Question

Given n enumerables of the same type that return distinct elements in ascending order, for example:

IEnumerable<char> s1 = "adhjlstxyz";
IEnumerable<char> s2 = "bdeijmnpsz";
IEnumerable<char> s3 = "dejlnopsvw";

I want to efficiently find all values that are elements of all enumerables:

IEnumerable<char> sx = Intersect(new[] { s1, s2, s3 });

Debug.Assert(sx.SequenceEqual("djs"));

“Efficiently” here means that

1. the input enumerables should each be enumerated only once,
2. the elements of the input enumerables should be retrieved only when needed, and
3. the algorithm should not recursively enumerate its own output.

I need some hints how to approach a solution.

---

Here is my (naive) attempt so far:

static IEnumerable<T> Intersect<T>(IEnumerable<T>[] enums)
{
    return enums[0].Intersect(
        enums.Length == 2 ? enums[1] : Intersect(enums.Skip(1).ToArray()));
}

Enumerable.Intersect collects the first enumerable into a HashSet, then enumerates the second enumerable and yields all matching elements.
Intersect then recursively intersects the result with the next enumerable.
This obviously isn’t very efficient (it doesn’t meet the constraints). And it doesn’t exploit the fact that the elements are sorted at all.

---

Here is my attempt to intersect two enumerables. Maybe it can be generalized for n enumerables?

static IEnumerable<T> Intersect<T>(IEnumerable<T> first, IEnumerable<T> second)
{
    using (var left = first.GetEnumerator())
    using (var right = second.GetEnumerator())
    {
        var leftHasNext = left.MoveNext();
        var rightHasNext = right.MoveNext();

        var comparer = Comparer<T>.Default;

        while (leftHasNext && rightHasNext)
        {
            switch (Math.Sign(comparer.Compare(left.Current, right.Current)))
            {
            case -1:
                leftHasNext = left.MoveNext();
                break;
            case 0:
                yield return left.Current;
                leftHasNext = left.MoveNext();
                rightHasNext = right.MoveNext();
                break;
            case 1:
                rightHasNext = right.MoveNext();
                break;
            }
        }
    }
}

Answer 1

OK; more complex answer:

public static IEnumerable<T> Intersect<T>(params IEnumerable<T>[] enums) {
    return Intersect<T>(null, enums);
}
public static IEnumerable<T> Intersect<T>(IComparer<T> comparer, params IEnumerable<T>[] enums) {
    if(enums == null) throw new ArgumentNullException("enums");
    if(enums.Length == 0) return Enumerable.Empty<T>();
    if(enums.Length == 1) return enums[0];
    if(comparer == null) comparer = Comparer<T>.Default;
    return IntersectImpl(comparer, enums);
}
public static IEnumerable<T> IntersectImpl<T>(IComparer<T> comparer, IEnumerable<T>[] enums) {
    IEnumerator<T>[] iters = new IEnumerator<T>[enums.Length];
    try {
        // create iterators and move as far as the first item
        for (int i = 0; i < enums.Length; i++) {
            if(!(iters[i] = enums[i].GetEnumerator()).MoveNext()) {
                yield break; // no data for one of the iterators
            }
        }
        bool first = true;
        T lastValue = default(T);
        do { // get the next item from the first sequence
            T value = iters[0].Current;
            if (!first && comparer.Compare(value, lastValue) == 0) continue; // dup in first source
            bool allTrue = true;
            for (int i = 1; i < iters.Length; i++) {
                var iter = iters[i];
                // if any sequence isn't there yet, progress it; if any sequence
                // ends, we're all done
                while (comparer.Compare(iter.Current, value) < 0) {
                    if (!iter.MoveNext()) goto alldone; // nasty, but
                }
                // if any sequence is now **past** value, then short-circuit
                if (comparer.Compare(iter.Current, value) > 0) {
                    allTrue = false;
                    break;
                }
            }
            // so all sequences have this value
            if (allTrue) yield return value;
            first = false;
            lastValue = value;
        } while (iters[0].MoveNext());
    alldone:
        ;
    } finally { // clean up all iterators
        for (int i = 0; i < iters.Length; i++) {
            if (iters[i] != null) {
                try { iters[i].Dispose(); }
                catch { }
            }
        }
    }
}