permutations.tsx

interface Permutation extends Array<number> {}
interface PermutationList extends Array<Permutation> {}

/**
 * Gets all permutations of a given list of digits. Results are not guaranteed
 * to be uniqe.
 */
export const getPermutations = (input: Permutation): PermutationList => (
	input.length <= 1 ? [input] : input.reduce((result, n, i, arr) => ([
		...result,
		...getPermutations([
			...arr.slice(0, i),
			...arr.slice(i + 1),
		]).map((permutation) => (
			[n, ...permutation]
		)),
	]), [])
);

/**
 * Determines if two input lists contain the same items in the same order.
 */
export const arraysEqual = <T>(left: T[], right: T[]) => (
	left.length === right.length &&
	left.every((item, i) => item === right[i])
);

/**
 * Compares two permutations and orders them such that the one with lower
 * numbers early on come first.
 *
 * @example
 *
 *     permutationComparator(
 *       [0, 1, 3, 2],
 *       [0, 1, 2, 3]
 *     ) === -1
 *
 *     permutationComparator(
 *       [0, 1, 3, 2],
 *       [1, 0, 2, 3]
 *     ) === 1
 */
export const permutationComparator = (left: Permutation, right: Permutation) => (
	left.reduce((result, item, i) => result || (
		item < right[i] ? -1 :
		item > right[i] ? 1 :
		0
	), 0)
);

/**
 * Given an input list of digits, returns a list of all unique permutations of
 * those digits.
 */
export const getUniquePermutations = (input: Permutation): PermutationList => (
	getPermutations(input)
		.sort(permutationComparator)
		.filter((permutation, i, arr) => (
			i === 0 ||
			!arraysEqual(permutation, arr[i - 1])
		))
);