const TESTS: [string, RegExp][] = [
    [ 'an', /^[aefhilmnirsx]([-.]|$|th$)/ui ],
    [ 'a',  /^[bcdgjkpqtuvwyz]([-.]|$|th$)/ui ],
    [ 'an', /^(euler|hour(?!i)|heir|honest|hono)/ui ],
    [ 'an', /^(?!FJO|[HLMNS]Y.|RY[EO]|SQU|(F[LR]?|[HL]|MN?|N|RH?|S[CHKLMNPTVW]?|X(YL)?)[AEIOU])[FHLMNRSX][A-Z]/u ],
    [ 'a',  /^[^aeiouy]/ui ],
    [ 'a',  /^e[uw]/ui ],
    [ 'a',  /^onc?e\b/u ],
    [ 'a',  /^uni([^nmd]|mo)/ui ],
    [ 'an', /^ut[th]/ui ],
    [ 'a',  /^u[bcfhjkqrst][aeiou]/ui ],
    [ 'a',  /^U[NK][AIEO]?/u ],
    [ 'an', /^[aeiou]/ui ],
    [ 'an', /^y(b[lor]|cl[ea]|fere|gg|p[ios]|rou|tt)/ui ],
];

type Options = {
    /** only return the indefinite article, do not combine with the input */
    only?: boolean;
};

/**
 * Determine the appropriate indefinite article to use with a word.
 *
 * @remarks The answer is derived from a simple rules engine, it attempts to cover most exceptions
 * to the rules, but the English language has lots of quirks, and this rules engine can not cover them
 * all
 *
 * @param word The word
 * @param __namedParameters see {@link Options}
 * @default only false
 * @returns The appropriate indefinite article ("a" or "an") combined with the input word.  If the only
 * option is used, only the indefinite article is returned.
 */
export function indefiniteArticle(word: string, { only = false }: Options = {}): string {
    let result = 'a';
    for(const [ article, rule ] of TESTS) {
        if(rule.test(word)) {
            result = article;
            break;
        }
    }

    return only ? result : `${result} ${word}`;
}

export default indefiniteArticle;

