const expect = require('chai').expect;

// Implementation of flatten(array)
function flatten(array) {
    // early return if it is not an array or if empty
    if (!array || !Array.isArray(array)) {
        return;
    }
    return array.reduce((flattened, element) => {
        return flattened.concat(Array.isArray(element) ? flatten(element) : [element]);
    }, []);
}

// Unit tests to run with Mocha and Chai.
// In order to install, run
// "npm install --save-dev mocha chai"
// and then run "node_modules/.bin/mocha flatten.js"
describe('flatten(array)', () => {

    const assertResult = (input, output) => expect(flatten(input)).to.deep.equals(output);

    it('should return undefined if the argument is not an array', () => {
        assertResult(undefined, undefined);
        assertResult({}, undefined);
        assertResult(1, undefined);
        assertResult("1", undefined);
        assertResult(true, undefined);
    });

    it('should return an empty array if the argument is an empty array', () => {
        assertResult([], []);
    });

    it('should return the same array if it is already flattened', () => {
        let input = [1, 2, 3];
        assertResult(input, input);
    });

    it('should return a flattened array if it has one array in it', () => {
        let input = [1, [1, 2, 3], 4];
        let output = [1, 1, 2, 3, 4];
        assertResult(input, output);
    });

    it('should return a flattened array for 3 nested levels', () => {
        let input = [1, [1, [2, [3]]], 4];
        let output = [1, 1, 2, 3, 4];
        assertResult(input, output);
    });

    it('should return a flattened array for 99 nested levels', () => {
        let output = Array.from({ length: 99 }, (v, index) => index);
        // here we do not use the output of reduce, because each callback will be returning a reference to the last "nested"
        // array. So maintain the reference from the inital state
        let input = [];
        output.reduce((accum, element) => {
            let lastElementArray = [element];
            accum.push(lastElementArray);
            return lastElementArray;
        }, input);
        assertResult(input, output);
    });
});