c# 倒序读取txt文件

曾经终败给现在 2024-02-17 17:49 43阅读 0赞

倒序读取txt文件，基本思路：  
1.使用 FileStream 文件流，可以设置流的位置 position；  
2.每次读取一行：行的标记就是两个 \\n 之间的内容,占位符在流中的值为 ；  
byte n = 0xD;//回车符  
byte r = 0xA;//换行符  
3.读取时每一行从后向前读，所以输出的字符串为倒的，故读完一行后要将内容反转；

4.倒序读行时同理，行的记录也是倒的，需要将行记录反转；

调用方法：  
1.获取文件夹下的所有文件信息  
var files = GetFile("c:\\logs");  
2.获取指定文件最后200行记录  
var result = LastLog("c:\\\\logs\\log.txt",0);  
result.data ;//文件内容  
result.position; //本次读取流的起始位置

主要方法：

/// <summary>
    /// 获取目录下所有文件信息
    /// </summary>
    /// <param name="path"></param>
    /// <returns></returns>
    private List<LogFileModel> GetFile(string path)
    {
    	List<LogFileModel> list = new List<LogFileModel>();
    	var files = Directory.GetFiles(path);
    	
    	if (files.IsNullOrDbNull())
    		return null;
    	LogFileModel item;
    	FileInfo f;
    	Func<long, string> getFileSize = (x) =>
    	{
    		if (x <= 0)
    			return "0KB";
    		var d = Math.Round(x*1.0/1024/1024, 2);
    		return string.Format("{0}KB", d);
    	};
    	var fileInfos = new List<FileInfo>();
    	files.All(x =>
    	{
    		fileInfos.Add(new FileInfo(x));
    		return true;
    	});
    	int i = 1;
    	//按照创建时间倒序
    	fileInfos
    		.OrderByDescending(x=>x.CreationTime)
    		.ToList()
    		.ForEach(x =>
    	{
    		item = new LogFileModel
    		{
    			name = x.Name,
    			size=x.Length,
    			desc = getFileSize(x.Length)
    		};
    
    		list.Add(item);
    		
    		if (i++ >= 20)
    			return ;
    	});
    	return list;
    }
    
    /// <summary>
    /// 倒序读取文本文件
    /// </summary>
    /// <param name="file">文件路径</param>
    /// <param name="position">读取流的起始位置</param>
    /// <returns></returns>
    public LastLogResult LastLog(string file, long position)
    {
    	if (!File.Exists(file))
    	{
    		result.msg = "文件路径无效！";
    		return result;
    	}
    	StringBuilder txt = new StringBuilder();
    	long ps = position;
    	using (var fs = new FileStream(file,FileMode.Open,FileAccess.Read))
    	{
    		if (ps <= 0)
    			ps = fs.Length;
    		//读取200行
    		for (int i = 0; i < 200; i++)
    		{
    			ps = InverseReadRow(fs, ps, ref txt);
    			if (ps <= 0)
    				break;
    		}
    	}
    	result.data = txt.ToString();
    	result.position = ps;
    	result.success = true;
    	return result;
    }
    
    
    /// <summary>
    /// 从后向前按行读取文本文件，最多读取 10kb
    /// </summary>
    /// <param name="fs"></param>
    /// <param name="position"></param>
    /// <param name="s"></param>
    /// <param name="maxRead">默认每次最多读取10kb数据</param>
    /// <returns>返回读取位置</returns>
    long InverseReadRow(FileStream fs
    	, long position
    	, ref StringBuilder s
    	, int maxRead = 10240)
    {
    	byte n = 0xD;//回车符
    	byte r = 0xA;//换行符
    	if (fs.Length == 0)
    		return 0;
    	var newPos = position - 1;
    	int curVal = 0;
    	var readLength = 0;
    	List<byte> arr = new List<byte>();
    	var str = "";
    	//bool brk = false;
    	do
    	{
    		readLength++;
    		if (newPos <= 0)
    			newPos = 0;
    
    		fs.Position = newPos;
    		curVal = fs.ReadByte();
    		if (curVal == -1)
    			break;
    
    		arr.Insert(0, (byte)curVal);
    
    		if (newPos <= 0)
    			break;
    		//
    		if (readLength == maxRead)
    			break;
    
    
    		if (curVal != n)
    			newPos--;
    
    	} while (curVal != n);
    	str = Encoding.UTF8.GetString(arr.ToArray());
    	s.Insert(0, str);
    
    	return newPos;
    }

使用的对象

public class LogFileModel
    {
    	public string name { set; get; }
    	public long size { set; get; }
    	public string desc { set; get; }
    }
    public class ResultModel<T>
    {
    	public ResultModel()
    	{
    		data = default(T);
    	}
    
    	public string msg { set; get; }
    	public bool success
    	{
    		set;
    		get;
    	}
    	public T data { set; get; }
    }
    
    public class LastLogResult : ResultModel<string>
    {
    	public long position { set; get; }
    }