/* Truncate input, keeping only the first few lines.

   Usage:
      ./head {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 head(1) tool,
   except we optimize for code size.
*/

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

#define USE_BUFFERED_INPUT

int main(int argc, char **argv)
{
   int line_count, current_line = 1;
   int i;
#ifdef USE_BUFFERED_INPUT
   int capacity = 0, size = 0, eof = 0;
   char *buffer = NULL;
#endif

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

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

#ifdef USE_BUFFERED_INPUT
   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, 1, capacity, stdin);
      eof = size < capacity;
      if( current_line <= line_count )
      {
         for(i = 0; i < size; i++)
         {
            if( buffer[i] == '\n' )
            {
               current_line++;
               if( current_line > line_count )
               {
                  size = i + 1;
                  break;
               }
            }
         }
         fwrite(buffer, size, 1, stdout);
         size = 0;
      }
   }

   if( current_line <= line_count )
      fwrite(buffer, size, 1, stdout);
   free(buffer);

#else
   while( (i = getchar()) != EOF )
   {
      if( current_line <= line_count )
         putchar(i);
      if( i == '\n' )
         current_line++;
   }
#endif

   return 0;
}