// Global object to hold drag information.
var dragObj = new Object();
dragObj.zIndex = 0;

function drag(event, id) {
  var x, y;

  // If an element id was given, find it. Otherwise use the element being
  // clicked on.
  if (id)
    dragObj.elNode = document.getElementById(id);
  else {
    dragObj.elNode = event.target;
    // If this is a text node, use its parent element.
    if (dragObj.elNode.nodeType == 3)
      dragObj.elNode = dragObj.elNode.parentNode;
  } 

  // Get cursor position with respect to the page.
  x = event.clientX + window.scrollX;
  y = event.clientY + window.scrollY;

  // Save starting positions of cursor and element.
  dragObj.cursorStartX = x;
  dragObj.cursorStartY = y;
  dragObj.elStartLeft  = parseInt(dragObj.elNode.style.left, 10);
  dragObj.elStartTop   = parseInt(dragObj.elNode.style.top,  10);

  if (isNaN(dragObj.elStartLeft)) dragObj.elStartLeft = 0;
  if (isNaN(dragObj.elStartTop))  dragObj.elStartTop  = 0;

  // Update element's z-index.
  dragObj.elNode.style.zIndex = ++dragObj.zIndex;

  // Capture mousemove and mouseup events on the page.
  document.addEventListener("mousemove", dragGo,   true);
  document.addEventListener("mouseup",   dragStop, true);
  event.preventDefault();
}

function dragGo(event) {
  var x, y;

  // Get cursor position with respect to the page.
  x = event.clientX + window.scrollX;
  y = event.clientY + window.scrollY;

  // Move drag element by the same amount the cursor has moved.

  dragObj.elNode.style.left =
    (dragObj.elStartLeft + x - dragObj.cursorStartX) + "px";
  dragObj.elNode.style.top  =
    (dragObj.elStartTop  + y - dragObj.cursorStartY) + "px";
  event.preventDefault();
}

function dragStop(event) {
  // Stop capturing mousemove and mouseup events.
  document.removeEventListener("mousemove", dragGo,   true);
  document.removeEventListener("mouseup",   dragStop, true);
}