/**
 * Convert IPv4 to Decimal
 * @param {String} ip - a String represents an IPv4
 * @returns a decimal if the input is valid. Otherwise, it returns -1.
 */
function convertIPv4toDecimal(ip) {
  // Simply validate input as IPv4
  const ipv4RegEx = /^(\d{1,3}\.){3}(\d{1,3})$/;
  const valid = ipv4RegEx.test(ip);
  if (!valid) {
    return -1;
  }

  let ipDecimal = 0;
  const octets = ip.split('.');

  for (let i = 0; i < 4; i++) {
    const octet = Number(octets[i]);

    // make sure each value is between 0 and 255
    if (octet > 255 || octet < 0) {
      return -1;
    }

    ipDecimal = ipDecimal * 256 + octet;
  }

  return ipDecimal;
}

/**
 * Convert Decimal to IPv4
 * @param {Number} decimal - a Number represents a Decimal
 * @returns an IPv4 as String if the input is valid. 
 * Otherwise, it returns an empty string ("").
 */
function convertDecimalToIPv4(decimal) {
  if (typeof decimal !== "number" || 
      Number.isInteger(decimal) === false ||
      decimal < 0 || 
      decimal > 4294967295
  ) {
    return "";
  }

  const p1 = decimal >>> 24;
  const p2 = (decimal & 0x00ff0000) >>> 16;
  const p3 = (decimal & 0x0000ff00) >>> 8;
  const p4 = (decimal & 0x000000ff);

  if (p1 > 255 || p2 > 255 || p3 > 255 || p4 > 255) {
     return "";
  }

  return `${p1}.${p2}.${p3}.${p4}`;
}

/**
 * Get the value corresponding with a variable in the URL's query string.
 * 
 * @param {String} variable - a variable to get in the URL's query string.
 * @returns the value of the variable.
 * 
 * Assume the URL is "http://www.example.com/index.php?id=1&image=awesome.jpg".
 * The query string is the result of calling "window.location.search"
 * So queryString = "?id=1&image=awesome.jpg".
 * 
 * Calling getQueryVariable("id") - would return "1".
 * Calling getQueryVariable(queryString, "image") - would return "awesome.jpg".
 * 
 * Ref: https://css-tricks.com/snippets/javascript/get-url-variables/
 */
function getURLVariable(variable) {
  const query = window.location.search.substring(1);
  const vars = query.split("&");
  const length = vars.length;

  for (let i = 0; i < length; i++) {
    const pair = vars[i].split("=");
    if (pair[0] === variable) { 
      return pair[1]; 
    }
  }

  return (false);
}