Lyra/web/src/app/api/discover/route.ts

import { prisma } from "@/lib/db";

// One Discover row = a suggested artist plus that artist's suggested albums (newest /
// most-played), since album suggestions are always derived from a surfaced artist. Pending
// suggestions are grouped by artistMbid so the UI shows a single artist-centric row.
type Row = {
  id: string | null; // the artist suggestion id (Follow/Dismiss); null if only albums exist
  artistMbid: string;
  artistName: string;
  score: number;
  seedCount: number;
  sources: string[];
  seeds: string[];
  albums: {
    id: string;
    rgMbid: string | null;
    album: string | null;
    primaryType: string | null;
    secondaryTypes: string[];
    firstReleaseDate: string | null;
    albumReason: string | null;
  }[];
};

export async function GET() {
  const rows = await prisma.discoverySuggestion.findMany({
    where: { status: "pending" },
    orderBy: [{ score: "desc" }, { createdAt: "asc" }],
  });

  // "Why suggested": the followed artists (seeds) that surfaced each candidate, from the
  // per-seed contributions joined to the current watched roster (an unfollowed seed drops out).
  const candidateMbids = [...new Set(rows.map((r) => r.artistMbid))];
  const contribs = candidateMbids.length
    ? await prisma.discoverySeedContribution.findMany({
        where: { candidateMbid: { in: candidateMbids } },
        orderBy: { score: "desc" },
      })
    : [];
  const seedNames = new Map(
    (await prisma.watchedArtist.findMany({ select: { mbid: true, name: true } })).map((w) => [w.mbid, w.name]),
  );
  const seedsByCandidate = new Map<string, string[]>();
  for (const c of contribs) {
    const name = seedNames.get(c.seedMbid);
    if (!name) continue;
    const list = seedsByCandidate.get(c.candidateMbid) ?? [];
    if (!list.includes(name)) list.push(name);
    seedsByCandidate.set(c.candidateMbid, list);
  }

  // Group by artist. An artist suggestion sets the row header; album suggestions fill albums[].
  // An album whose artist has no pending artist suggestion still yields a header (never orphaned).
  const byArtist = new Map<string, Row>();
  const ensure = (r: (typeof rows)[number]): Row => {
    let row = byArtist.get(r.artistMbid);
    if (!row) {
      row = {
        id: null,
        artistMbid: r.artistMbid,
        artistName: r.artistName,
        score: r.score,
        seedCount: r.seedCount,
        sources: r.sources,
        seeds: seedsByCandidate.get(r.artistMbid) ?? [],
        albums: [],
      };
      byArtist.set(r.artistMbid, row);
    }
    return row;
  };

  for (const r of rows) {
    const row = ensure(r);
    if (r.kind === "artist") {
      row.id = r.id;
      row.score = r.score;
      row.seedCount = r.seedCount;
      row.sources = r.sources;
    } else {
      row.albums.push({
        id: r.id,
        rgMbid: r.rgMbid,
        album: r.album,
        primaryType: r.primaryType,
        secondaryTypes: r.secondaryTypes,
        firstReleaseDate: r.firstReleaseDate,
        albumReason: r.albumReason,
      });
      row.score = Math.max(row.score, r.score); // album-only row scores by its best album
    }
  }

  for (const row of byArtist.values()) {
    row.albums.sort((a, b) => (b.firstReleaseDate ?? "").localeCompare(a.firstReleaseDate ?? ""));
  }
  const out = [...byArtist.values()].sort((a, b) => b.score - a.score);
  return Response.json({ rows: out });
}