import { createHash } from "node:crypto";
import { mkdir, readFile, readdir, stat, writeFile } from "node:fs/promises";
import path from "node:path";

import {
  SERVICE_WORKER_BOUNDS,
  SERVICE_WORKER_SCRIPT_PATH,
  type StaticAssetManifestV1,
} from "../src/contracts/service-worker.ts";

/**
 * §17.2.2 step 4. Scans the completed app `dist` and emits the exact hashed
 * asset list the Service Worker will verify at install time.
 *
 * `service-worker.js` itself and `index.html` are excluded (§17.2.2), as are
 * the runtime config and release manifest, which are network-only (§18.4).
 */

const OUTPUT = ".generated/frontend-runtime/service-worker-assets.ts";

const CACHEABLE_EXTENSIONS: Readonly<Record<string, string>> = Object.freeze({
  ".js": "text/javascript",
  ".mjs": "text/javascript",
  ".css": "text/css",
  ".woff2": "font/woff2",
  ".svg": "image/svg+xml",
  ".png": "image/png",
  ".webp": "image/webp",
});

const EXCLUDED_FILES: ReadonlySet<string> = new Set([
  "index.html",
  SERVICE_WORKER_SCRIPT_PATH,
  "config.json",
  "release-manifest.json",
  "runtime-config.schema.json",
]);

/** Vite emits content-hashed names; only those may be treated as immutable. */
const HASHED_NAME = /-[A-Za-z0-9_-]{8,}\.[a-z0-9]+$/;

export async function collectStaticAssets(
  distDirectory: string,
  buildId: string,
  releaseId: string,
): Promise<StaticAssetManifestV1> {
  const files = await walk(distDirectory, distDirectory);
  const assets: StaticAssetManifestV1["assets"][number][] = [];

  for (const relative of files.sort()) {
    const base = path.basename(relative);
    if (EXCLUDED_FILES.has(base) || relative.startsWith(".vite/")) continue;
    const contentType = CACHEABLE_EXTENSIONS[path.extname(base).toLowerCase()];
    if (!contentType || !HASHED_NAME.test(base)) continue;

    const absolute = path.join(distDirectory, relative);
    const bytes = await readFile(absolute);
    if (bytes.byteLength > SERVICE_WORKER_BOUNDS.singleAssetBytes) {
      throw new Error(`Static asset exceeds its byte bound: ${relative}`);
    }
    assets.push({
      url: `/${relative.split(path.sep).join("/")}`,
      sha256: `sha256:${createHash("sha256").update(bytes).digest("hex")}`,
      bytes: bytes.byteLength,
      contentType,
    });
  }

  if (assets.length > SERVICE_WORKER_BOUNDS.assets) {
    throw new Error("Static asset count exceeds its bound.");
  }
  const totalBytes = assets.reduce((sum, asset) => sum + asset.bytes, 0);
  if (totalBytes > SERVICE_WORKER_BOUNDS.assetSetBytes) {
    throw new Error("Static asset set exceeds its byte bound.");
  }

  // The set digest is a length-prefixed hash over the sorted asset identities,
  // so a reordered directory listing cannot change it.
  const hash = createHash("sha256");
  hash.update("CA_STATIC_ASSET_SET_V1\0");
  for (const asset of assets) {
    hash.update(lengthPrefixed(asset.url));
    hash.update(lengthPrefixed(asset.sha256));
    hash.update(lengthPrefixed(String(asset.bytes)));
    hash.update(lengthPrefixed(asset.contentType));
  }

  return {
    schemaVersion: 1,
    buildId,
    releaseId,
    setDigest: `sha256:${hash.digest("hex")}`,
    assets,
  };
}

function lengthPrefixed(value: string): Buffer {
  const bytes = Buffer.from(value, "utf8");
  const prefix = Buffer.alloc(4);
  prefix.writeUInt32BE(bytes.byteLength, 0);
  return Buffer.concat([prefix, bytes]);
}

async function walk(root: string, current: string): Promise<string[]> {
  const entries = await readdir(current, { withFileTypes: true });
  const files: string[] = [];
  for (const entry of entries) {
    const absolute = path.join(current, entry.name);
    if (entry.isDirectory()) {
      files.push(...(await walk(root, absolute)));
    } else if ((await stat(absolute)).isFile()) {
      files.push(path.relative(root, absolute));
    }
  }
  return files;
}

async function main(): Promise<void> {
  const distDirectory = process.argv[2] ?? "dist";
  const buildId = process.env.VITE_BUILD_ID ?? "local-build";
  const releaseId = process.env.RELEASE_ID ?? "local-release";
  const manifest = await collectStaticAssets(distDirectory, buildId, releaseId);
  const source = [
    "// Generated by scripts/generate-service-worker-assets.ts. Do not edit.",
    "",
    'import type { StaticAssetManifestV1 } from "../../src/contracts/service-worker.ts";',
    "",
    `export const SERVICE_WORKER_ASSETS: StaticAssetManifestV1 = ${JSON.stringify(
      manifest,
      null,
      2,
    )} as const;`,
    "",
  ].join("\n");
  await mkdir(path.dirname(OUTPUT), { recursive: true });
  await writeFile(OUTPUT, source, "utf8");
  process.stdout.write(
    `service worker assets: ${manifest.assets.length} file(s) ${manifest.setDigest}\n`,
  );
}

if (process.argv[1]?.endsWith("generate-service-worker-assets.ts")) {
  await main();
}