content: /*
   * Copyright 2024-2025 the original author or authors.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      https://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.alibaba.cloud.ai.graph.utils;
  
  import java.sql.Timestamp;
  import java.util.concurrent.Executors;
  import java.util.concurrent.ScheduledExecutorService;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * <p>
   * Optimization for performance issues with System.currentTimeMillis() in high concurrency
   * scenarios. This approach is also used to implement timestamps in Redis's LRU list (very
   * efficient and useful)
   * </p>
   * <p>
   * Invoking System.currentTimeMillis() is much more time-consuming compared to creating a
   * regular object<br>
   * The reason System.currentTimeMillis() is slow is because it interacts with the
   * operating system<br>
   * The clock is updated periodically in the background, and the thread is automatically
   * reclaimed when the JVM exits<br>
   * Performance comparison data:<br>
   * 1 Billion calls: 43410 vs 206, a difference of 210.73%<br>
   * 100 Million calls: 4699 vs 29, a difference of 162.03%<br>
   * 10 Million calls: 480 vs 12, a difference of 40.0%<br>
   * 1 Million calls: 50 vs 10, a difference of 5.0%<br>
   * </p>
   *
   * @author disaster
   * @version 1.0.0.1
   */
  public class SystemClock {
  
  	private final long period;
  
  	private final AtomicLong now;
  
  	private SystemClock(long period) {
  		this.period = period;
  		this.now = new AtomicLong(System.currentTimeMillis());
  		scheduleClockUpdating();
  	}
  
  	private static SystemClock instance() {
  		return InstanceHolder.INSTANCE;
  	}
  
  	public static long now() {
  		return instance().currentTimeMillis();
  	}
  
  	public static String nowDate() {
  		return new Timestamp(instance().currentTimeMillis()).toString();
  	}
  
  	private void scheduleClockUpdating() {
  		ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor(runnable -> {
  			Thread thread = new Thread(runnable, "System Clock");
  			thread.setDaemon(true);
  			return thread;
  		});
  		scheduler.scheduleAtFixedRate(() -> now.set(System.currentTimeMillis()), period, period, TimeUnit.MILLISECONDS);
  	}
  
  	private long currentTimeMillis() {
  		return now.get();
  	}
  
  	private static class InstanceHolder {
  
  		public static final SystemClock INSTANCE = new SystemClock(1);
  
  	}
  
  }
  