slice-ring.js

/**
 * function sliceRing make circular slice of array
 * @param {Array} arr
 * @param {Number} start
 * @param {Number} length
 * @returns {Array}
 */

export default function sliceRing(arr, start, length) {
	start = start % arr.length;
	if (start < 0) start = arr.length + start;
	var end = (start + length) % arr.length;
	if (start < end) {
		return arr.slice(start, end);
	}
	return arr.slice(start).concat(arr.slice(0, end));
}

describe('Ring slice', function() {
	var arr = [0, 1, 2, 3, 4, 5];

	it('should slice normaly', function() {
		expect(sliceRing(arr, 2, 3)).eql([2, 3, 4]);
	});

	it('should slice ring when start + length is bigger then length', function() {
		expect(sliceRing(arr, 4, 4)).eql([4, 5, 0, 1]);
	});

	it('should slice when start is bigger then length', function() {
		expect(sliceRing(arr, 8, 4)).eql([2, 3, 4, 5]);
	});

	it('should slice ring when start is bigger then length', function() {
		expect(sliceRing(arr, 10, 4)).eql([4, 5, 0, 1]);
	});

	it('should slice ring when start is below 0', function() {
		expect(sliceRing(arr, -2, 4)).eql([4, 5, 0, 1]);
	});

	it('should slice ring when start and further then length', function() {
		expect(sliceRing(arr, -8, 4)).eql([4, 5, 0, 1]);
		expect(sliceRing(arr, -14, 4)).eql([4, 5, 0, 1]);
	});

	it('should slice ring length is equal to array length', function() {
		expect(sliceRing(arr, 2, 6)).eql([2, 3, 4, 5, 0, 1]);
		expect(sliceRing(arr, -2, 6)).eql([4, 5, 0, 1, 2, 3]);
	});
});