forto-sorter/src/algorithms/mergeSort.ts

Fast and powerful array sorting. Sort by any property in any direction with easy to read syntax.

export const mergeSort = ({ list }, { shouldSwap }) => {
	/**
	 * @return {number}
	 */
	const getMiddleIndex = (startIndex, endIndex) => (
		startIndex + Math.floor((endIndex - startIndex) / 2)
	);

	/**
	 * merges two sorted partitions using a temporary array
	 * @return {array} the start and end of a partition
	 */
	const partitionAndSort = (startIndex = 0, endIndex = list.length - 1) => {
		if (endIndex - startIndex <= 0) return [startIndex, startIndex + 1];

		const middleIndex = getMiddleIndex(startIndex, endIndex);
		const leftPartition = partitionAndSort(startIndex, middleIndex);
		const rightPartition = partitionAndSort(middleIndex + 1, endIndex);

		const [leftStart, leftEnd] = leftPartition;
		const [rightStart, rightEnd] = rightPartition;

		const merged = [];
		let leftIndex = leftStart;
		let rightIndex = rightStart;

		while (leftIndex < leftEnd || rightIndex < rightEnd) {
			if (rightIndex === rightEnd) {
				merged.push(list[leftIndex]);
				leftIndex += 1;
			} else if (leftIndex === leftEnd) {
				merged.push(list[rightIndex]);
				rightIndex += 1;
			} else if (shouldSwap(leftIndex, rightIndex)) {
				merged.push(list[rightIndex]);
				rightIndex += 1;
			} else {
				merged.push(list[leftIndex]);
				leftIndex += 1;
			}
		}

		// update list segment with merged partition
		let j = 0;
		for (let i = leftStart; i < rightEnd; i += 1) {
			list[i] = merged[j]; // eslint-disable-line no-param-reassign
			j += 1;
		}

		return [leftStart, rightEnd];
	};

	partitionAndSort();
	return list;
};