sub SLASH() { 47 }

sub _common_path {
   if ( $_[0] eq $_[1] ) {
      return(
         $_[0],      # all path components are common
         "",         # nothing remains of first
         "",         # nothing remains of second
      );
   } else {
      use bytes;
      my ( $len0, $len1 ) = ( length($_[0]), length($_[1]) );
      # find the offset of the first byte that differs
      my $pos = $_[0] ^ $_[1];
      $pos =~ m/[^\x00]/g;
      $pos = pos($pos) - 1;
      # if some bytes are common but the last one wasn't the separator
      # we must decide which path components are common
      if ( $pos > 0 && vec($_[0], ($pos - 1), 8) != SLASH ) {
         # check if first path is just longer than the second
         if ( $pos == $len1 && vec($_[0], $pos, 8) == SLASH ) {
            $pos++;
            return(
               substr($_[0], 0, $pos),    # common path with slash
               substr($_[0], $pos),       # extra in first
               "",                        # nothing remains of second
            );
         }
         # check if second path is just longer than the first
         if ( $pos == $len0 && vec($_[1], $pos, 8) == SLASH ) {
            $pos++;
            return(
               substr($_[1], 0, $pos),    # common path with slash
               "",                        # nothing remains of first
               substr($_[1], $pos),       # extra in second
            );
         }
         # otherwise, rewind until last common path component
         while ( $pos > 0 ) {
            $pos--;
            if ( vec($_[0], $pos, 8) == SLASH ) {
               $pos++;                    # and keep the common slash
               last;
            }
         }
      }
      return(
         substr($_[0], 0, $pos), # common path components (with slash)
         substr($_[0], $pos),    # extra in first
         substr($_[1], $pos),    # extra in second
      );
   }
}