/**
 * waitlist-promotion — the cron task that honours the "we will email you if a
 * place becomes available" promise.
 *
 * Registrations are stamped `confirmed` OR `waitlisted` at signup
 * (event-registration/middlewares/sanitize-submission.ts). Nothing ever moved
 * a `waitlisted` row to `confirmed` when a seat freed up, so the waitlist was
 * a queue nobody drained. This job does it, first-come-first-served:
 *
 *   1. find every event that still has `waitlisted` registrations;
 *   2. for each, lock the event row (the same serialisation the registration
 *      middleware uses, so a concurrent POST cannot race the count) and
 *      compute how many seats are left below capacity;
 *   3. promote the OLDEST waitlisted registrations (`submittedAt` asc) into
 *      those seats, sending the standard confirmation mail first and flipping
 *      the row to `confirmed` ONLY after the mail was accepted.
 *
 * Semantics are at-least-once, like the reminder job: a row whose mail failed
 * stays `waitlisted` and is retried on the next run. A waitlisted row with no
 * email cannot be told anything, so it is left as-is for staff to deal with.
 *
 * Registered from src/index.ts only when `server.cron.enabled` is true.
 */

import type { Core } from '@strapi/strapi';

import { sendRegistrationConfirmation } from './event-mail';

const EVENT_UID = 'api::event.event';
const REGISTRATION_UID = 'api::event-registration.event-registration';

const LOG_PREFIX = '[waitlist-promotion]';

const PAGE_SIZE = 50;

/** The subset of the event that decides how many seats can be promoted into. */
type EventCapacity = {
  documentId: string;
  capacity: number | null;
};

/** How many places on this event are already taken, mirroring the middleware. */
const countConfirmed = async (strapi: Core.Strapi, documentId: string): Promise<number> =>
  strapi.db.query(REGISTRATION_UID).count({
    where: {
      event: { documentId },
      registrationState: 'confirmed',
    },
  });

/**
 * Promote the oldest waiting registrants of ONE event into its free seats.
 *
 * The lock mirrors `sanitize-submission`: registering and promoting both
 * serialize on the event row, so a concurrent request cannot steal a seat the
 * promotion just counted. Count and updates run inside the same transaction.
 */
async function promoteEvent(
  strapi: Core.Strapi,
  eventDocumentId: string,
): Promise<{ promoted: number; failed: number }> {
  return strapi.db.transaction(async ({ trx }) => {
    await trx('events')
      .select('id')
      .where({ document_id: eventDocumentId })
      .orderBy('id')
      .forUpdate()
      .first();

    const event = (await strapi.documents(EVENT_UID).findFirst({
      filters: { documentId: eventDocumentId },
      status: 'published',
    })) as EventCapacity | null;

    if (!event) {
      return { promoted: 0, failed: 0 };
    }

    const capacity =
      typeof event.capacity === 'number' && event.capacity > 0 ? event.capacity : null;

    // No finite capacity means there is never a seat to free up.
    if (capacity === null) {
      return { promoted: 0, failed: 0 };
    }

    const taken = await countConfirmed(strapi, eventDocumentId);
    let slots = capacity - taken;
    if (slots <= 0) {
      return { promoted: 0, failed: 0 };
    }

    // Oldest waiters first; the page size is a safety bound, the loop stops
    // at `slots` anyway.
    const registrations = await strapi
      .documents(REGISTRATION_UID)
      .findMany({
        filters: {
          event: { documentId: eventDocumentId },
          registrationState: { $eq: 'waitlisted' },
        },
        sort: 'submittedAt:asc',
        pagination: { page: 1, pageSize: PAGE_SIZE },
      });

    let promoted = 0;
    let failed = 0;

    for (const registration of registrations) {
      if (slots <= 0) break;

      const email = (registration as { email?: unknown }).email;
      const fullName = (registration as { fullName?: unknown }).fullName;
      if (typeof email !== 'string' || email.length === 0) {
        strapi.log.warn(
          `${LOG_PREFIX} registration ${registration.documentId} has no email; left waitlisted.`,
        );
        continue;
      }

      const ok = await sendRegistrationConfirmation({
        strapi,
        to: email,
        state: 'confirmed',
        eventReference: { documentId: eventDocumentId },
        reservation: {
          id: registration.documentId as string,
          name: typeof fullName === 'string' ? fullName : undefined,
        },
      });

      if (!ok) {
        failed += 1;
        continue;
      }

      await strapi
        .documents(REGISTRATION_UID)
        .update({
          documentId: registration.documentId as string,
          data: { registrationState: 'confirmed' },
        });

      promoted += 1;
      slots -= 1;
    }

    return { promoted, failed };
  });
}

/**
 * Run one pass over every event that still has waitlisted registrations.
 * One failing event never stops the batch; totals are logged.
 */
export async function runWaitlistPromotion({
  strapi,
}: {
  strapi: Core.Strapi;
}): Promise<void> {
  const eventIds = new Set<string>();

  let page = 1;
  while (true) {
    const registrations = await strapi
      .documents(REGISTRATION_UID)
      .findMany({
        filters: { registrationState: { $eq: 'waitlisted' } },
        populate: { event: true },
        pagination: { page, pageSize: PAGE_SIZE },
      });

    for (const registration of registrations) {
      const event = (registration as { event?: { documentId?: unknown } }).event;
      if (typeof event?.documentId === 'string' && event.documentId.length > 0) {
        eventIds.add(event.documentId);
      }
    }

    if (registrations.length < PAGE_SIZE) break;
    page += 1;
  }

  if (eventIds.size === 0) {
    strapi.log.info(`${LOG_PREFIX} no events with a waitlist, skipping.`);
    return;
  }

  let promotedTotal = 0;
  let failedTotal = 0;

  for (const eventId of eventIds) {
    try {
      const { promoted, failed } = await promoteEvent(strapi, eventId);
      promotedTotal += promoted;
      failedTotal += failed;
    } catch (error) {
      failedTotal += 1;
      strapi.log.error(
        `${LOG_PREFIX} failed for event ${eventId}: ${String(error)}`,
      );
    }
  }

  strapi.log.info(
    `${LOG_PREFIX} finished: ${promotedTotal} waitlisted registration(s) promoted, ${failedTotal} mail(s) failed (retried next run).`,
  );
}