Implement your own tail (Read last n lines of a huge file) - GeeksforGeeks

Implement your own tail (Read last n lines of a huge file) - GeeksforGeeks
Given a huge file having dynamic data, write a program to read last n lines from the file at any point without reading the entire file. The problem is similar to tail command in linux which displays the last few lines of a file. It is mostly used for viewing log file updates as these updates are appended to the log files.

The problem mainly focuses on below things –

1. The program should not read entire file.
2. The program should handle incoming dynamic data and returns last n lines at any point.
3. The program should not close input stream before reading last n lines.

void tail(FILE* in, int n)

{

    int count = 0;  // To count '\n' characters

    // unsigned long long pos (stores upto 2^64 – 1

    // chars) assuming that long long int takes 8

    // bytes

    unsigned long long pos;

    char str[2*SIZE];

    // Go to End of file

    if (fseek(in, 0, SEEK_END))

        perror("fseek() failed");

    else

    {

        // pos will contain no. of chars in

        // input file.

        pos = ftell(in);

        // search for '\n' characters

        while (pos)

        {

            // Move 'pos' away from end of file.

            if (!fseek(in, --pos, SEEK_SET))

            {

                if (fgetc(in) == '\n')

                    // stop reading when n newlines

                    // is found

                    if (count++ == n)

                        break;

            }

            else

                perror("fseek() failed");

        }

        // print last n lines

        printf("Printing last %d lines -\n", n);

        while (fgets(str, sizeof(str), in))

            printf("%s", str);

    }

    printf("\n\n");

}

http://ideone.com/cJSByU

http://netjs.blogspot.in/2016/02/how-to-read-file-from-last-line-in-java.html

 public void readFromLast(File file, int lines){
  int readLines = 0;
        StringBuilder builder = new StringBuilder();
        RandomAccessFile randomAccessFile = null;
        try {
         randomAccessFile = new RandomAccessFile(file, "r");
         long fileLength = file.length() - 1;
         // Set the pointer at the last of the file
         randomAccessFile.seek(fileLength);
         for(long pointer = fileLength; pointer >= 0; pointer--){
          randomAccessFile.seek(pointer);
          char c;
          // read from the last one char at the time
    c = (char)randomAccessFile.read(); 
    // break when end of the line
             if(c == '\n'){
              readLines++;
              if(readLines == lines)
               break;
             }
             builder.append(c);
         }
         // Since line is read from the last so it 
         // is in reverse so use reverse method to make it right
         builder.reverse();
         System.out.println("Line - " + builder.toString());
        } catch (FileNotFoundException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
 }
        catch (IOException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
 }finally{
  if(randomAccessFile != null){
   try {
    randomAccessFile.close();
   } catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
   }
  }
   }

Using Apache Commons IO

public void reverseLines(File file){
 ReversedLinesFileReader object = null;
 try {
  object = new ReversedLinesFileReader(file);
  System.out.println("Line - " + object.readLine());
 } catch (IOException e) {
  // TODO Auto-generated catch block
  e.printStackTrace();
 }finally{
  try {
   object.close();
  } catch (IOException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
 }
}

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