packUnpackFile.js

const fs = require('fs');

function packFile(filename, outfilename, noOfBytesToPrefix = 100) {
    const filenameEncoded = new Buffer.from(filename, 'utf8');
    const filenameLength = filenameEncoded.length;
    if (filenameLength > noOfBytesToPrefix) {
        throw new Error("file name after utf-8 encoding is too long");
    }
    
    let prefixBuffer = new Buffer.allocUnsafe(noOfBytesToPrefix).fill(0);
    filenameEncoded.copy(prefixBuffer)
    // console.log(filenameEncoded);
    // console.log(prefixBuffer);

    if (!fs.existsSync(filename)) {
        throw new Error(`File does not exist: [${filename}]`);
    }
    if (!fs.statSync(filename).isFile()) {
        throw new Error(`Is not a file: [${filename}]`);
    }
    let rs = fs.createReadStream(filename, {
        encoding: null,
        autoClose: true,
    });
    let  ws = fs.createWriteStream(outfilename, {
        encoding: null,
        autoClose: true,
    });
    ws.write(prefixBuffer);
    rs.on('error', (err) => {
        rs.close();
        ws.close();
        throw(new Error(`error piping ${filename} to ${outfilename}`));
    });
    rs.pipe(ws);
}

function unpackFile(filename, noOfBytesPrefixed = 100) {
    if (!fs.existsSync(filename)) {
        throw new Error(`File does not exist: [${filename}]`);
    }
    if (!fs.statSync(filename).isFile()) {
        throw new Error(`Is not a file: [${filename}]`);
    }

    let rs = fs.createReadStream(filename, {
        encoding: null,
        autoClose: true,
    });
    // get the first noOfBytesPrefixed octets to get the original file name
    rs.once('readable', () => {
        let prefixBuffer = rs.read(noOfBytesPrefixed);
        if (prefixBuffer) {
            let idx = prefixBuffer.indexOf(0);
            if (idx === -1) {
                idx = prefixBuffer.length;
            }
            const filenameEncoded = prefixBuffer.subarray(0, idx);
            const outfilename = filenameEncoded.toString('utf8');

            let  ws = fs.createWriteStream(outfilename, {
                encoding: null,
                autoClose: true,
            });
        
            rs.resume();
            rs.on('error', (err) => {
                rs.close();
                ws.close();
                throw(new Error(`error piping ${filename} to ${outfilename}`));
            });
            rs.pipe(ws);
        }
    });
    
}

function testStream() {
    let crypto = require('crypto');
    let fs = require('fs');
    let readStream = fs.createReadStream('test.js');
    let hash = crypto.createHash('sha256');
    readStream.on('readable', function () {
        let chunk;
        while (null !== (chunk = readStream.read())) {
        hash.update(chunk);
        }
    }).on('end', function () {
        console.log(hash.digest('hex'));
    });
}