Parse IPv6

ipv6.js

// https://tools.ietf.org/html/rfc5952
function parseIPv6(addr) {
  // does it have an IPv4 suffix
  let ipv4 = /^([\s\S]*):(\d+)\.(\d+)\.(\d+)\.(\d+)$/.exec(addr);
  if (ipv4) {
    // dot-decimal to ints
    let dec = ipv4.slice(2).map(s => parseInt(s, 10));
    if (dec.some(i => i > 255)) return null;
    // ints to hexs
    let hex = dec.map(i => ('0' + i.toString(16)).substr(-2));
    // rebuild
    addr = ipv4[1] + ':' + hex[0] + hex[1] + ':' + hex[2] + hex[3];
  }

  // where's the ::, is there a single one?
  let ellipsis = addr.indexOf('::');
  if (ellipsis !== addr.lastIndexOf('::')) return null;

  let groups;
  if (ellipsis < 0) {
    // must have exactly 8 groups
    groups = addr.split(':', 9);
    if (groups.length != 8) return null;
  } else {
    // must have less than 8 groups
    let head = [];
    let tail = [];
    if (ellipsis > 0)
      head = addr.slice(0, ellipsis).split(':', 8);
    if (ellipsis < addr.length - 2)
      tail = addr.slice(ellipsis + 2).split(':', 8);
    if (head.length + tail.length > 7) return null;
    // fill in ellipsis, concat tail
    head.length = 8 - tail.length;
    groups = head.concat(tail);
  }

  // convert to an array of 8 16-bit ints
  let ipv6 = [];
  for (let i = 0; i < 8; ++i) {
    let g = groups[i] || '0';
    if (!/^[0-9a-fA-F]{1,4}$/.test(g)) return null;
    ipv6.push(parseInt(g, 16));
  }

  Object.defineProperty(ipv6, 'getByte', {
    value: function(i) {
      let b = this[i / 2 >>> 0];
      return (i & 1) === 0 ? b >>> 8 : b & 0xff;
    }
  });

  Object.defineProperty(ipv6, 'toString', {
    value: function() {
      // find longest run of zeros
      let e0 = -1;
      let e1 = -1;
      for (let i = 0; i < 8; ++i) {
        let j = i;
        while (this[j] === 0) ++j;
        if (j - i > e1 - e0) {
          e0 = i;
          e1 = j;
        }
        if (j > i) i = j;
      }
      // don't replace single group
      if (e1 - e0 <= 1) {
        e0 = -1;
        e1 = -1;
      }

      let str = '';
      for (let i = 0; i < 8; ++i) {
        if (i === e0) {
          str += '::';
          if (e1 >= 8) break;
          i = e1;
        } else if (i > 0) {
          str += ':';
        }
        str += this[i].toString(16);
      }
      return str;
    }
  });

  return ipv6;
}