#include<stdio.h>
#include<stdlib.h>
#include<string.h>

int main(int argc, char **argv)
{
   /* Line number or count associated with operation.
      This is positive if measured from start of file, or
      negative if measured from end of file.              */
   int line;

   /* Current read position. */
   int current_line = 1;

   char *buffer = NULL; /* Input buffer. */
   int capacity = 0;    /* Buffer capacity. */
   int size = 0;        /* Number of bytes read. */
   int eof = 0;         /* End of file. */

   char *r, *w;   /* General purpose pointers inside buffer. */
   char *e;       /* End of previous line (1 byte after newline). */

   if( argc != 2 )
   {
      #if !defined(HEAD) && !defined(TAIL)
         return printf("%s {line_number}\n", *argv);
      #else
         return printf("%s {line_count}\n", *argv);
      #endif
   }

   #ifdef HEAD
      line = abs(atoi(argv[1]));
   #elif defined(TAIL)
      line = -abs(atoi(argv[1]));
   #else
      line = atoi(argv[1]);
   #endif

   while( !eof )
   {
      /* Extend input buffer. */
      if( size == capacity )
      {
         if( (buffer = (char*)realloc(buffer, capacity += 4096)) == NULL )
         {
            fprintf(stderr, "Out of memory (need %d)\n", capacity);
            return 1;
         }
      }
      size += fread(buffer + size, 1, capacity - size, stdin);
      eof = size < capacity;

      if( line >= 0 )
      {
         /* Scanning forward. */
         for(r = w = buffer; r != buffer + size; r++)
         {
            #ifdef HEAD
               /* Copy bytes up to the end of the requested line number. */
               if( current_line <= line )
                  *w++ = *r;
            #else
               /* Copy bytes, skipping over requested line number. */
               if( current_line != line )
                  *w++ = *r;
            #endif
            if( *r == '\n' )
               current_line++;
         }
         #ifndef TAIL
            /* If tail mode is called with line count set to zero,
               we will end up in the forward scanning branch.
               We will drop all output bytes in that case.         */
            fwrite(buffer, w - buffer, 1, stdout);
         #endif
         size = 0;
      }
      else
      {
         /* Scanning backward. */
         if( size == 0 )
            continue;
         r = e = buffer + size;
         if( r[-1] == '\n' )
            r--;

         current_line = -1;
         for(; r-- != buffer;)
         {
            if( *r == '\n' )
            {
               current_line--;
               if( current_line < line )
               {
                  current_line++;
                  break;
               }
               e = r + 1;
            }
         }
         r++;

         /* "r" now points at the first byte of the desired line, or at the
            start of the buffer.  "e" points at the end of the desired line. */
         #ifndef TAIL
            /* Write prefix in slash mode, since we know none of these
               bytes would be cut.                                     */
            fwrite(buffer, r - buffer, 1, stdout);
         #endif

         /* Move start of desired line and all subsequent bytes to start
            of buffer.                                                   */
         memmove(buffer, r, size -= (r - buffer));

         /* If we have read everything, we can write the trailing parts
            out, optionally skipping over the first line if we are in
            slash mode.                                                 */
         if( eof )
         {
            #ifndef TAIL
               if( current_line == line )
               {
                  e -= r - buffer;
                  memmove(buffer, e, size -= (e - buffer));
               }
            #endif
            fwrite(buffer, size, 1, stdout);
         }
      }
   }

   return 0;
}