import { generateKeyPairFromSeed } from "@stablelib/x25519";
import { blake3 } from "hash-wasm";

const SYNC_IDENTITY_SALT = new TextEncoder().encode("peach-sync-identity-v1");
const VAULT_ID_INFO = new TextEncoder().encode("peach-vault-id");
const VAULT_ID_HEX_LENGTH = 16; // 8 bytes → 16 hex chars

export interface SyncIdentityKeypair {
  publicKey: Uint8Array; // 32 bytes
  secretKey: Uint8Array; // 32 bytes — sensitive; treat as key material
}

/**
 * Derive the deterministic sync identity keypair for a vault on a given
 * installation. Always returns the same keypair for the same
 * (masterKey, deviceId) pair across runs and reinstalls.
 */
export function deriveSyncIdentityKeypair(
  masterKey: Uint8Array,
  deviceId: string,
): SyncIdentityKeypair {
  if (masterKey.length === 0) throw new Error("masterKey must not be empty");
  if (!deviceId) throw new Error("deviceId must not be empty");
  const info = new TextEncoder().encode(deviceId);
  // HKDF-SHA256(salt="peach-sync-identity-v1", ikm=master_key, info=device_id)
  // → 32 bytes used directly as the X25519 secret key seed.
  const seed = new HKDF(SHA256, masterKey, SYNC_IDENTITY_SALT, info).expand(32);
  const kp = generateKeyPairFromSeed(seed);
  return { publicKey: kp.publicKey, secretKey: kp.secretKey };
}

/**
 * Stable per-vault identifier carried in the mDNS TXT record and the
 * first WebSocket handshake. Receiver rejects the connection if its
 * own vault_id differs — prevents a paired-but-different-vault peer
 * from corrupting the registry.
 */
export async function computeVaultId(masterKey: Uint8Array): Promise<string> {
  if (masterKey.length === 0) throw new Error("masterKey must not be empty");
  const buf = new Uint8Array(masterKey.length + VAULT_ID_INFO.length);
  buf.set(masterKey, 0);
  buf.set(VAULT_ID_INFO, masterKey.length);
  const fullHex = await blake3(buf);
  return fullHex.slice(0, VAULT_ID_HEX_LENGTH);
}

export function pubkeyToHex(pk: Uint8Array): string {
  let s = "";
  for (let i = 0; i < pk.length; i++) {
    s += pk[i].toString(16).padStart(2, "0");
  }
  return s;
}

export function hexToPubkey(hex: string): Uint8Array {
  if (hex.length % 2 !== 0) throw new Error("hex length must be even");
  const out = new Uint8Array(hex.length / 2);
  for (let i = 0; i < out.length; i++) {
    const byte = parseInt(hex.slice(i * 2, i * 2 + 2), 16);
    if (Number.isNaN(byte)) throw new Error(`invalid hex at offset ${i * 2}`);
    out[i] = byte;
  }
  return out;
}