Source

// What the two devices say to each other over the WebRTC data channel. Text messages are JSON;
// binary messages are the bytes of the file being sent, in order. Nothing here touches a server.

export interface FileInfo {
  /** Position in the sender's list; stays the same when the sender adds more files. */
  i: number;
  name: string;
  size: number;
  type: string;
  /** A folder, zipped on the sender's device as it is sent. */
  zip?: boolean;
}

/** Sender to receiver. */
export type FromSender =
  | { t: "files"; from: string; files: FileInfo[] }
  // Every byte of file i from the requested offset has been sent.
  | { t: "end"; i: number }
  // SHA-256 of file i, in hex, worked out on the sender's device.
  | { t: "sum"; i: number; hex: string }
  | { t: "error"; i: number; message: string }
  | { t: "bye" };

/** Receiver to sender. */
export type FromReceiver =
  // Send file i, starting at byte `from` (above 0 after a dropped connection).
  | { t: "get"; i: number; from: number }
  // Bytes of file i saved so far: the sender stays at most a window ahead of this.
  | { t: "ack"; i: number; at: number }
  | { t: "sum"; i: number }
  // Progress, so the sender can show it: bytes received and wanted, files done and wanted.
  | { t: "progress"; got: number; total: number; done: number; count: number };

/** Bytes per data channel message. 64 KiB works between every browser. */
export const CHUNK = 64 * 1024;
/** How far the sender may run ahead of what the receiver has saved. */
export const WINDOW = 16 * 1024 * 1024;

export function parse<T>(raw: unknown): T | null {
  if (typeof raw !== "string") return null;
  try {
    const m = JSON.parse(raw);
    return m && typeof m.t === "string" ? (m as T) : null;
  } catch {
    return null;
  }
}