extract overlapping regions

I have a file characterizing genomic regions that looks like this:

chrom   chromStart  chromEnd    PGB
chr1    12874   28371   2
chr1    15765   21765   1
chr1    15795   28371   2
chr1    18759   24759   1
chr1    28370   34961   1
chr3    233278  240325  1
chr3    239279  440831  2
chr3    356365  362365  1

Basically PGB describes the category of the genomic region characterised by its chromosome number (chrom), start (chromStart) and end (chromEnd) coordinates.

I wish to collapse the overlapping regions such that overlapping regions of PGB = 1 and 2 are in a new category, PGB = 3. Output being:

chrom   chromStart  chromEnd    PGB
chr1    12874   15764   2
chr1    15765   24759   3
chr1    24760   28369   2
chr1    28370   28371   3
chr1    28372   34961   1
chr3    233278  239278  1
chr3    239279  240325  3
chr3    240326  356364  2
chr3    356365  440831  3

Basically I wish to obtain an output file which reports unique regions. There are a two criteria.

First, if PGB (column 4) is identical between rows, merge range. eg.

chrom   chromStart  chromEnd    PGB
chr1    1   10  1
chr1    5   15  1

output

chrom   chromStart  chromEnd    PGB
chr1    1   15  1

Second, if PGB is different between rows, chr (column 1) is identical, and the ranges overlap (col2 and 3), report overlapping range as PGB = 3 as well as the ranges unique to their individual categories.

eg.

chrom   chromStart  chromEnd    PGB
chr1    30  100 1
chr1    50  150 2

output

chrom   chromStart  chromEnd    PGB
chr1    30  49  1
chr1    50  100 3
chr1    101 150 2

I hope that illustrates the problem better.

I've created a script that I believe accomplishes this goal.

use List::Util qw(max);

use strict;
use warnings;

# Skip Header row
<DATA>;

# With only 60k rows, going to just slurp all the data
my %chroms;
while (<DATA>) {
    chomp;
    my ($chrom, $start, $end, $pgb) = split ' ';

    # Basic Data Validation.
    warn "chromStart is NaN, '$start'" if $start =~ /\D/;
    warn "chromEnd is NaN, '$end'" if $end =~ /\D/;
    warn "range not ordered, '$start' to '$end'" if $start > $end;
    warn "PGB only can be 1 or 2, '$pgb'" if ($pgb ne '1' && $pgb ne '2');

    push @{$chroms{$chrom}{$pgb}}, [$start, $end];
}

print "chrom    chromStart  chromEnd    PGB\n";

# Process each Chrom
for my $chrom (sort keys %chroms) {
    for my $pgb (sort keys %{$chroms{$chrom}}) {
        my $ranges = $chroms{$chrom}{$pgb};

        # Sort Ranges
        @$ranges = sort {$a->[0] <=> $b->[0] || $a->[1] <=> $b->[1]} @$ranges;

        # Combine overlapping and continguous ranges.
        # - Note because we're dealing with integer ranges, 1-4 & 5-8 are contiguous
        for my $i (0..$#$ranges-1) {
            if ($ranges->[$i][1] >= $ranges->[$i+1][0] - 1) {
                $ranges->[$i+1][0] = $ranges->[$i][0];
                $ranges->[$i+1][1] = max($ranges->[$i][1], $ranges->[$i+1][1]);
                $ranges->[$i] = undef;
            }
        }
        @$ranges = grep {$_} @$ranges;
    }

    # Create pgb=3 for overlaps.
    # - Save old ranges into aliases, and then start fresh
    my $pgb1array = $chroms{$chrom}{1};
    my $pgb2array = $chroms{$chrom}{2};
    my @ranges;

    # Always working on the first range in each array, until one of the arrays is empty
    while (@$pgb1array && @$pgb2array) {
        # Aliases to first element
        my $pgb1 = $pgb1array->[0];
        my $pgb2 = $pgb2array->[0];

        # PGB1 < PGB2
        if ($pgb1->[1] < $pgb2->[0]) {
            push @ranges, [@{shift @$pgb1array}, 1]

        # PGB2 < PGB1
        } elsif ($pgb2->[1] < $pgb1->[0]) {
            push @ranges, [@{shift @$pgb2array}, 2]

        # There's overlap for all rest 
        } else {
            # PGB1start < PGB2start
            if ($pgb1->[0] < $pgb2->[0]) {
                push @ranges, [$pgb1->[0], $pgb2->[0]-1, 1];
                $pgb1->[0] = $pgb2->[0];

            # PGB2start < PGB1start
            } elsif ($pgb2->[0] < $pgb1->[0]) {
                push @ranges, [$pgb2->[0], $pgb1->[0]-1, 2];
                $pgb2->[0] = $pgb1->[0];
            }
            # (Starts are equal now)

            # PGB1end < PGB2end
            if ($pgb1->[1] < $pgb2->[1]) {
                $pgb2->[0] = $pgb1->[1] + 1;
                push @ranges, [@{shift @$pgb1array}, 3];

            # PGB2end < PGB1end
            } elsif ($pgb2->[1] < $pgb1->[1]) {
                $pgb1->[0] = $pgb2->[1] + 1;
                push @ranges, [@{shift @$pgb2array}, 3];

            # PGB2end = PGB1end
            } else {
                push @ranges, [@$pgb1, 3];
                shift @$pgb1array;
                shift @$pgb2array;
            }
        }
    }

    # Append whichever is left over
    push @ranges, map {$_->[2] = 1; $_} @$pgb1array;
    push @ranges, map {$_->[2] = 2; $_} @$pgb2array;

    printf "%-8s %-11s %-11s %s\n", $chrom, @$_ for (@ranges);
}

1;

__DATA__
chrom   chromStart  chromEnd    PGB
chr1    12871   12873   2
chr1    12874   28371   2
chr1    15765   21765   1
chr1    15795   28371   2
chr1    18759   24759   1
chr1    28370   34961   1
chr3    233278  240325  1
chr3    239279  440831  2
chr3    356365  362365  1

And the result:

chrom    chromStart  chromEnd    PGB
chr1     12871       15764       2
chr1     15765       24759       3
chr1     24760       28369       2
chr1     28370       28371       3
chr1     28372       34961       1
chr3     233278      239278      1
chr3     239279      240325      3
chr3     240326      356364      2
chr3     356365      362365      3
chr3     362366      440831      2

Note: I created this mostly as an exercise for myself, but if you use it in any way, please let me know.