如何阅读最后＆QUOT; N＆QUOT;日志文件的行 [英] How to read last "n" lines of log file

问题描述

需要code片断这将读出日志文件的最后一个n行。我想出了从net.I以下code我还挺新的C锋利。由于日志文件可能是 蛮大的，我想避免的读取整个file.Can有人提出任何性能增强开销。 我真的不想要阅读的每个字符并改变位置。

  VAR读卡器=新的StreamReader（文件路径，Encoding.ASCII）;
            reader.BaseStream.Seek（0，SeekOrigin.End）;
            变种数= 0;
            而（计数＆LT; = tailCount）
            {
                如果（reader.BaseStream.Position＆LT; = 0）中断;
                reader.BaseStream.Position--;
                INT C = reader.Read（）;
                如果（reader.BaseStream.Position＆LT; = 0）中断;
                reader.BaseStream.Position--;
                如果（C =='\ N'）
                {
                    ++计数;
                }
            }

            VAR海峡= reader.ReadToEnd（）;
 

解决方案

您code会执行得非常糟糕，因为你没有允许任何缓存的情况发生。
此外，它不会工作的所有的为统一code。

我写了下面执行：

  ///＆LT;总结＆gt;返回文本阅读器的结束和LT; /总结＆gt;
///＆LT; PARAM NAME =读者＆GT;读者从阅读＆LT; /参数＆GT;
///＆LT; PARAM NAME =lineCount＆GT;行返回的数量和LT; /参数＆GT;
///＆LT;返回＆gt;在最后lneCount线从阅读器＆LT; /回报＆GT;
公共静态字符串[]尾（这TextReader的读者，INT lineCount）{
    VAR缓冲区=新的名单，其中，串＆GT;（lineCount）;
    串线;
    的for（int i = 0; I＆LT; lineCount;我++）{
        行= reader.ReadLine（）;
        如果（线== NULL）返回buffer.ToArray（）;
        buffer.Add（线）;
    }

    INT lastLine所= lineCount  -  1; //从缓冲器中读取的最后一行的索引。一切＆GT;该指数早于一切都被读取＆LT; =这个东印度

    而（空！=（行= reader.ReadLine（）））{
        lastLine所++;
        如果（lastLine所== lineCount）lastLine所= 0;
        缓冲区[lastLine所] =行;
    }

    如果（lastLine所== lineCount  -  1）返回buffer.ToArray（）;
    VAR retVal的=新的字符串[lineCount]
    buffer.CopyTo（lastLine所+ 1，retVal的，0，lineCount  -  lastLine所 -  1）;
    buffer.CopyTo（0，retVal的，lineCount  -  lastLine所 -  1，lastLine所+ 1）;
    返回retVal的;
}
 

need a snippet of code which would read out last "n lines" of a log file. I came up with the following code from the net.I am kinda new to C sharp. Since the log file might be quite large, I want to avoid overhead of reading the entire file.Can someone suggest any performance enhancement. I do not really want to read each character and change position.

   var reader = new StreamReader(filePath, Encoding.ASCII);
            reader.BaseStream.Seek(0, SeekOrigin.End);
            var count = 0;
            while (count <= tailCount)
            {
                if (reader.BaseStream.Position <= 0) break;
                reader.BaseStream.Position--;
                int c = reader.Read();
                if (reader.BaseStream.Position <= 0) break;
                reader.BaseStream.Position--;
                if (c == '\n')
                {
                    ++count;
                }
            }

            var str = reader.ReadToEnd();

解决方案

Your code will perform very poorly, since you aren't allowing any caching to happen.
In addition, it will not work at all for Unicode.

I wrote the following implementation:

///<summary>Returns the end of a text reader.</summary>
///<param name="reader">The reader to read from.</param>
///<param name="lineCount">The number of lines to return.</param>
///<returns>The last lneCount lines from the reader.</returns>
public static string[] Tail(this TextReader reader, int lineCount) {
    var buffer = new List<string>(lineCount);
    string line;
    for (int i = 0; i < lineCount; i++) {
        line = reader.ReadLine();
        if (line == null) return buffer.ToArray();
        buffer.Add(line);
    }

    int lastLine = lineCount - 1;           //The index of the last line read from the buffer.  Everything > this index was read earlier than everything <= this indes

    while (null != (line = reader.ReadLine())) {
        lastLine++;
        if (lastLine == lineCount) lastLine = 0;
        buffer[lastLine] = line;
    }

    if (lastLine == lineCount - 1) return buffer.ToArray();
    var retVal = new string[lineCount];
    buffer.CopyTo(lastLine + 1, retVal, 0, lineCount - lastLine - 1);
    buffer.CopyTo(0, retVal, lineCount - lastLine - 1, lastLine + 1);
    return retVal;
}

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