sub find_seq {
    my ($num,$array)=@_;

    # assert that @$array is sorted numerically increasing

    return if $num > @$array; # ensure array is big enough
           

    $num--;  # num is one based, we need a 0 based value
    
    # the default return will be the first N values,
    # if we find better then we will overwrite these
    my $min_start = 0;
    my $min_end = $num;
    my $min = $array->[$num] - $array->[0];

    # start at num as below that nothing can match
    for my $end ( $num .. $#$array) {
        my $start = $end - $num;
        my $diff = $array->[$end] - $array->[$start];
        if ( $min > $diff ) {
            # we have found a better sequence, 
            # so remember where it is
            $min = $diff;
            $min_end = $end;
            $min_start = $start;
        }
        # if the sequence is contiguous we can finish
        last if $diff == $num; 
    }
    return ( $min_start, $min_end );
}