/**
 * Decides which of a playlist's dumps a viewer may see. The playlist service
 * filters its listings with it, the WebSocket service filters each client's
 * `playlist_dumps_updated` message the same way, and the queries that list
 * playlists count their dumps by the same rule.
 *
 * @module
 */

import type { Playlist } from "../../shared/playlist.ts";

/**
 * The fields of a playlist's dump that {@link isMemberVisibleTo} needs. A
 * `Dump` has them. For its broadcasts, the playlist service reads only these.
 */
export interface PlaylistMember {
  id: string;
  isPrivate: boolean;
  /** The dump's poster, not the playlist's owner. */
  userId: string;
}

/**
 * Checks whether `viewerId` may see `member` in `playlist`. A public dump is
 * visible to all. A private dump is visible only to the playlist's owner, and
 * only if they posted it: moderators don't see it, and neither does the owner
 * once someone else's dump in their playlist goes private. A null or undefined
 * `viewerId` is a signed-out viewer.
 */
export function isMemberVisibleTo(
  member: PlaylistMember,
  playlist: Pick<Playlist, "userId">,
  viewerId: string | null | undefined,
): boolean {
  return !member.isPrivate ||
    (member.userId === viewerId && viewerId === playlist.userId);
}

/**
 * The fields of a playlist's dump that {@link dumpFiguresFor} needs: those
 * that tell who may see it, and when it was added. The playlist service reads
 * them for its broadcasts.
 */
export interface DatedPlaylistMember extends PlaylistMember {
  addedAt: Date;
}

/** A figure of the dumps of `playlists p` that the viewer `?1` may see. */
const visibleDumps = (figure: string) =>
  `(SELECT ${figure} FROM playlist_dumps pd
     INNER JOIN dumps d ON d.id = pd.dump_id
    WHERE pd.playlist_id = p.id
      AND (d.is_private = 0 OR (d.user_id = p.user_id AND p.user_id = ?1))
   )`;

/**
 * The `dump_count` and `last_dump_at` columns of a query on `playlists p`:
 * how many of the playlist's dumps the viewer may see, by
 * {@link isMemberVisibleTo}'s rule, and when the latest of them was added,
 * null without any. Both read the viewer's id, or `""` for a signed-out one,
 * as `?1`: it is the query's first parameter, and each `?` after it takes the
 * next.
 */
export const VISIBLE_DUMP_FIGURES = `${visibleDumps("COUNT(*)")} as dump_count,
   ${visibleDumps("MAX(pd.added_at)")} as last_dump_at`;

/**
 * Returns what `playlist` says of its dumps to the viewer, `members` being
 * all of them: the `dumpCount` and `lastDumpAt` that
 * {@link VISIBLE_DUMP_FIGURES} reads in a query. The WebSocket service puts
 * them on the playlist each socket is sent.
 */
export function dumpFiguresFor(
  members: DatedPlaylistMember[],
  playlist: Pick<Playlist, "userId">,
  viewerId: string | null | undefined,
): Required<Pick<Playlist, "dumpCount">> & Pick<Playlist, "lastDumpAt"> {
  const visible = members.filter((m) =>
    isMemberVisibleTo(m, playlist, viewerId)
  );
  return {
    dumpCount: visible.length,
    lastDumpAt: visible.length
      ? new Date(Math.max(...visible.map((m) => m.addedAt.getTime())))
      : undefined,
  };
}