#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 *r, *w;   /* General purpose pointers inside buffer. */
static char *e;       /* End of previous line (1 byte after newline). */
static char *s;       /* Source address for ShiftToFront. */

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

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

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 )
      {
         /* Scanning forward. */
         for(r = w = buffer; r != buffer + size; r++)
         {
            if( (head && current_line <= line) ||
                (!head && current_line != line) )
            {
               *w++ = *r;
            }
            if( *r == '\n' )
               current_line++;
         }
         if( !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.         */
            Flush();
         }
         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. */
         if( !tail )
         {
            /* Write prefix in slash mode, since we know none of these
               bytes would be cut.                                     */
            w = r;
            Flush();
         }

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

         /* 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 )
            {
               e -= r - buffer;
               size -= (e - buffer);
               s = e;
               ShiftToFront();
            }
            w = buffer + size;
            Flush();
         }
      }
   }

   return 0;
}