/// <summary> Returns all combinations of the specified elements of a given size. </summary>
/// <typeparam name="T"> The elements of the sequence. </typeparam>
/// <param name="elements"> The elements to combine. Each element is assumed to be unique. </param>
/// <param name="combinationSize"> The number of elements to choose per combination. </param>
public static IEnumerable <T[]> AllCombinations <T>(this IReadOnlyList <T> elements, int combinationSize)
{
Contract.Requires(elements != null);
Contract.Requires(combinationSize > 0);
if (elements.Count < combinationSize)
{
return(EmptyCollection <T[]> .Enumerable);
}
var ranges = EnumerableExtensions.AllCombinationsInRange(elements.Count, combinationSize);
EnsureSingleEnumerationDEBUG(ref ranges);
return(ranges.Select(indices => indices.Map(i => elements[i])));
}