/**
 * Lets `digest-service.ts` know when each user's next digest is due.
 * `digest_schedule` holds, for each user and cadence they receive, the start
 * of the period their next digest covers; it is due once a full period has
 * passed.
 *
 * @module
 */

import type { DatabaseSync } from "node:sqlite";

const DAY_MS = 86_400_000;
// A frozen copy of digest-service's periods: a migration must keep doing
// what it did when it shipped, whatever the service later becomes.
const PERIOD_DAYS = { weekly: 7, monthly: 30 } as const;

/** Spreads `userId` over `[0, days)` days, the same way each run. */
function offsetOf(userId: string, days: number): number {
  let hash = 2166136261;
  for (const ch of userId) hash = Math.imul(hash ^ ch.charCodeAt(0), 16777619);
  return ((hash >>> 0) % (days * 24)) * 3_600_000;
}

/**
 * Run once by `runMigrations`. Every user present gets both cadences, with a
 * first period that ends at some hour of the coming week or month, picked
 * from their id: the digests start trickling out after the deploy instead of
 * all leaving a week later. Users created later are scheduled by the service
 * when it first sees them.
 */
export function up(db: DatabaseSync): void {
  db.exec(`
    CREATE TABLE IF NOT EXISTS digest_schedule (
      user_id      TEXT NOT NULL,
      cadence      TEXT NOT NULL,
      window_start TEXT NOT NULL,
      PRIMARY KEY (user_id, cadence),
      FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
    );
  `);
  const users = db.prepare(`SELECT id FROM users;`).all() as { id: string }[];
  const insert = db.prepare(
    `INSERT OR IGNORE INTO digest_schedule (user_id, cadence, window_start) VALUES (?, ?, ?);`,
  );
  const now = Date.now();
  for (const { id } of users) {
    for (const [cadence, days] of Object.entries(PERIOD_DAYS)) {
      const start = now - days * DAY_MS + offsetOf(id, days);
      insert.run(id, cadence, new Date(start).toISOString());
    }
  }
}