纯真IP数据库格式详解

/**
 * 给定一个ip国家地区记录的偏移，返回一个IPLocation结构
 * @param offset 国家记录的起始偏移
 * @return IPLocation对象
 */
private IPLocation getIPLocation(long offset) {
    try {
        // 跳过4字节ip

        ipFile.seek(offset + 4);
        // 读取第一个字节判断是否标志字节

        byte b = ipFile.readByte();
        if(b == REDIRECT_MODE_1) {
            // 读取国家偏移

            long countryOffset = readLong3();
            // 跳转至偏移处

            ipFile.seek(countryOffset);
            // 再检查一次标志字节，因为这个时候这个地方仍然可能是个重定向

            b = ipFile.readByte();
            if(b == REDIRECT_MODE_2) {
                loc.country = readString(readLong3());
                ipFile.seek(countryOffset + 4);
            } else
                loc.country = readString(countryOffset);
            // 读取地区标志

            loc.area = readArea(ipFile.getFilePointer());
        } else if(b == REDIRECT_MODE_2) {
            loc.country = readString(readLong3());
            loc.area = readArea(offset + 8);
        } else {
            loc.country = readString(ipFile.getFilePointer() - 1);
            loc.area = readArea(ipFile.getFilePointer());
        }
        return loc;
    }
    catch (IOException e) {
        return null;
    }
}    

/**
 * 从offset偏移开始解析后面的字节，读出一个地区名
 * @param offset 地区记录的起始偏移
 * @return 地区名字符串
 * @throws IOException 地区名字符串
 */
private String readArea(long offset) throws IOException {
    ipFile.seek(offset);
    byte b = ipFile.readByte();
    if(b == REDIRECT_MODE_1 || b == REDIRECT_MODE_2) {
        long areaOffset = readLong3(offset + 1);
        if(areaOffset == 0)
            return LumaQQ.getString("unknown.area");
        else
            return readString(areaOffset);
    }
    else
        return readString(offset);
}

/**
 * 从offset位置读取3个字节为一个long，因为java为big-endian格式，所以没办法
 * 用了这么一个函数来做转换
 * @param offset 整数的起始偏移
 * @return 读取的long值，返回-1表示读取文件失败
 */
private long readLong3(long offset) {
    long ret = 0;
    try {
        ipFile.seek(offset);
        ipFile.readFully(b3);
        ret |= (b3[0] & 0xFF);
        ret |= ((b3[1] << 8) & 0xFF00);
        ret |= ((b3[2] << 16) & 0xFF0000);
        return ret;
    }
    catch (IOException e) {
        return -1;
    }
}    
    
/**
 * 从当前位置读取3个字节转换成long
 * @return 读取的long值，返回-1表示读取文件失败
 */
private long readLong3() {
    long ret = 0;
    try {
        ipFile.readFully(b3);
        ret |= (b3[0] & 0xFF);
        ret |= ((b3[1] << 8) & 0xFF00);
        ret |= ((b3[2] << 16) & 0xFF0000);
        return ret;
    }
    catch (IOException e) {
        return -1;
    }
}

/**
 * 从offset偏移处读取一个以0结束的字符串
 * @param offset 字符串起始偏移
 * @return 读取的字符串，出错返回空字符串
 */
private String readString(long offset) {
    try {
        ipFile.seek(offset);
        int i;
        for(i = 0, buf[i] = ipFile.readByte(); buf[i] != 0; buf[++i] = ipFile.readByte());
        if(i != 0)
         return Utils.getString(buf, 0, i, "GBK");
    }
    catch (IOException e) {            
     log.error(e.getMessage());
    }
    return "";
}