function init()
{
  document.getElementById("target_textarea").value = "";
  document.getElementById("raw_source_textarea").value = "";
  document.getElementById("source").value = "";
  document.getElementById("current_seg_id").value = "";
  document.getElementById("paused").value = "";
  document.getElementById("next").removeAttribute("disabled");

  return false;
}

function CreateCORSRequest(method, url)
{
  var xhr = new XMLHttpRequest();
  if ("withCredentials" in xhr) {
    xhr.open(method, url, true);
  } else {
    xhr = null;
  }

  return xhr;
}

function catch_return(e)
{
  if (e.keyCode == 13) {
    e.preventDefault();
    Next();
  }

  return false;
}

function pause()
{
  var paused = document.getElementById("paused");
  var button = document.getElementById("pause_button");
  var next_button = document.getElementById("next");
  if (paused.value == 0) {
    button.innerHTML = "Unpause";
    paused.value = 1;
    next.setAttribute("disabled", "disabled");
  } else {
    button.innerHTML = "Pause";
    paused.value = 0;
    next.removeAttribute("disabled");
  }
}

function Next()
{
  // elements
  var button = document.getElementById("next");
  var target_textarea = document.getElementById("target_textarea")
  var raw_source_textarea = document.getElementById("raw_source_textarea");
  var current_seg_id = document.getElementById("current_seg_id");
  var source = document.getElementById("source");
  var status = document.getElementById("status");

  // disable button and textarea
  button.setAttribute("disabled", "disabled");
  target_textarea.setAttribute("disabled", "disabled");

  var port = document.getElementById("port").value;
  var base_url = "http://coltrane.cl.uni-heidelberg.de:"+port;

  var key = document.getElementById("key").value;
  next_url = base_url+"/next?key="+key;
  var post_edit = target_textarea.value;
  if (post_edit != "") {
    // compose request
    next_url += "&example="+source.value+" %7C%7C%7C "+post_edit;
    // update document overview
    document.getElementById("seg_"+(current_seg_id.value)+"_t").innerHTML=post_edit;
  } else {
    if (source.value != "") {
      alert("Error: 1");
    }
    // FIXME: do something reasonable
  }

  // show 'working' message
  status.style.display = "block";

  // build request
  var xhr = CreateCORSRequest('get', next_url);
  if (!xhr) {
    alert("Error: 2");
    // FIXME: do something reasonable
  }

  // 'next' request's callbacks
  xhr.onload = function() {
    /*
     * translation system is currently handling another request
     * FIXME: maybe poll server for result?
     *
     */
    if (xhr.responseText == "locked") {
      alert("Translation system is locked, try again in a moment (reload page and click 'Start/Continue' again).");
      document.getElementById("status").style.display = "none";

      return;
    }
    // got response: seg id\tsource\ttranslation\traw source
    //                   0   1        2           3
    var x = xhr.responseText.split("\t");
    if (x == "fi") { // done, hide/disable functional elements
      raw_source_textarea.style.display = "none";
      target_textarea.style.display = "none";
      status.style.display = "none";
      button.innerHTML = "Session finished, thank you!";
      button.setAttribute("disabled", "disabled");
      document.getElementById("pause_button").setAttribute("disabled", "disabled");
      document.getElementById("seg_"+current_seg_id.value).className = "";
    } else {
      var id = x[0];
      var src = x[1];
      var translation = x[2];
      var raw_source = x[3];

      // update interface
      status.style.display = "none";
      target_textarea.value = translation;
      raw_source_textarea.value = raw_source;
      button.innerHTML = "Next";
      button.removeAttribute("disabled");
      target_textarea.removeAttribute("disabled", "disabled");
      document.getElementById("seg_"+id).className = "bold";
      if (x[0] > 0) {
        document.getElementById("seg_"+(id-1)).className = "";
      }
      target_textarea.rows = Math.round(translation.length/80)+1;
      raw_source_textarea.rows = Math.round(raw_source.length/80)+1;
      target_textarea.focus();
      target_textarea.selectionStart = 0;
      target_textarea.selectionEnd = 0;

      // remember aux data in DOM
      current_seg_id.value = id;
      source.value = src;

      // confirm to server
      var xhr_confirm = CreateCORSRequest('get', base_url+"/confirm");
      xhr_confirm.send(); // FIXME: handle errors
    }
  };

  xhr.onerror = function() {
    // FIXME: do something reasonable
  };

  xhr.send(); // send 'next' request
}