// Lays a .br and a .gz beside every compressible file of the editor's
// bundle, once, when the image is built (precompressMain.ts); F sends them
// instead of the file (editorFiles.ts). At the strongest level (brotli's is
// its default), because it costs only the build: the editor's bundle shrinks to about a third, and a
// tablet on the school's network feels that at every opening. A packed file
// that is not smaller is not laid.

import { readdir, readFile, writeFile } from 'node:fs/promises';
import { extname, join } from 'node:path';
import { promisify } from 'node:util';
import { brotliCompress, constants, gzip } from 'node:zlib';

export const COMPRESSIBLE = new Set(['.html', '.js', '.mjs', '.css', '.json', '.svg', '.txt']);

const packers = [
  { suffix: '.br', pack: (bytes: Buffer) => promisify(brotliCompress)(bytes) },
  { suffix: '.gz', pack: (bytes: Buffer) => promisify(gzip)(bytes, { level: constants.Z_BEST_COMPRESSION }) },
];

/** Packs the compressible files under `folder`; returns how many packed files it laid. */
export async function precompress(folder: string): Promise<number> {
  let laid = 0;
  for (const entry of await readdir(folder, { withFileTypes: true })) {
    const path = join(folder, entry.name);
    if (entry.isDirectory()) laid += await precompress(path);
    else if (entry.isFile() && COMPRESSIBLE.has(extname(entry.name))) {
      const bytes = await readFile(path);
      for (const { suffix, pack } of packers) {
        const packed = await pack(bytes);
        if (packed.length >= bytes.length) continue;
        await writeFile(path + suffix, packed);
        laid += 1;
      }
    }
  }
  return laid;
}