test.js

function getShortcodeFromTag (tag) {
	let id = bigInt(tag.split('_', 1)[0]);
	const alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';
	let remainder;
	let shortcode = '';
	
	while (id.greater(0)) {
		let division = id.divmod(64);
		id = division.quotient;
		shortcode = `${alphabet.charAt(division.remainder)}${shortcode}`;
	}
	
	return shortcode;
}