/* Delete input prefix, keeping only the last few lines.

   Usage:
      ./tail {line_count} < input.txt > output.txt

   Where {line_count} is the number of lines.  Sign is ignored, so
   "-10" is interpreted the same as "10".

   This tool basically implements a subset of the standard tail(1) tool,
   except we optimize for code size.
*/

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

int main(int argc, char **argv)
{
   int line_count, i, c;
   int capacity = 0, size = 0, eof = 0;
   char *buffer = NULL;

   if( argc != 2 )
      return printf("%s {line_count}\n", *argv);

   line_count = abs(atoi(argv[1]));

   while( !eof )
   {
      /* Extend input buffer. */
      if( size == capacity )
      {
         capacity = (capacity != 0 ? capacity * 2 : 256);
         if( (buffer = (char*)realloc(buffer, capacity)) == NULL )
         {
            fprintf(stderr, "Out of memory (need %d)\n", capacity);
            return 1;
         }
      }

      size += fread(buffer + size, 1, capacity - size, stdin);
      eof = size < capacity;

      if( size > 0 )
      {
         /* Scan backwards until we got enough lines. */
         c = 0;
         i = size - 1;
         if( buffer[i] != '\n' )
            c++;
         for(; i >= 0; i--)
         {
            if( buffer[i] == '\n' )
            {
               c++;
               if( c > line_count )
                  break;
            }
         }

         /* Discard prefix, keeping only the suffix that has enough lines. */
         i++;
         if( i > 0 )
            memmove(buffer, buffer + i, size -= i);
      }
   }

   /* Write output. */
   fwrite(buffer, size, 1, stdout);
   free(buffer);
   return 0;
}