Merging overlapping time intervals?

Question

I have the following:

public class Interval
{
   DateTime Start;
   DateTime End; 
}

I have a List<Interval> object containing multiple intervals. I am trying to achieve the following (I used numbers to make it easy to understand):

[(1, 5), (2, 4), (3, 6)] --->  [(1,6)]
[(1, 3), (2, 4), (5, 8)] --->  [(1, 4), (5,8)]

I currently do this in Python as follows:

def merge(times):
    saved = list(times[0])
    for st, en in sorted([sorted(t) for t in times]):
        if st <= saved[1]:
            saved[1] = max(saved[1], en)
        else:
            yield tuple(saved)
            saved[0] = st
            saved[1] = en
    yield tuple(saved)

but am trying to achieve the same in C# (LINQ would be best but optional). Any suggestions on how to do this efficiently?

Answer 1

Here's a version using yield return - I find it easier to read than doing an Aggregate query, although it's still lazy evaluated. This assumes you've ordered the list already, if not, just add that step.

IEnumerable<Interval> MergeOverlappingIntervals(IEnumerable<Interval> intervals)
{
  var accumulator = intervals.First();  
  intervals = intervals.Skip(1);

  foreach(var interval in intervals)
  {
    if ( interval.Start <= accumulator.End )
    {
        accumulator = Combine(accumulator, interval);
    }
    else
    {
        yield return accumulator;
        accumulator = interval;     
    }       
  }

  yield return accumulator;
}

Interval  Combine(Interval start, Interval end)
{
  return new Interval 
  {
    Start = start.Start,
    End = Max(start.End, end.End),
  };
}

private static DateTime Max(DateTime left, DateTime right) 
{
    return (left > right) ? left : right;
}