Split a List into groups of consecutive numbers [closed]

Answer 1

The correct implementation of @Bradley Uffner and @NetMage non allocating iterator method is like this:

public static IEnumerable> GroupConsecutive(this IEnumerable source)
{
    using (var e = source.GetEnumerator())
    {
        for (bool more = e.MoveNext(); more; )
        {
            int first = e.Current, last = first, next;
            while ((more = e.MoveNext()) && (next = e.Current) > last && next - last == 1)
                last = next;
            yield return Enumerable.Range(first, last - first + 1);
        }
    }
}

It works correctly even for unordered input, iterates the source sequence only once and handles correctly all corner cases and integer over/underflow. The only case it fails is for consecutive range count bigger than int.MaxValue.

But looking at your follow up question, probably the following implementation will better suit your needs:

public static IEnumerable<(int First, int Last)> ConsecutiveRanges(this IEnumerable source)
{
    using (var e = source.GetEnumerator())
    {
        for (bool more = e.MoveNext(); more;)
        {
            int first = e.Current, last = first, next;
            while ((more = e.MoveNext()) && (next = e.Current) > last && next - last == 1)
                last = next;
            yield return (first, last);
        }
    }
}