'use strict';

const internals = {};


module.exports = function (array1, array2, options = {}) {

    if (!array1 ||
        !array2) {

        return (options.first ? null : []);
    }

    const common = [];
    const hash = (Array.isArray(array1) ? new Set(array1) : array1);
    const found = new Set();
    for (const value of array2) {
        if (internals.has(hash, value) &&
            !found.has(value)) {

            if (options.first) {
                return value;
            }

            common.push(value);
            found.add(value);
        }
    }

    return (options.first ? null : common);
};


internals.has = function (ref, key) {

    if (typeof ref.has === 'function') {
        return ref.has(key);
    }

    return ref[key] !== undefined;
};