import fetch from "node-fetch";
import * as fs from "fs";
import { strict as assert } from "assert";

/**
 * Downloads file from `url` and stores it at `destFilePath`.
 * `onProgress` callback is called on recieveing each chunk of bytes
 * to track the progress of downloading, it gets the already read and total
 * amount of bytes to read as its parameters.
 */
export async function downloadFile(
    url: string,
    destFilePath: fs.PathLike,
    onProgress: (readBytes: number, totalBytes: number) => void
): Promise<void> {
    const response = await fetch(url);

    const totalBytes = Number(response.headers.get('content-length'));
    assert(!Number.isNaN(totalBytes), "Sanity check of content-length protocol");

    let readBytes = 0;

    console.log("Downloading file of", totalBytes, "bytes size from", url, "to", destFilePath);

    return new Promise<void>((resolve, reject) => response.body
        .on("data", (chunk: Buffer) => {
            readBytes += chunk.length;
            onProgress(readBytes, totalBytes);
        })
        .on("error", reject)
        .pipe(fs.createWriteStream(destFilePath).on("close", resolve))
    );
}