Question

I have two sets of ranges, represented by [ start, stop ] values. Some of the ranges overlap, meaning that the start of one range is in between the [ start, stop ] of the other range. I’d like to make a new set of ranges that has no such overlap, and also doesn’t include any new values in a range.

The ranges look like this:

@starts  @ends
      5    108 
      5    187
     44    187
     44    229 
     44    236 
     64    236 
    104    236
    580    644
    632    770

The output that I expect would be this:

@starts  @ends
      5    236
    580    770

This is because the first seven ranges overlap with the interval from 5 => 236, and the last two overlap with the interval from 632 => 770.

Here’s the code that I tried:

$fix = 0;
foreach (@ends) {  
    if ($starts[$fix + 1] < $ends[$fix]) {
        splice(@ends, $fix, $fix);
        splice(@starts, $fix + 1, $fix + 1);
    } else {
        $fix += 1;
    }
}

I can print out the values myself, I just need help with the algorithm for merging.

Answer 1

This edits your arrays in-place, simply collapsing boundaries when they overlap.

# Since they're sorted by @starts, accept the 0th interval, start at 1
for (1..$#starts) {
    # extra check on array bounds, since we edit in-place
    last unless $_ < @starts;
    # don't need to collapse if no overlap with previous end
    next unless $starts[$_] <= $ends[$_-1];
    # delete this start and the previous end
    splice(@starts,$_,1);
    splice(@ends,$_-1,1);
    # rerun this loop for the same value of $_ since it was deleted
    redo;
}