/*
for(const [a, w] of [
['an', 'Ath'],
['a',  'Bth'],
['a',  'Cth'],
['a',  'Dth'],
['an', 'Eth'],
['an', 'Fth'],
['a',  'Gth'],
['an', 'Hth'],
['an', 'Ith'],
['a',  'Jth'],
['a',  'Kth'],
['an', 'Lth'],
['an', 'Mth'],
['an', 'Nth'],
['an', 'Oth'],
['a',  'Pth'],
['a',  'Qth'],
['an', 'Rth'],
['an', 'Sth'],
['a',  'Tth'],
['a',  'Uth'],
['a',  'Vth'],
['a',  'Wth'],
['an', 'Xth'],
['a',  'Yth'],
['a',  'Zth'],
['an', 'a-th'],
['a',  'b-th'],
['a',  'c-th'],
['a',  'd-th'],
['an', 'e-th'],
['an', 'f-th'],
['a',  'g-th'],
['an', 'h-th'],
['an', 'i-th'],
['a',  'j-th'],
['a',  'k-th'],
['an', 'l-th'],
['an', 'm-th'],
['an', 'n-th'],
['an', 'o-th'],
['a',  'p-th'],
['a',  'q-th'],
['an', 'r-th'],
['an', 's-th'],
['a',  't-th'],
['a',  'u-th'],
['a',  'v-th'],
['a',  'w-th'],
['an', 'x-th'],
['a',  'y-th'],
['a',  'z-th'],
['an', 'A.B.C'],
['an', 'AI'],
['an', 'AGE'],
['an', 'agendum'],
['an', 'aide-de-camp'],
['an', 'albino'],
['a',  'B.L.T. sandwich'],
['a',  'BMW'],
['a',  'BLANK'],
['a',  'bacterium'],
['a',  'Burmese restaurant'],
['a',  'C.O.'],
['a',  'CCD'],
['a',  'COLON'],
['a',  'cameo'],
['a',  'CAPITAL'],
['a',  'D.S.M.'],
['a',  'DNR'],
['a',  'DINNER'],
['a',  'dynamo'],
['an', 'E.K.G.'],
['an', 'ECG'],
['an', 'EGG'],
['an', 'embryo'],
['an', 'erratum'],
['a',  'eucalyptus'],
['an', 'Euler number'],
['a',  'eulogy'],
['a',  'euphemism'],
['a',  'euphoria'],
['a',  'ewe'],
['a',  'ewer'],
['an', 'extremum'],
['an', 'eye'],
['an', 'F.B.I. agent'],
['an', 'FSM'],
['a',  'FACT'],
['a',  'FAQ'],
['an', 'F.A.Q.'],
['a',  'fish'],
['a',  'G-string'],
['a',  'GSM phone'],
['a',  'GOD'],
['a',  'genus'],
['a',  'Governor General'],
['an', 'H-Bomb'],
['an', 'H.M.S Ark Royal'],
['an', 'HSL colour space'],
['a',  'HAL 9000'],
['an', 'H.A.L. 9000'],
['a',  'has-been'],
['a',  'height'],
['an', 'heir'],
['a',  'honed blade'],
['an', 'honest man'],
['a',  'honeymoon'],
['an', 'honorarium'],
['an', 'honorary degree'],
['an', 'honoree'],
['an', 'honorific'],
['a',  'Hough transform'],
['a',  'hound'],
['an', 'hour'],
['an', 'hourglass'],
['a',  'houri'],
['a',  'house'],
['an', 'I.O.U.'],
['an', 'IQ'],
['an', 'IDEA'],
['an', 'inferno'],
['an', 'Inspector General'],
['a',  'jumbo'],
['a',  'knife'],
['an', 'L.E.D.'],
['a',  'LED'],
['an', 'LCD'],
['a',  'lady in waiting'],
['a',  'leaf'],
['an', 'M.I.A.'],
['a',  'MIASMA'],
['an', 'MTV channel'],
['a',  'Major General'],
['an', 'N.C.O.'],
['an', 'NCO'],
['a',  'NATO country'],
['a',  'note'],
['an', 'O.K.'],
['an', 'OK'],
['an', 'OLE'],
['an', 'octavo'],
['an', 'octopus'],
['an', 'okay'],
['a',  'once-and-future-king'],
['an', 'oncologist'],
['a',  'one night stand'],
['an', 'onerous task'],
['an', 'opera'],
['an', 'optimum'],
['an', 'opus'],
['an', 'ox'],
['a',  'Ph.D.'],
['a',  'PET'],
['a',  'P.E.T. scan'],
['a',  'plateau'],
['a',  'quantum'],
['an', 'R.S.V.P.'],
['an', 'RSVP'],
['a',  'REST'],
['a',  'reindeer'],
['an', 'S.O.S.'],
['a',  'SUM'],
['an', 'SST'],
['a',  'salmon'],
['a',  'T.N.T. bomb'],
['a',  'TNT bomb'],
['a',  'TENT'],
['a',  'thought'],
['a',  'tomato'],
['a',  'U-boat'],
['a',  'UNESCO representative'],
['a',  'U.F.O.'],
['a',  'UFO'],
['a',  'UK citizen'],
['a',  'ubiquity'],
['a',  'unicorn'],
['an', 'unidentified flying object'],
['a',  'uniform'],
['a',  'unimodal system'],
['an', 'unimpressive record'],
['an', 'uninformed opinion'],
['an', 'uninvited guest'],
['a',  'union'],
['a',  'uniplex'],
['a',  'uniprocessor'],
['a',  'unique opportunity'],
['a',  'unisex hairdresser'],
['a',  'unison'],
['a',  'unit'],
['a',  'unitarian'],
['a',  'united front'],
['a',  'unity'],
['a',  'univalent bond'],
['a',  'univariate statistic'],
['a',  'universe'],
['an', 'unordered meal'],
['a',  'uranium atom'],
['an', 'urban myth'],
['an', 'urbane miss'],
['an', 'urchin'],
['a',  'urea detector'],
['a',  'urethane monomer'],
['an', 'urge'],
['an', 'urgency'],
['a',  'urinal'],
['an', 'urn'],
['a',  'usage'],
['a',  'use'],
['an', 'usher'],
['a',  'usual suspect'],
['a',  'usurer'],
['a',  'usurper'],
['a',  'utensil'],
['a',  'utility'],
['an', 'utmost urgency'],
['a',  'utopia'],
['an', 'utterance'],
['a',  'V.I.P.'],
['a',  'VIPER'],
['a',  'viper'],
['an', 'X-ray'],
['an', 'X.O.'],
['a',  'XYLOPHONE'],
['an', 'XY chromosome'],
['a',  'xenophobe'],
['a',  'Y-shaped pipe'],
['a',  'Y.Z. plane'],
['a',  'YMCA'],
['an', 'YBLENT eye'],
['an', 'yblent eye'],
['an', 'yclad body'],
['a',  'yellowing'],
['a',  'yield'],
['a',  'youth'],
['a',  'youth'],
['an', 'ypsiliform junction'],
['an', 'yttrium atom'],
['a',  'zoo'],
]) {
    const ia = indefiniteArticle(w);
}
*/