package policy;//package pbook;

import policy.Command;

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);
   }
}