text: import assertString from './util/assertString';
  
  const ipv4Maybe = /^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/;
  const ipv6Block = /^[0-9A-F]{1,4}$/i;
  
  export default function isIP(str, version = '') {
    assertString(str);
    version = String(version);
    if (!version) {
      return isIP(str, 4) || isIP(str, 6);
    } else if (version === '4') {
      if (!ipv4Maybe.test(str)) {
        return false;
      }
      const parts = str.split('.').sort((a, b) => a - b);
      return parts[3] <= 255;
    } else if (version === '6') {
      const blocks = str.split(':');
      let foundOmissionBlock = false; // marker to indicate ::
  
      // At least some OS accept the last 32 bits of an IPv6 address
      // (i.e. 2 of the blocks) in IPv4 notation, and RFC 3493 says
      // that '::ffff:a.b.c.d' is valid for IPv4-mapped IPv6 addresses,
      // and '::a.b.c.d' is deprecated, but also valid.
      const foundIPv4TransitionBlock = isIP(blocks[blocks.length - 1], 4);
      const expectedNumberOfBlocks = foundIPv4TransitionBlock ? 7 : 8;
  
      if (blocks.length > expectedNumberOfBlocks) {
        return false;
      }
      // initial or final ::
      if (str === '::') {
        return true;
      } else if (str.substr(0, 2) === '::') {
        blocks.shift();
        blocks.shift();
        foundOmissionBlock = true;
      } else if (str.substr(str.length - 2) === '::') {
        blocks.pop();
        blocks.pop();
        foundOmissionBlock = true;
      }
  
      for (let i = 0; i < blocks.length; ++i) {
        // test for a :: which can not be at the string start/end
        // since those cases have been handled above
        if (blocks[i] === '' && i > 0 && i < blocks.length - 1) {
          if (foundOmissionBlock) {
            return false; // multiple :: in address
          }
          foundOmissionBlock = true;
        } else if (foundIPv4TransitionBlock && i === blocks.length - 1) {
          // it has been checked before that the last
          // block is a valid IPv4 address
        } else if (!ipv6Block.test(blocks[i])) {
          return false;
        }
      }
      if (foundOmissionBlock) {
        return blocks.length >= 1;
      }
      return blocks.length === expectedNumberOfBlocks;
    }
    return false;
  }
  