package shared;

import java.io.*;
import java.nio.*;
import java.nio.channels.*;

public class SharedMemory {
	private final static int DATA_UNIT = 64;
	private final static int HEAD_LENGTH = DATA_UNIT;
	private final static int TAIL_LENGTH = DATA_UNIT;
	private final static byte[] zero = new byte[DATA_UNIT];
	private final static int FILE_BEG_TAG = 0XF1ABABF2;
	private final static int FILE_END_TAG = 0XF3ABABF4;
	private String filename;
	private File file;
	private RandomAccessFile rafile;
	private FileChannel channel;
	private MappedByteBuffer mapBuf;
	private int size;
	private int dataSize;
	
	public static class SharedMemoryException extends RuntimeException {
		public SharedMemoryException(Exception ex) {
			super(String.format("%s: %s", ex.getClass().getName(), ex.getMessage()));
			setStackTrace(ex.getStackTrace());
		}
		public String toString() {
			return super.toString();
		}
	}
	
	public SharedMemory(String filename, int size) {
		long extra = HEAD_LENGTH + TAIL_LENGTH;
		if (size<=extra || size%DATA_UNIT!=0) {
			throw new RuntimeException(String.format(
					"size must be multiple of %d and bigger than %d.", DATA_UNIT, extra));
		}
		this.filename = filename;
		this.size = size;
		this.dataSize = size - HEAD_LENGTH - TAIL_LENGTH;
		initFile();
	}
	
	private void buildMapBuffer() throws Exception {
		channel = rafile.getChannel();
		long filesize = channel.size();
		System.out.println(String.format("map size %d.", filesize));
		mapBuf = channel.map(FileChannel.MapMode.READ_WRITE, 0, filesize);
	}
	
	private void initFile() {
		try {
			file = new File(filename);
			if (!file.exists()) {
				rafile = new RandomAccessFile(file, "rw");
				int cnt = size/DATA_UNIT;
				while (cnt-->0)
					rafile.write(zero);
				buildMapBuffer();
				mapBuf.position(0);
				mapBuf.putInt(FILE_BEG_TAG);
				mapBuf.putInt(size);
				mapBuf.position(HEAD_LENGTH+dataSize);
				mapBuf.putInt(FILE_END_TAG);
				mapBuf.force();
			} else {
				rafile = new RandomAccessFile(file, "rw");
				buildMapBuffer();
			}
		} catch (Exception e) {
			throw new SharedMemoryException(e);
		}
	}
	
	public int getSize() {
		return size;
	}
	
	public int getStartPos() {
		return HEAD_LENGTH;
	}
	
	public int getDataSize() {
		return dataSize;
	}
	
	public FileChannel getChannel() {
		return channel;
	}
	
	public MappedByteBuffer getMapBuffer() {
		return mapBuf;
	}
	
	public FileLock lock() throws Exception {
		return channel.lock();
	}
	
	public FileLock lock(long position, long size, boolean shared) throws Exception {
		return channel.lock(position, size, shared);
	}
	
	public FileLock tryLock() throws Exception {
		return channel.tryLock();
	}
	
	public FileLock tryLock(long position, long size, boolean shared) throws Exception {
		return channel.tryLock(position, size, shared);
	}
	
	public static void main(String[] args) {
		String filename = "sharetest.txt";
		int size = 1024;
		SharedMemory shareMemory = new SharedMemory(filename, size);
		System.out.println(String.format("dataSize is %d.", shareMemory.getDataSize()));
	}

}