/**
 * Returns the plural of an English word.
 *
 * @export
 * @param {string} word
 * @param {number} [amount]
 * @returns {string}
 */
export function pluralize(word?: string, amount?: number): string | undefined {
    if (!word) return undefined;

    if (amount !== undefined && amount === 1) {
        return word;
    }
    const plural: { [key: string]: string } = {
        '(quiz)$': '$1zes',
        '^(ox)$': '$1en',
        '([m|l])ouse$': '$1ice',
        '(matr|vert|ind)ix|ex$': '$1ices',
        '(x|ch|ss|sh)$': '$1es',
        '([^aeiouy]|qu)y$': '$1ies',
        '(hive)$': '$1s',
        '(?:([^f])fe|([lr])f)$': '$1$2ves',
        '(shea|lea|loa|thie)f$': '$1ves',
        sis$: 'ses',
        '([ti])um$': '$1a',
        '(tomat|potat|ech|her|vet)o$': '$1oes',
        '(bu)s$': '$1ses',
        '(alias)$': '$1es',
        '(octop)us$': '$1i',
        '(ax|test)is$': '$1es',
        '(us)$': '$1es',
        '([^s]+)$': '$1s',
    };
    const irregular: { [key: string]: string } = {
        move: 'moves',
        foot: 'feet',
        goose: 'geese',
        sex: 'sexes',
        child: 'children',
        man: 'men',
        tooth: 'teeth',
        person: 'people',
    };
    const uncountable: string[] = [
        'sheep',
        'fish',
        'deer',
        'moose',
        'series',
        'species',
        'money',
        'rice',
        'information',
        'equipment',
        'bison',
        'cod',
        'offspring',
        'pike',
        'salmon',
        'shrimp',
        'swine',
        'trout',
        'aircraft',
        'hovercraft',
        'spacecraft',
        'sugar',
        'tuna',
        'you',
        'wood',
    ];
    // save some time in the case that singular and plural are the same
    if (uncountable.indexOf(word.toLowerCase()) >= 0) {
        return word;
    }
    // check for irregular forms
    for (const w in irregular) {
        const pattern = new RegExp(`${w}$`, 'i');
        const replace = irregular[w];
        if (pattern.test(word)) {
            return word.replace(pattern, replace);
        }
    }
    // check for matches using regular expressions
    for (const reg in plural) {
        const pattern = new RegExp(reg, 'i');
        if (pattern.test(word)) {
            return word.replace(pattern, plural[reg]);
        }
    }
    return word;
}