'use strict'

module.exports = function 找出最长公子串 (...strings) {

    let setsOfSubstrings = []

    strings.reduce((accumulator, currentValue) => {

        setsOfSubstrings.push(findAllSubStringsFromTwoString(accumulator, currentValue))

        return accumulator

    })

    return findTheLongest(findIntersection(...setsOfSubstrings))

}

function findIntersection (...sets) {

    /**

     *

     *

     * @param {sets} a list of sets

     * @returns {array} the elements of the intersection of given sets.

     */

    return sets.reduce((accumulator, currentValue) => [...accumulator].filter(value => currentValue.has(value)))

}

function findAllSubStringsFromTwoString (string1, string2) {

    let allSubstrings = new Set()

    for (let index = 0; index < string1.length; index++) {

        let length = 1, substring = string1.substr(index, length)

        while (string2.indexOf(substring) !== -1 && length <= string1.length) { // 当string2内含有substring时

            allSubstrings.add(substring)

            substring = string1.substr(index, length++)

        }

    }

    return allSubstrings

}

function findTheLongest (strings) {

    return strings.reduce((accumulator, currentValue) => findTheLonger(accumulator, currentValue), '')

}

function findTheLonger (string1, string2) {

    return (string1.length > string2.length) ? (string1) : (string2)

}