wg-lua/src/base64.ts

const { slice } = require<{
	slice: <T extends defined>(arr: T[], start: number, stop?: number) => T[];
}>("util");

function stringToBytes(str: string) {
	const result = [];

	for (let i = 0; i < str.size(); i++) {
		result.push(string.byte(str, i + 1)[0]);
	}

	return result;
}

// Adapted from https://github.com/un-ts/ab64/blob/main/src/ponyfill.ts#L24
const _atob = (asc: string) => {
	const b64CharList = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

	const b64Chars = string.split(b64CharList, "");

	const b64Table = b64Chars.reduce<Record<string, number>>((acc, char, index) => {
		acc[char] = index;
		return acc;
	}, {});

	const fromCharCode = string.char;

	asc = string.gsub(asc, "%s+", "")[0];
	asc += string.char(...slice(stringToBytes("=="), 2 - (asc.size() & 3)));

	let u24: number;
	let binary = "";
	let r1: number;
	let r2: number;

	for (let i = 0; i < asc.size(); i++) {
		u24 =
			(b64Table[string.byte(asc, i++)[0]] << 18) |
			(b64Table[string.byte(asc, i++)[0]] << 12) |
			((r1 = b64Table[string.byte(asc, i++)[0]]) << 6) |
			(r2 = b64Table[string.byte(asc, i++)[0]]);
		binary +=
			r1 === 64
				? fromCharCode((u24 >> 16) & 255)
				: r2 === 64
					? fromCharCode((u24 >> 16) & 255, (u24 >> 8) & 255)
					: fromCharCode((u24 >> 16) & 255, (u24 >> 8) & 255, u24 & 255);
	}

	return binary;
};

// Adapted from https://gist.github.com/jonleighton/958841
export function atob(buf: number[]): string {
	let base64 = "";
	const encodings = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

	const byteLength = buf.size();
	const byteRemainder = byteLength % 3;
	const mainLength = byteLength - byteRemainder;

	let a: number, b: number, c: number, d: number;
	let chunk;

	// Main loop deals with bytes in chunks of 3
	for (let i = 0; i < mainLength; i = i + 3) {
		// Combine the three bytes into a single integer
		chunk = (buf[i] << 16) | (buf[i + 1] << 8) | buf[i + 2];

		// Use bitmasks to extract 6-bit segments from the triplet
		a = (chunk & 16515072) >> 18;
		b = (chunk & 258048) >> 12;
		c = (chunk & 4032) >> 6;
		d = chunk & 63;

		// Convert the raw binary segments to the appropriate ASCII encoding
		base64 +=
			string.char(string.byte(encodings, a + 1)[0]) +
			string.char(string.byte(encodings, b + 1)[0]) +
			string.char(string.byte(encodings, c + 1)[0]) +
			string.char(string.byte(encodings, d + 1)[0]);
	}

	// Deal with the remaining bytes and padding
	if (byteRemainder === 1) {
		chunk = buf[mainLength];

		a = (chunk & 252) >> 2;

		// Set the 4 least significant bits to zero
		b = (chunk & 3) << 4;

		base64 += string.byte(encodings, a)[0] + string.byte(encodings, b)[0] + "==";
	} else if (byteRemainder === 2) {
		chunk = (buf[mainLength] << 8) | buf[mainLength + 1];

		a = (chunk & 64512) >> 10;
		b = (chunk & 1008) >> 4;

		// Set the 2 least significant bits to zero
		c = (chunk & 15) << 2;

		base64 +=
			string.char(string.byte(encodings, a + 1)[0]) +
			string.char(string.byte(encodings, b + 1)[0]) +
			string.char(string.byte(encodings, c + 1)[0]) +
			"=";
	}

	return base64;
}