Question

In a table I have columns with to and from dates, I highlight overlaps between rows taking into account the periods, this is done exhaustively in nested loops. This is not the issue.
I need the same colour for the rows that overlap.

sub highlight_overlaps {
    my $date_from1;
    my $date_to1;
    my $date_from2;
    my $date_to2;

    my $i = 0;
    my $j = 0;

    for ($i; $i < $#DATE_HOLDER; $i++) {
        $date_from1 = $DATE_HOLDER[$i][0];
        $date_to1   = $DATE_HOLDER[$i][1];

        my $red   = int(rand(65))  + 190;
        my $green = int(rand(290)) - 55;
        my $blue  = int(rand(290)) - 55;

        for ($j=$i+1; $j<=$#DATE_HOLDER; $j++) {
            $date_from2 = $DATE_HOLDER[$j][0];
            $date_to2   = $DATE_HOLDER[$j][1];

            if (($date_from1 le $date_to2   && $date_to1 ge $date_to2) ||
                ($date_from1 le $date_from2 && $date_to1 le $date_to2) ||
                ($date_from1 gt $date_from2 && $date_from1 lt $date_to2)) {

                $tb->setCellStyle($i+2, 6, "background-color:rgb($red,$green,$blue);font-size:9pt");
                $tb->setCellStyle($i+2, 7, "background-color:rgb($red,$green,$blue);font-size:9pt");

                $tb->setCellStyle($j+2, 6, "background-color:rgb($red,$green,$blue);font-size:9pt");
                $tb->setCellStyle($j+2, 7, "background-color:rgb($red,$green,$blue);font-size:9pt");
            }
        }
    }
}

This works fine if it’s just a pair of dates; say:

1) 25-06-2012 27-06-2012

2) 18-06-2012 29-06-2012

Will get the same colour

If though I have

0) 26-06-2012 28-06-2012

1) 25-06-2012 27-06-2012

2) 18-06-2012 29-06-2012

0 will get a different colour while 1 & 2 are paired as intended.

When and how to pick colours so that different colours are only applied to different overlaps?

Following up on the first answer; how may I represent overlaps in order to store them in a data structure, so that I can colour them after their detection?

Answer 1

You’ll have to compare each interval against each other interval, and put them in ‘buckets’ when they are equal. Now when you compare an interval to a third interval, you put the third in the same bucket as the interval.

Then you print the buckets.

Perl’s hash would make for fine buckets.

About your overlap detection

There is no overlap if

• date1_to < date2_from OR
• date2_to < date1_from

Or, in Perl:

if ($date_to1 lt $date_from2 || $date_to2 lt $date_from1) {
#overlap
}

Invert that either using Perl’s unless, or using de Morgan:

if ($date_to1 ge $date_from2 && $date_to2 ge $date_from1) {
#overlap
}