c# - What's a good, generic algorithm for collapsing a set of potentially-overlapping ranges?

Question

I have a method that gets a number of objects of this class

class Range<T>
{
    public T Start;
    public T End;
}

In my case T is DateTime, but lets use int for simplicity. I would like a method that collapses those ranges into ones that cover the same "area" but that do not overlap.

So if I had the following ranges

• 1 to 5
• 3 to 9
• 11 to 15
• 12 to 14
• 13 to 20

The method should give me

• 1 to 9
• 11 to 20

Guess it would be called a union? I imagine the method signature could look something like this:

public static IEnumerable<Range<T>> Collapse<T>(
    this IEnumerable<Range<T>>, 
    IComparable<T> comparer)
{
    ...
}

I have looked at some other questions here that are kind of similar, but I haven't found an implementation of this yet. This answer and some other answers to the same question describes algorithms, but I am not quite sure if I understand the algorithms. Not especially good at implementing algorithms either, so I was hoping someone here could help me out.

Answer 1

This seems to works and is easy to understand.

    public static IEnumerable<Range<T>> Collapse<T>(this IEnumerable<Range<T>> me, IComparer<T> comparer)
    {
        List<Range<T>> orderdList = me.OrderBy(r => r.Start).ToList();
        List<Range<T>> newList = new List<Range<T>>();

        T max = orderdList[0].End;
        T min = orderdList[0].Start;

        foreach (var item in orderdList.Skip(1))
        {
            if (comparer.Compare(item.End, max) > 0 && comparer.Compare(item.Start, max) > 0)
            {
                newList.Add(new Range<T> { Start = min, End = max });
                min = item.Start;
            }
            max = comparer.Compare(max, item.End) > 0 ? max : item.End;
        }
        newList.Add(new Range<T>{Start=min,End=max});

        return newList;
    }

---

Here is the variation which I mentioned in the comments. It's basically the same thing, but with some checking and yielding of the results instead of collecting in a list before returning.

    public static IEnumerable<Range<T>> Collapse<T>(this IEnumerable<Range<T>> ranges, IComparer<T> comparer)
    {
        if(ranges == null || !ranges.Any())
            yield break;

        if (comparer == null)
            comparer = Comparer<T>.Default;

        var orderdList = ranges.OrderBy(r => r.Start);
        var firstRange = orderdList.First();

        T min = firstRange.Start;
        T max = firstRange.End;

        foreach (var current in orderdList.Skip(1))
        {
            if (comparer.Compare(current.End, max) > 0 && comparer.Compare(current.Start, max) > 0)
            {
                yield return Create(min, max);
                min = current.Start;
            }
            max = comparer.Compare(max, current.End) > 0 ? max : current.End;
        }
        yield return Create(min, max);
    }