function check_dimensions(l, w, h)
{
  var length, width, height;

  if (l != null && w != null && h != null)
  {
    // check if any dimensions have been entered
    if (l.value.length > 0 || w.value.length > 0 || h.value.length > 0)
    {
      if (l.value.length == 0)
      {
        focusOn(l);
        alert(enter_length_dimension);
        return false;
      }

      if (w.value.length == 0)
      {
        focusOn(w);
        alert(enter_width_dimension);
        return false;
      }

      if (h.value.length == 0)
      {
        focusOn(h);
        alert(enter_height_dimension);
        return false;
      }

      // normalize decimal points
      length = l.value.replace(',', '.');
      width  = w.value.replace(',', '.');
      height = h.value.replace(',', '.');

      // must be valid numbers
      if (isNaN(length))
      {
        focusOn(l);
        alert(length_not_a_number);
        return false;
      }

      if (isNaN(width))
      {
        focusOn(w);
        alert(width_not_a_number);
        return false;
      }

      if (isNaN(height))
      {
        focusOn(h);
        alert(height_not_a_number);
        return false;
      }

      length = Number(length);
      width  = Number(width);
      height = Number(height);

      // cannot be zero or negative
      if (length <= 0)
      {
        focusOn(l);
        alert(invalid_dimension);
        return false;
      }

      if (width <= 0)
      {
        focusOn(w);
        alert(invalid_dimension);
        return false;
      }

      if (height <= 0)
      {
        focusOn(h);
        alert(invalid_dimension);
        return false;
      }

      // cannot be smaller than 0.01
      if (length < 0.01)
      {
        focusOn(l);
        alert(dimension_too_small);
        return false;
      }

      if (width < 0.01)
      {
        focusOn(w);
        alert(dimension_too_small);
        return false;
      }

      if (height < 0.01)
      {
        focusOn(h);
        alert(dimension_too_small);
        return false;
      }

      // length must be the larger dimension
      if (length < width || length < height)
      {
        focusOn(l);
        alert(length_must_be_greater);
        return false;
      }
    }
  }

  return true;
}

function focusOn(element)
{
  var elem = $(element);

  if (elem.is(':hidden'))
    toggle_section(elem.closest('div.collapsible_section').prev().prev(), false);

  elem.focus();
}