package cn.edu.bjut.textmining.chapter2;

import cn.edu.bjut.textmining.util.FileUtil;

import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.StandardCharsets;
import java.nio.file.Path;

public class MappedFileDemo {
    private static final int MAP_BYTES = 2048;
    private static final int PREVIEW_CHARS = 220;

    public static void main(String[] args) {
        Path path = FileUtil.path("data", "chapter2", "tang-poems-classics.txt");

        try (RandomAccessFile file = new RandomAccessFile(path.toFile(), "r");
             FileChannel channel = file.getChannel()) {

            long fileSize = channel.size();
            long mapSize = Math.min(fileSize, MAP_BYTES);
            MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, mapSize);

            String mappedText = StandardCharsets.UTF_8.decode(buffer).toString();
            int previewEnd = Math.min(mappedText.length(), PREVIEW_CHARS);
            int lastLineBreak = mappedText.lastIndexOf('\n', previewEnd);
            if (lastLineBreak > 0) {
                previewEnd = lastLineBreak;
            }

            System.out.println("文件大小：" + fileSize + " 字节");
            System.out.println("本次映射：" + mapSize + " 字节");
            System.out.println("文件头部内容：");
            System.out.println(mappedText.substring(0, previewEnd));
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}