/**
 * The Log: one chronological feed of a user's engagements (reads, plays, watches) and listens, newest first.
 *
 * Each item has an `at` sort key: a listen's `listened_at`; an engagement's finish date, else its start
 * date, else when it was logged. Partial dates sort as strings ("2025-03" sits after "2025-03-31" and
 * before "2025-04"), never through `Date`. Pages use a keyset cursor over (at, kind, id), all descending,
 * so equal timestamps across the two kinds page without gaps or repeats.
 */
import type { Db } from "@sttracker/db/db";
import { fromJson } from "@sttracker/db/json";
import type { PartialDate } from "@sttracker/plugin-sdk/types";
import { sql } from "kysely";
import { fromStoredDate } from "./entries.ts";
import { ValidationError } from "./errors.ts";

export type LogItem =
  | {
      kind: "engagement";
      id: string;
      at: string;
      entryId: string;
      workId: string;
      title: string;
      mediaType: string;
      coverUrl: string;
      startedAt: PartialDate | null;
      finishedAt: PartialDate | null;
      platform: string | null;
      rating: number | null;
    }
  | {
      kind: "listen";
      id: string;
      at: string;
      workId: string | null;
      title: string;
      mediaType: "track";
      artist: string;
      track: string;
      album: string | null;
      source: string;
    };

type Kind = LogItem["kind"];
interface Cursor {
  at: string;
  kind: Kind;
  id: string;
}

function encodeCursor(c: Cursor): string {
  return Buffer.from(JSON.stringify([c.at, c.kind, c.id])).toString("base64url");
}

function decodeCursor(s: string): Cursor {
  try {
    const [at, kind, id] = JSON.parse(Buffer.from(s, "base64url").toString("utf8")) as unknown[];
    if (typeof at === "string" && (kind === "engagement" || kind === "listen") && typeof id === "string") return { at, kind, id };
  } catch {
    // Fall through to the validation error.
  }
  throw new ValidationError("cursor", "invalid");
}

/** (at, kind, id) < cursor, with this query's fixed kind: kinds sort "listen" > "engagement". */
function after(kind: Kind, c: Cursor): "lt" | "le" | "tie" {
  if (kind === c.kind) return "tie";
  return kind < c.kind ? "le" : "lt";
}

export async function listLog(
  db: Db,
  userId: string,
  opts: { limit?: number; cursor?: string } = {},
): Promise<{ items: LogItem[]; nextCursor: string | null }> {
  const limit = Math.min(Math.max(opts.limit ?? 50, 1), 200);
  const cursor = opts.cursor ? decodeCursor(opts.cursor) : null;

  const at = sql<string>`coalesce(nullif(g.finished_at, ''), nullif(g.started_at, ''), g.created_at)`;
  let eq = db.k
    .selectFrom("engagements as g")
    .innerJoin("entries as e", "e.id", "g.entry_id")
    .innerJoin("works as w", "w.id", "e.work_id")
    .select([
      "g.id",
      "g.entry_id",
      "g.started_at",
      "g.started_precision",
      "g.finished_at",
      "g.finished_precision",
      "g.platform",
      "g.rating",
      "e.work_id",
      "e.overrides",
      "e.media_type_override",
      "w.media_type",
      "w.title",
      "w.cover_url",
      at.as("at"),
    ])
    .where("e.user_id", "=", userId)
    .where("e.deleted_at", "is", null)
    .where("g.deleted_at", "is", null);
  let lq = db.k
    .selectFrom("listens")
    .select(["id", "listened_at", "artist", "track", "album", "work_id", "source"])
    .where("user_id", "=", userId);

  if (cursor) {
    const ge = after("engagement", cursor);
    eq = eq.where((x) =>
      ge === "lt" ? x(at, "<", cursor.at) : ge === "le" ? x(at, "<=", cursor.at) : x.or([x(at, "<", cursor.at), x.and([x(at, "=", cursor.at), x("g.id", "<", cursor.id)])]),
    );
    const gl = after("listen", cursor);
    lq = lq.where((x) =>
      gl === "lt"
        ? x("listened_at", "<", cursor.at)
        : gl === "le"
          ? x("listened_at", "<=", cursor.at)
          : x.or([x("listened_at", "<", cursor.at), x.and([x("listened_at", "=", cursor.at), x("id", "<", cursor.id)])]),
    );
  }

  const [engagements, listens] = await Promise.all([
    eq.orderBy(at, "desc").orderBy("g.id", "desc").limit(limit + 1).execute(),
    lq.orderBy("listened_at", "desc").orderBy("id", "desc").limit(limit + 1).execute(),
  ]);

  const merged: LogItem[] = [
    ...engagements.map((r): LogItem => {
      const o = fromJson<{ title?: unknown; coverUrl?: unknown }>(r.overrides, {});
      return {
        kind: "engagement",
        id: r.id,
        at: r.at,
        entryId: r.entry_id,
        workId: r.work_id,
        title: typeof o.title === "string" && o.title ? o.title : r.title,
        mediaType: r.media_type_override ?? r.media_type,
        coverUrl: typeof o.coverUrl === "string" ? o.coverUrl : r.cover_url,
        startedAt: fromStoredDate(r.started_at, r.started_precision),
        finishedAt: fromStoredDate(r.finished_at, r.finished_precision),
        platform: r.platform,
        rating: r.rating,
      };
    }),
    ...listens.map(
      (r): LogItem => ({
        kind: "listen",
        id: r.id,
        at: r.listened_at,
        workId: r.work_id,
        title: r.track,
        mediaType: "track",
        artist: r.artist,
        track: r.track,
        album: r.album,
        source: r.source,
      }),
    ),
  ];
  // Same order as the SQL: at, then kind, then id, all descending. Plain code-unit comparison on both sides.
  const cmp = (a: string, b: string) => (a < b ? 1 : a > b ? -1 : 0);
  merged.sort((a, b) => cmp(a.at, b.at) || cmp(a.kind, b.kind) || cmp(a.id, b.id));
  const items = merged.slice(0, limit);
  const last = items[items.length - 1];
  return { items, nextCursor: merged.length > limit && last ? encodeCursor(last) : null };
}