// Apply the chosen theme, and save that choice in a cookie on the user's browser
//
function SelectTheme(which) {
	reloadCSS(which);
	setCookie('theme', which, 10);	// Remember which theme they like
}

// Apply the CSS, which hosts the theme.
//
function reloadCSS(which) {
	var links = document.getElementsByTagName("link");
	for (var cl in links) {
		var link = links[cl];
		if (link.rel === "stylesheet")
			link.href = "inc/theme-" + which + ".css";
	}
}

//
//
function includeHTML() {
  var z, i, elmnt, file, xhttp;
  /* Loop through a collection of all HTML elements: */
  z = document.getElementsByTagName("*");
  for (i = 0; i < z.length; i++) {
    elmnt = z[i];
    /*search for elements with a certain atrribute:*/
    file = elmnt.getAttribute("w3-include-html");
    if (file) {
      /* Make an HTTP request using the attribute value as the file name: */
      xhttp = new XMLHttpRequest();
      xhttp.onreadystatechange = function() {
        if (this.readyState == 4) {
          if (this.status == 200) {elmnt.innerHTML = this.responseText;}
          if (this.status == 404) {elmnt.innerHTML = "Page not found.";}
          /* Remove the attribute, and call this function once more: */
          elmnt.removeAttribute("w3-include-html");
          includeHTML();
        }
      }
      xhttp.open("GET", file, true);
      xhttp.send();
      return;
    }
  }
}

// Save the cookie on the user's browser - this is for the color theme
// and it has a timeout of some number of days, after which it expires.
//
function setCookie(cname, cvalue, exdays) {
  const d = new Date();
  d.setTime(d.getTime() + (exdays*24*60*60*1000));
  let expires = "expires="+ d.toUTCString();
  document.cookie = cname + "=" + cvalue + ";" + expires + ";path=/";
}

// Try to get the cookie. If the user has one, then it will use that
// to apply a specific theme.
//
function getCookie(cname) {
	let name = cname + "=";
	let decodedCookie = decodeURIComponent(document.cookie);
	let ca = decodedCookie.split(';');
	for(let i = 0; i <ca.length; i++) {
		let c = ca[i];
		while (c.charAt(0) == ' ') {
			c = c.substring(1);
		}
		if (c.indexOf(name) == 0) {
			return c.substring(name.length, c.length);
		}
	}
	return "";
}