groupLowercaseRotations.js

// Put strings from the input array
// which can be rotated to match each other
// into groups.
//
// input: ['Hello', 'World', 'lohEl', 'orld']
// output: [['Hello', 'lohEl'], ['World'], ['orld']]

function groupRotations(strings, transform) {
	if (typeof transform !== 'function') {
		transform = string => string.toLowerCase();
	}
	const buckets = []; // contains all buckets in the correct order
	const bucketMap = new Map(); // maps all possible rotations to the correct bucket
	strings.forEach(string => {
		const transformed = String(transform(string));
		const bucket = bucketMap.get(transformed) || [];
		if (bucket.length === 0) {
			// a new bucket!
			buckets.push(bucket);
			for (let i = 0; i < transformed.length; ++i) {
				bucketMap.set(transformed.slice(i) + transformed.slice(0, i), bucket);
			}
		}
		bucket.push(string); // regardless
	});
	return buckets;
}