function pick_heap(heap_id)
{
  var picked_beans = document.getElementById("picked_beans");
  var picked_heap = document.getElementById("picked_heap");
  var cur_heap =  document.getElementById(heap_id);
  var sub_cur_heap = document.getElementById("sub_" + heap_id);
  var heap_index;
  
  switch(heap_id)
  {
    case "heap_a" : heap_index = 0; break;
    case "heap_b" : heap_index = 1; break;
    case "heap_c" : heap_index = 2; break;
    case "heap_d" : heap_index = 3; break;
  }
  
  
  if(cur_heap.value > 0)
  {
    if(picked_heap.value == "")
    {
      picked_heap.value = heap_index;
    }
    else
    {
      if(heap_index != picked_heap.value)
      {
        return;
      }
    }
    if(!isNaN(parseInt(picked_beans.value)))
		{
			picked_beans.value = parseInt(picked_beans.value) + 1;
			sub_cur_heap.innerHTML = "-"+ picked_beans.value;
			cur_heap.value = parseInt(cur_heap.value) - 1;
		}
  }
}

function has_picked()
{
  var picked_beans = document.getElementById("picked_beans");
  if(picked_beans.value == 0)
  {
    alert("请拣取豆子。");
    return false;
  }
  return true;
}