Combinations of IEnumerable Source

SOURCE CODE

/// <summary>
///   Returns all combinations of a chosen amount of selected elements in the sequence.
/// </summary>
/// <typeparam name = “T”>The type of the elements of the input sequence.</typeparam>
/// <param name = “source”>The source for this extension method.</param>
/// <param name = “select”>The amount of elements to select for every combination.</param>
/// <param name = “repetition”>True when repetition of elements is allowed.</param>
/// <returns>All combinations of a chosen amount of selected elements in the sequence.</returns>
public static IEnumerable<IEnumerable<T>> Combinations<T>( this IEnumerable<T> source, int select, bool repetition = false )
{   
    Contract.Requires( source != null );   
    Contract.Requires( select >= 0 );    
    return select == 0       
        ? new[] { new T[0] }       
        : source.SelectMany( ( element, index ) =>            source
                .Skip( repetition ? index : index + 1 )
                .Combinations( select – 1, repetition )               
                .Select( c => new[] { element }.Concat( c ) ) );
}

EXAMPLE

int[] numbers = new[] { 0, 1 };
var result = numbers.Combinations( 2, true );
// result == {{0, 0}, {0, 1}, {1, 0}, {1, 1}}