typescript source code of FileWithEmbeddedBlockMapDifferentialDownloader

import { BlockMap } from "builder-util-runtime/out/blockMapApi";
import { close, fstat, open, read } from "fs-extra";
import { DifferentialDownloader } from "./DifferentialDownloader";
import { inflateRawSync } from "zlib";

export class FileWithEmbeddedBlockMapDifferentialDownloader extends DifferentialDownloader {
  async download(): Promise<void> {
    const packageInfo = this.blockAwareFileInfo;
    const fileSize = packageInfo.size!!;
    const offset = fileSize - (packageInfo.blockMapSize!! + 4);
    this.fileMetadataBuffer = await this.readRemoteBytes(offset, fileSize - 1);
    const newBlockMap = readBlockMap(
      this.fileMetadataBuffer.slice(0, this.fileMetadataBuffer.length - 4)
    );
    const emit = () => {};
    await this.doDownload(
      await readEmbeddedBlockMapData(this.options.oldFile),
      newBlockMap,
      emit
    );
  }
}

function readBlockMap(data: Buffer): BlockMap {
  return JSON.parse(inflateRawSync(data).toString());
}

async function readEmbeddedBlockMapData(file: string): Promise<BlockMap> {
  const fd = await open(file, "r");
  try {
    const fileSize = (await fstat(fd)).size;
    const sizeBuffer = Buffer.allocUnsafe(4);
    await read(
      fd,
      sizeBuffer,
      0,
      sizeBuffer.length,
      fileSize - sizeBuffer.length
    );

    const dataBuffer = Buffer.allocUnsafe(sizeBuffer.readUInt32BE(0));
    await read(
      fd,
      dataBuffer,
      0,
      dataBuffer.length,
      fileSize - sizeBuffer.length - dataBuffer.length
    );
    await close(fd);

    return readBlockMap(dataBuffer);
  } catch (e) {
    await close(fd);
    throw e;
  }
}