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

/* Operation mode. */
#ifdef HEAD
static int head = 1;
#else
static int head = 0;
#endif
#ifdef TAIL
static int tail = 1;
#else
static int tail = 0;
#endif

/* Line number or count associated with operation.
   This is positive if measured from start of file, or
   negative if measured from end of file.              */
static int line;

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

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

static char *p, *q;
static char *e;

static void Flush(void)
{
   fwrite(buffer, q - buffer, 1, stdout);
}

static void ShiftToFront(char *s)
{
   size -= s - buffer;
   for(p = buffer; p != buffer + size;)
      *p++ = *s++;
}

int main(int argc, char **argv)
{
   if( argc != 2 )
   {
      return printf("%s {line_%s}\n",
                    *argv, head || tail ? "count" : "number");
   }

   line = atoi(argv[1]);
   if( head )
      line = abs(line);
   else if( tail )
      line = -abs(line);

   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 && !tail )
      {
         /* Scanning forward. */
         for(p = q = buffer; p != buffer + size; p++)
         {
            if( head ? current_line <= line : current_line != line )
               *q++ = *p;
            if( *p == '\n' )
               current_line++;
         }
         Flush();
         size = 0;
      }
      else if( size && line )
      {
         /* Scanning backward.

            This block is skipped if we didn't read anything (size==0),
            or if there is nothing to output (tail!=0 and line==0).     */
         q = e = buffer + size;
         if( q[-1] == '\n' )
            q--;

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

         /* "q" 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. */
         if( !tail )
         {
            /* Write prefix in slash mode, since we know none of these
               bytes would be cut.                                     */
            Flush();
         }

         /* Move start of desired line and all subsequent bytes to start
            of buffer.                                                   */
         ShiftToFront(q);

         /* 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 )
         {
            if( !tail && current_line == line )
            {
               ShiftToFront(e -= q - buffer);
            }
            q = buffer + size;
            Flush();
         }
      }
   }

   return 0;
}