package com.gimi.common.cinema.utils;

import java.io.IOException;
import java.io.RandomAccessFile;
import java.security.MessageDigest;

/**
 * Created by wugian on 2016/7/15
 */
public class FileHashUtils {

    /**
     * 射手网做法，取特定位置字节转化计算成hash值
     *
     * @param filePath
     * @return
     */
    public static String getFileHash(String filePath) {
        try {
            RandomAccessFile f = new RandomAccessFile(filePath, "r");
            long total_size = f.length();
            String md5 = "";
            long seek_positions[] = {0, 0, 0, 0};
            seek_positions[0] = 4096;
            seek_positions[1] = total_size / 3 * 2;
            seek_positions[2] = total_size / 3;
            seek_positions[3] = total_size - 8192;

            for (int i = 0; i < 4; i++) {
                byte[] buffer = new byte[4096];
                f.seek(seek_positions[i]);
                f.read(buffer, 0, 4096);
                if (md5.length() != 0)
                    md5 = md5 + ";" + bytesToMD5(buffer);
                else
                    md5 = md5 + bytesToMD5(buffer);
            }
            return md5;
        } catch (IOException e) {
            e.printStackTrace();
        }
        return "";
    }

    private static String bytesToHex(byte[] bytes) {
        StringBuffer md5str = new StringBuffer();
        // 把数组每一字节换成16进制连成md5字符串
        int digital;
        for (int i = 0; i < bytes.length; i++) {
            digital = bytes[i];
            if (digital < 0) {
                digital += 256;
            }
            if (digital < 16) {
                md5str.append("0");
            }
            md5str.append(Integer.toHexString(digital));
        }
        return md5str.toString().toLowerCase();
    }

    private static String bytesToMD5(byte[] input) {
        String md5str = null;
        try {
            // 创建一个提供信息摘要算法的对象，初始化为md5算法对象
            MessageDigest md = MessageDigest.getInstance("MD5");
            // 计算后获得字节数组
            byte[] buff = md.digest(input);
            // 把数组每一字节换成16进制连成md5字符串
            md5str = bytesToHex(buff);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return md5str;
    }
}