// @flow

const EMOJI_REGEX = /(?:[\u2700-\u27bf]|(?:\ud83c[\udde6-\uddff]){2}|[\ud800-\udbff][\udc00-\udfff])[\ufe0e\ufe0f]?(?:[\u0300-\u036f\ufe20-\ufe23\u20d0-\u20f0]|\ud83c[\udffb-\udfff])?(?:\u200d(?:[^\ud800-\udfff]|(?:\ud83c[\udde6-\uddff]){2}|[\ud800-\udbff][\udc00-\udfff])[\ufe0e\ufe0f]?(?:[\u0300-\u036f\ufe20-\ufe23\u20d0-\u20f0]|\ud83c[\udffb-\udfff])?)*/g;

export function toHex(str: string): string {
  const array = Array.from(str);

  let result = '';

  for (var i = 0; i < array.length; i++) {
    const val = array[i];

    const isEmoji = EMOJI_REGEX.test(val);

    const utf = isEmoji
      ? toUTF8Array(val)
          .map((num) => num.toString(16))
          .join('')
      : val.charCodeAt(0).toString(16);

    result += utf;
  }

  return result;
}

function toUTF8Array(str) {
  var utf8 = [];
  for (var i = 0; i < str.length; i++) {
    var charcode = str.charCodeAt(i);
    if (charcode < 0x80) utf8.push(charcode);
    else if (charcode < 0x800) {
      utf8.push(0xc0 | (charcode >> 6), 0x80 | (charcode & 0x3f));
    } else if (charcode < 0xd800 || charcode >= 0xe000) {
      utf8.push(0xe0 | (charcode >> 12), 0x80 | ((charcode >> 6) & 0x3f), 0x80 | (charcode & 0x3f));
    }
    // surrogate pair
    else {
      i++;
      charcode = (((charcode & 0x3ff) << 10) | (str.charCodeAt(i) & 0x3ff)) + 0x010000;
      utf8.push(
        0xf0 | (charcode >> 18),
        0x80 | ((charcode >> 12) & 0x3f),
        0x80 | ((charcode >> 6) & 0x3f),
        0x80 | (charcode & 0x3f)
      );
    }
  }
  return utf8;
}