package class8_3_PhonebookCommands_start;//package pbook;

import java.util.Stack;

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

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

   // returns null if no command to redo, the command otherwise
   public Command nextToRedo()
   {
      if ( undoneCmds.isEmpty() )
         return null;
      else
         return undoneCmds.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();
      undoneCmds.clear();
      doneCmds.push(new_cmd);
   }

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

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