similarity = (a, b) => {
	const l = Math.min(a.length, b.length);
	let dist = 0;
	for (let i = 0; i < l; i++) {
		const ca = a.charCodeAt(i);
		const cb = b.charCodeAt(i);
		dist += Math.max(0, 10 - Math.abs(ca - cb));
	}
	return dist;
}
@param {string} a key
@param {string} b key
@returns {number} the similarity as number