util.js

'use strict';

exports.stringToBytes = string => [...string].map(character => character.charCodeAt(0));

const uint8ArrayUtf8ByteString = (array, start, end) => {
	return String.fromCharCode(...array.slice(start, end));
};

exports.tarHeaderChecksumMatches = buffer => { // Does not check if checksum field characters are valid
	if (buffer.length < 512) { // `tar` header size, cannot compute checksum without it
		return false;
	}

	const MASK_8TH_BIT = 0x80;

	let sum = 256; // Intitalize sum, with 256 as sum of 8 spaces in checksum field
	let signedBitSum = 0; // Initialize signed bit sum

	for (let i = 0; i < 148; i++) {
		const byte = buffer[i];
		sum += byte;
		signedBitSum += byte & MASK_8TH_BIT; // Add signed bit to signed bit sum
	}

	// Skip checksum field

	for (let i = 156; i < 512; i++) {
		const byte = buffer[i];
		sum += byte;
		signedBitSum += byte & MASK_8TH_BIT; // Add signed bit to signed bit sum
	}

	const readSum = parseInt(uint8ArrayUtf8ByteString(buffer, 148, 154), 8); // Read sum in header

	// Some implementations compute checksum incorrectly using signed bytes
	return (
		// Checksum in header equals the sum we calculated
		readSum === sum ||

		// Checksum in header equals sum we calculated plus signed-to-unsigned delta
		readSum === (sum - (signedBitSum << 1))
	);
};

exports.uint8ArrayUtf8ByteString = uint8ArrayUtf8ByteString;

/**
ID3 UINT32 sync-safe tokenizer token.
28 bits (representing up to 256MB) integer, the msb is 0 to avoid "false syncsignals".
*/
exports.uint32SyncSafeToken = {
	get: (buffer, offset) => {
		return (buffer[offset + 3] & 0x7F) | ((buffer[offset + 2]) << 7) | ((buffer[offset + 1]) << 14) | ((buffer[offset]) << 21);
	},
	len: 4
};