LINQ operator that maps over adjacent items in the source sequence using the given selector function

Enumerable.Pairwise.cs

static IEnumerable<R> Pairwise<T, R>(this IEnumerable<T> xs, Func<T, T, R> selector)
{
    using (var enumerator = xs.GetEnumerator())
    {
        if (enumerator.MoveNext())
        {
            T x = enumerator.Current;
            while (enumerator.MoveNext())
            {
                T y = enumerator.Current;
                yield return selector(x, y);
                x = y;
            }
        }
    }
}