import { isSegment, readSong } from "@/lib/browse";
import { DEFAULT_TOLERANCE, diffPlans, summariseDiff } from "@/lib/plandiff";

export const dynamic = "force-dynamic";

// `?song=&a=&b=` — two plans of one song, aligned and compared.
//
// A 880-note plan produces a lot of deltas, so `?deltas=` caps what comes back;
// the counts are always complete and the cap is reported, because a truncation
// you cannot see reads as "that is all of them".
export async function GET(request: Request) {
  const url = new URL(request.url);
  const id = url.searchParams.get("song") ?? "";
  const a = url.searchParams.get("a") ?? "";
  const b = url.searchParams.get("b") ?? "";
  const limit = Math.min(2000, Math.max(0, Number(url.searchParams.get("deltas")) || 400));
  const tol = Number(url.searchParams.get("tolerance"));

  if (!isSegment(id)) return Response.json({ error: "bad song" }, { status: 400 });
  if (!(await readSong(id))) return Response.json({ error: "no such song" }, { status: 404 });
  if (a === b) return Response.json({ error: "pick two different plans" }, { status: 400 });

  const diff = await diffPlans(id, a, b, Number.isFinite(tol) && tol > 0 ? tol : DEFAULT_TOLERANCE);
  if (!diff) {
    return Response.json({ error: "both names must be files in plan/" }, { status: 404 });
  }

  const shown = diff.deltas.slice(0, limit);
  return Response.json(
    {
      ...diff,
      deltas: shown,
      deltaCount: diff.deltas.length,
      truncated: diff.deltas.length - shown.length,
      summary: summariseDiff(diff),
    },
    { headers: { "cache-control": "no-store" } },
  );
}