export const REPLAY_JSON_MIME_TYPE = "application/vnd.proctorkit.replay+json";
export const REPLAY_GZIP_MIME_TYPE = "application/vnd.proctorkit.replay+json+gzip";

/** Encode one bounded replay window, preferring native streaming gzip. */
export async function encodeReplayEvents(events: readonly unknown[]): Promise<Blob> {
  return encodeSerializedReplayEvents(events.map((event) => JSON.stringify(event)));
}

/** Encode already-serialized events without parsing and stringifying a second
 * in-memory copy of the replay window. */
export async function encodeSerializedReplayEvents(
  serializedEvents: readonly string[],
): Promise<Blob> {
  const parts: BlobPart[] = ["["];
  serializedEvents.forEach((event, index) => {
    if (index > 0) parts.push(",");
    parts.push(event);
  });
  parts.push("]");
  const input = new Blob(parts, { type: REPLAY_JSON_MIME_TYPE });
  if (typeof CompressionStream === "undefined") {
    return input;
  }
  const compressed = input.stream().pipeThrough(new CompressionStream("gzip"));
  const bytes = await new Response(compressed).arrayBuffer();
  return new Blob([bytes], { type: REPLAY_GZIP_MIME_TYPE });
}

/** Decode a replay chunk. Exported for SDK diagnostics and codec tests. */
export async function decodeReplayChunk(blob: Blob): Promise<unknown[]> {
  let text: string;
  if (blob.type === REPLAY_GZIP_MIME_TYPE) {
    if (typeof DecompressionStream === "undefined") {
      throw new Error("This browser cannot decompress assessment replay data");
    }
    const decompressed = blob.stream().pipeThrough(new DecompressionStream("gzip"));
    text = await new Response(decompressed).text();
  } else {
    text = await blob.text();
  }
  const parsed: unknown = JSON.parse(text);
  if (!Array.isArray(parsed)) {
    throw new Error("Assessment replay chunk must contain an event array");
  }
  return parsed;
}