import type { SyncMessage } from "./sync-types";

const textEncoder = new TextEncoder();
const textDecoder = new TextDecoder("utf-8", { fatal: true });

export function encodeSyncMessage(message: SyncMessage): string {
  return base64urlEncode(textEncoder.encode(JSON.stringify(message)));
}

export function decodeSyncMessage(data: string): SyncMessage {
  try {
    const raw = textDecoder.decode(base64urlDecode(data));
    const parsed = JSON.parse(raw) as unknown;
    if (isSyncMessage(parsed)) {
      return parsed;
    }
  } catch {
    // Normalize malformed transport payloads so callers do not branch on parser details.
  }
  throw new Error("invalid sync message");
}

export function isSyncMessage(value: unknown): value is SyncMessage {
  if (!isRecord(value) || typeof value.type !== "string") return false;

  switch (value.type) {
    case "SYNC_VAULT_SEND":
      return (
        isString(value.vault_a) &&
        isString(value.vault_b) &&
        isFiniteNumber(value.sent_at)
      );
    case "SYNC_VAULT_ACK":
      return isFiniteNumber(value.received_at);
    case "SYNC_ERROR":
      return isString(value.reason);
    case "PAIR_REQUEST":
      return isString(value.public_key_hex) && isString(value.device_name);
    case "PAIR_ACCEPT":
      return isString(value.public_key_hex) && isString(value.device_name);
    case "PAIR_REJECT":
      return isString(value.reason);
    default:
      return false;
  }
}

function isRecord(value: unknown): value is Record<string, unknown> {
  return Boolean(value) && typeof value === "object" && !Array.isArray(value);
}

function isString(value: unknown): value is string {
  return typeof value === "string";
}

function isFiniteNumber(value: unknown): value is number {
  return typeof value === "number" && Number.isFinite(value);
}

function base64urlEncode(bytes: Uint8Array): string {
  let binary = "";
  for (const byte of bytes) binary += String.fromCharCode(byte);
  return btoa(binary).replace(/\+/g, "-").replace(/\//g, "_").replace(/=+$/g, "");
}

function base64urlDecode(value: string): Uint8Array {
  if (!/^[A-Za-z0-9_-]*$/.test(value) || value.length % 4 === 1) {
    throw new Error("invalid sync message");
  }
  const padded = value
    .replace(/-/g, "+")
    .replace(/_/g, "/")
    .padEnd(Math.ceil(value.length / 4) * 4, "=");
  const binary = atob(padded);
  const out = new Uint8Array(binary.length);
  for (let i = 0; i < binary.length; i += 1) out[i] = binary.charCodeAt(i);
  return out;
}