History.java

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/*
 * Turró i Cutiller Foundation. License notice.
 * Copyright (C) 2021 Lluis Turró Cutiller <http://www.turro.org/>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
 
package org.turro.elephant.context;
 
import java.util.Iterator;
import java.util.Objects;
import java.util.concurrent.BlockingDeque;
import java.util.concurrent.LinkedBlockingDeque;
 
public class History {
  
  /* Utilities */
  
  public static void add(IConstructor constructor, String path) {
    getInstance(constructor).add(path);
  }
          
  public static String get(IConstructor constructor) {
    return getInstance(constructor).get();
  }
  
  public static String peek(IConstructor constructor) {
    return getInstance(constructor).peek();
  }
  
  public static String back(IConstructor constructor) {
    History history = getInstance(constructor);
    history.get(); // remove current
    return history.get();
  }
          
  public static String peekBack(IConstructor constructor) {
    History history = getInstance(constructor);
    return history.back();
  }
  /* History */
  
  public void add(String path) {
    String last = HISTORY.peekLast();
    if(!Objects.equals(last, path)) {
      if(!HISTORY.offerLast(path)) {
        HISTORY.pollFirst();
        HISTORY.offerLast(path);
      }
    }
  }
  
  public String get() {
    return HISTORY.pollLast();
  }
  
  public String peek() {
    return HISTORY.peekLast();
  }
  
  public String back() {
    if(HISTORY.size() > 1) {
      Iterator<String> it = HISTORY.descendingIterator();
      it.next();
      return it.next();
    }
    return HISTORY.peekLast();
  }
  
  /* Stack pool */
  
  private final BlockingDeque<String> HISTORY = new LinkedBlockingDeque<>(50);
  
  /* Instance */
  
  public static History getInstance(IConstructor constructor) {
    History history = (History) constructor.getSessionAttribute(HISTORY_CONTEXT);
    if(history == null) {
      history = new History();
      constructor.setSessionAttribute(HISTORY_CONTEXT, history);
    }
    return history;
  }
  
  private History() {}
 
  private static final String HISTORY_CONTEXT = "EL_HIS_CTX";
  
}