package class8_1_Command_pbook_start;//package pbook;

import java.util.Stack;

public class History {
  // done_cmds:   those commands that are currently executed
  // undone_cmds: commands which were undone
  protected Stack<Command> done_cmds = new Stack<>(), undone_cmds = new Stack<>();

  // returns null if no command to undo, the command otherwise
  public Command nextToUndo()
  {
    if ( done_cmds.isEmpty() )
      return null;
    else
      return done_cmds.peek();
  }

  // returns null if no command to redo, the command otherwise
  public Command nextToRedo()
  {
    if ( undone_cmds.isEmpty() )
      return null;
    else
      return undone_cmds.peek();
  }

  // execute command and store it on the done commands list;
  //   old undone commands are permanently deleted
  public void doCommand(Command new_cmd)
  {
    new_cmd.execute();
    undone_cmds.clear();
    done_cmds.push(new_cmd);
  }

  // undoes last executed command; precondition: at least one command to undo
  public void undo()
  {
    Command to_undo = done_cmds.pop();
    to_undo.unexecute();
    undone_cmds.push(to_undo);
  }

  // redoes last undone command; precondition: at least one command to redo
  public void redo()
  {
    Command todo = undone_cmds.pop();
    todo.execute();
    done_cmds.push(todo);
  }
}