linker/service/resolver.js

// Tiny resolver service: candidate names -> artist links, for CONFIRMED artists only.
//
// Zero npm deps; Node 18+ (built-in fetch). The "which words are real artists" problem
// is delegated to a music API rather than guessed client-side:
//   * with SPOTIFY_CLIENT_ID/SECRET set -> Spotify Web API (client-credentials) for
//     DIRECT artist links;
//   * without creds -> MusicBrainz validation + Spotify/Bandcamp SEARCH links (zero setup).
// Either way a candidate only resolves when an artist's name matches it case-insensitively
// (the precision lever). The pure matching/link logic lives in lib.js (unit-tested).

import { createServer } from "node:http";
import { readFileSync } from "node:fs";
import { norm, pickArtist, spotifyResult, searchResult } from "./lib.js";

// --- minimal .env loader (no dependency) ------------------------------------
try {
  const env = readFileSync(new URL("./.env", import.meta.url), "utf8");
  for (const line of env.split("\n")) {
    const m = line.match(/^\s*([\w.-]+)\s*=\s*(.*?)\s*$/);
    if (m && !(m[1] in process.env)) process.env[m[1]] = m[2].replace(/^["']|["']$/g, "");
  }
} catch { /* no .env -> use real env / defaults */ }

const PORT = Number(process.env.PORT) || 8787;
const SPOTIFY_ID = process.env.SPOTIFY_CLIENT_ID;
const SPOTIFY_SECRET = process.env.SPOTIFY_CLIENT_SECRET;
const USE_SPOTIFY = Boolean(SPOTIFY_ID && SPOTIFY_SECRET);
const UA = "reddit-spotify-linker/0.1 (https://git.yapplesauce.com/paul/linker)";
const MAX_CANDIDATES = 100;

const cache = new Map(); // norm(name) -> { spotify, bandcamp, name } | null  (negatives cached too)

// --- Spotify (optional) -----------------------------------------------------
let spToken = null;
let spTokenExp = 0;
async function spotifyToken() {
  if (spToken && Date.now() < spTokenExp) return spToken;
  const auth = Buffer.from(`${SPOTIFY_ID}:${SPOTIFY_SECRET}`).toString("base64");
  const r = await fetch("https://accounts.spotify.com/api/token", {
    method: "POST",
    headers: { authorization: `Basic ${auth}`, "content-type": "application/x-www-form-urlencoded" },
    body: "grant_type=client_credentials",
  });
  if (!r.ok) throw new Error(`spotify token http ${r.status}`);
  const j = await r.json();
  spToken = j.access_token;
  spTokenExp = Date.now() + (j.expires_in - 60) * 1000;
  return spToken;
}

async function viaSpotify(name) {
  const token = await spotifyToken();
  const url = `https://api.spotify.com/v1/search?type=artist&limit=5&q=${encodeURIComponent(name)}`;
  const r = await fetch(url, { headers: { authorization: `Bearer ${token}` } });
  if (!r.ok) throw new Error(`spotify search http ${r.status}`);
  const j = await r.json();
  const hit = pickArtist(name, j.artists?.items);
  return hit ? spotifyResult(hit) : null;
}

// --- MusicBrainz (zero-cred fallback) ---------------------------------------
// MusicBrainz asks for <=1 request/second, so issuance is serialized + spaced.
const sleep = (ms) => new Promise((r) => setTimeout(r, ms));
let mbGate = Promise.resolve();
let mbLast = 0;
function mbThrottled(name) {
  const gate = mbGate.then(async () => {
    const wait = 1100 - (Date.now() - mbLast);
    if (wait > 0) await sleep(wait);
    mbLast = Date.now();
  });
  mbGate = gate.catch(() => {}); // a failure never wedges the queue
  return gate.then(() => viaMusicBrainz(name));
}

async function viaMusicBrainz(name) {
  const q = encodeURIComponent(`artist:"${name}"`);
  const url = `https://musicbrainz.org/ws/2/artist?fmt=json&limit=5&query=${q}`;
  const r = await fetch(url, { headers: { "user-agent": UA, accept: "application/json" } });
  if (!r.ok) throw new Error(`musicbrainz http ${r.status}`);
  const j = await r.json();
  const hit = pickArtist(name, j.artists, { minScore: 90 });
  return hit ? searchResult(hit) : null;
}

// --- resolve one candidate (cached) -----------------------------------------
async function resolveOne(name) {
  const key = norm(name);
  if (!key) return null;
  if (cache.has(key)) return cache.get(key);
  try {
    const result = USE_SPOTIFY ? await viaSpotify(name) : await mbThrottled(name);
    cache.set(key, result); // cache hits AND confirmed misses
    return result;
  } catch (e) {
    console.error(`resolve "${name}": ${e.message}`); // transient -> don't cache
    return null;
  }
}

async function resolveAll(candidates) {
  const uniq = [...new Set(candidates.map((c) => String(c).trim()).filter(Boolean))].slice(0, MAX_CANDIDATES);
  const out = {};
  await Promise.all(uniq.map(async (name) => { out[name] = await resolveOne(name); }));
  return out;
}

// --- HTTP -------------------------------------------------------------------
function readBody(req) {
  return new Promise((resolve, reject) => {
    let data = "";
    req.on("data", (c) => {
      data += c;
      if (data.length > 1e6) { reject(new Error("body too large")); req.destroy(); }
    });
    req.on("end", () => resolve(data));
    req.on("error", reject);
  });
}

const json = (res, code, obj) => {
  res.writeHead(code, { "content-type": "application/json" });
  res.end(JSON.stringify(obj));
};

const server = createServer(async (req, res) => {
  res.setHeader("access-control-allow-origin", "*");
  res.setHeader("access-control-allow-headers", "content-type");
  res.setHeader("access-control-allow-methods", "GET, POST, OPTIONS");
  if (req.method === "OPTIONS") { res.writeHead(204); return res.end(); }

  if (req.method === "GET" && req.url === "/health") {
    return json(res, 200, { ok: true, provider: USE_SPOTIFY ? "spotify" : "musicbrainz", cached: cache.size });
  }

  if (req.method === "POST" && req.url === "/resolve") {
    try {
      const { candidates } = JSON.parse((await readBody(req)) || "{}");
      if (!Array.isArray(candidates)) return json(res, 400, { error: "candidates must be an array" });
      return json(res, 200, await resolveAll(candidates));
    } catch (e) {
      return json(res, 500, { error: e.message });
    }
  }

  json(res, 404, { error: "not found" });
});

server.listen(PORT, () => {
  console.log(`resolver listening on http://localhost:${PORT}  (provider: ${USE_SPOTIFY ? "spotify" : "musicbrainz"})`);
});