...
__END__

__C__

#define DEBUG(x)
#define GATTACA(x)  (x)=='A' || (x)=='C' || (x)=='G' || (x)=='T' || \
                    (x)=='a' || (x)=='c' || (x)=='g' || (x)=='t'

int deltasum_acgt(char* a, char* b) {
  int i;
  int sum     = 0;
  int tab[8]  = {0};
  int ignored = 0;

  DEBUG( printf("IN (%d:%s) (%d:%s)!\n", strlen(a), a, strlen(b), b); )

  for ( ; *a ; ++a ) {
    if ( GATTACA( *a  ) ) ++tab[ *a & 0x07 ];
    else ++ignored;
  }

  for ( ; *b ; ++b ) {
    if ( GATTACA( *b ) ) --tab[ *b & 0x07 ];
    else ++ignored;
  }

  sum += abs( tab[1] ); /* A */
  sum += abs( tab[3] ); /* C */
  sum += abs( tab[7] ); /* G */
  sum += abs( tab[4] ); /* T */
  sum += ignored << 20; /* signal ill chars */

  DEBUG( for (i=0; i<7; ++i) printf("%d -> %d\n", i, tab[i]); )
  DEBUG( printf("OUT: %d  ign=%d delta=%d\n", sum, ignored, sum & 0xfffff); )

  return sum;
}