import { BadRequestException } from "@nestjs/common";

interface CursorPayload {
  priority: number;
  createdAt: Date;
  id: string;
}

export function decodeCursor(cursor: string): CursorPayload {
  try {
    const decoded = Buffer.from(cursor, "base64").toString("utf-8");
    const [priorityStr, timestamp, id] = decoded.split("|");
    if (!priorityStr || !timestamp || !id) throw new Error("Invalid format");
    const priority = parseInt(priorityStr, 10);
    if (isNaN(priority)) throw new Error("Priority not a number");
    const createdAt = new Date(timestamp);
    if (isNaN(createdAt.getTime())) throw new Error("Invalid date");
    return { priority, createdAt, id };
  } catch {
    throw new BadRequestException(
      "Invalid cursor. Expected base64‑encoded string containing priority, ISO date and id (separated by '|').",
    );
  }
}

export function encodeCursor(task: { priority: number; createdAt: Date; id: string }): string {
  return Buffer.from(`${task.priority}|${task.createdAt.toISOString()}|${task.id}`).toString("base64");
}