src/Pure/Concurrent/counter.scala

discontinued "isabelle usedir" option -r (reset session path);
simplified internal session identification: chapter / name;
clarified chapter index (of sessions) vs. session index (of theories);
discontinued "up" links, for improved modularity also wrt. partial browser_info (users can use "back" within the browser);
removed obsolete session parent_path;

/*  Title:      Pure/Concurrent/counter.scala
    Module:     PIDE
    Author:     Makarius

Synchronized counter for unique identifiers < 0.

NB: ML ticks forwards, JVM ticks backwards.
*/

package isabelle


object Counter
{
  type ID = Long
  def apply(): Counter = new Counter
}

final class Counter private
{
  private var count: Counter.ID = 0

  def apply(): Counter.ID = synchronized {
    require(count > java.lang.Long.MIN_VALUE)
    count -= 1
    count
  }
}