Task: Card shuffles
C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CardShuffles {
    public static class Helper {
        public static string AsString<T>(this ICollection<T> c) {
            StringBuilder sb = new StringBuilder("[");
            sb.Append(string.Join(", ", c));
            return sb.Append("]").ToString();
        }
    }

    class Program {
        private static Random rand = new Random();

        public static List<T> riffleShuffle<T>(ICollection<T> list, int flips) {
            List<T> newList = new List<T>(list);

            for (int n = 0; n < flips; n++) {
                
                int cutPoint = newList.Count / 2
                    + (rand.Next(0, 2) == 0 ? -1 : 1) * rand.Next((int)(newList.Count * 0.1));

                
                List<T> left = new List<T>(newList.Take(cutPoint));
                List<T> right = new List<T>(newList.Skip(cutPoint));

                newList.Clear();

                while (left.Count > 0 && right.Count > 0) {
                    
                    
                    
                    if (rand.NextDouble() >= ((double)left.Count / right.Count) / 2) {
                        newList.Add(right.First());
                        right.RemoveAt(0);
                    }
                    else {
                        newList.Add(left.First());
                        left.RemoveAt(0);
                    }
                }

                
                if (left.Count > 0) newList.AddRange(left);
                if (right.Count > 0) newList.AddRange(right);
            }

            return newList;
        }

        public static List<T> overhandShuffle<T>(List<T> list, int passes) {
            List<T> mainHand = new List<T>(list);

            for (int n = 0; n < passes; n++) {
                List<T> otherHand = new List<T>();

                while (mainHand.Count>0) {
                    
                    int cutSize = rand.Next((int)(list.Count * 0.2)) + 1;

                    List<T> temp = new List<T>();

                    
                    for (int i = 0; i < cutSize && mainHand.Count > 0; i++) {
                        temp.Add(mainHand.First());
                        mainHand.RemoveAt(0);
                    }

                    
                    if (rand.NextDouble()>=0.1) {
                        
                        temp.AddRange(otherHand);
                        otherHand = temp;
                    }
                    else {
                        
                        otherHand.AddRange(temp);
                    }
                }

                
                mainHand = otherHand;
            }

            return mainHand;
        }

        static void Main(string[] args) {
            List<int> list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
            Console.WriteLine(list.AsString());
            list = riffleShuffle(list, 10);
            Console.WriteLine(list.AsString());
            Console.WriteLine();

            list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
            Console.WriteLine(list.AsString());
            list = riffleShuffle(list, 1);
            Console.WriteLine(list.AsString());
            Console.WriteLine();

            list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
            Console.WriteLine(list.AsString());
            list = overhandShuffle(list, 10);
            Console.WriteLine(list.AsString());
            Console.WriteLine();

            list = new List<int>() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
            Console.WriteLine(list.AsString());
            list = overhandShuffle(list, 1);
            Console.WriteLine(list.AsString());
            Console.WriteLine();
        }
    }
}

C++: #include <time.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <deque>


class riffle
{
public:
    void shuffle( std::deque<int>* v, int tm )
    {
        std::deque<int> tmp;
	bool fl;
	size_t len;
	std::deque<int>::iterator it;

	copyTo( v, &tmp );

	for( int t = 0; t < tm; t++ )
	{
	    std::deque<int> lHand( rand() % ( tmp.size() / 3 ) + ( tmp.size() >> 1 ) ), rHand( tmp.size() - lHand.size() );

	    std::copy( tmp.begin(), tmp.begin() + lHand.size(), lHand.begin() );
	    std::copy( tmp.begin() + lHand.size(), tmp.end(), rHand.begin() );
	    tmp.clear();

	    while( lHand.size() && rHand.size() )
	    {
		fl = rand() % 10 < 5;
		if( fl )
    		    len = 1 + lHand.size() > 3 ? rand() % 3 + 1 : rand() % ( lHand.size() ) + 1;
		else
		    len = 1 + rHand.size() > 3 ? rand() % 3 + 1 : rand() % ( rHand.size() ) + 1;

		while( len )
		{
		    if( fl ) 
		    {
			tmp.push_front( *lHand.begin() );
			lHand.erase( lHand.begin() );
		    }
		    else 
		    {
			tmp.push_front( *rHand.begin() );
			rHand.erase( rHand.begin() );
		    }
		    len--;
		}
	    }

	    if( lHand.size() < 1 ) 
	    {
		for( std::deque<int>::iterator x = rHand.begin(); x != rHand.end(); x++ )
		    tmp.push_front( *x );
	    }
	    if( rHand.size() < 1 ) 
	    {
		for( std::deque<int>::iterator x = lHand.begin(); x != lHand.end(); x++ )
		    tmp.push_front( *x );
	    }
	}
	copyTo( &tmp, v );
    }
private:
    void copyTo( std::deque<int>* a, std::deque<int>* b )
    {
	for( std::deque<int>::iterator x = a->begin(); x != a->end(); x++ )
	    b->push_back( *x );
	a->clear();
    }
};

class overhand
{
public:
    void shuffle( std::deque<int>* v, int tm )
    {
	std::deque<int> tmp;
	bool top;
	for( int t = 0; t < tm; t++ )
	{
	    while( v->size() )
	    {
		size_t len = rand() % ( v->size() ) + 1;
		top = rand() % 10 < 5;
		while( len )
		{
		    if( top ) tmp.push_back( *v->begin() );
		    else tmp.push_front( *v->begin() );
		    v->erase( v->begin() );
		    len--;
		}
	    }
	    for( std::deque<int>::iterator x = tmp.begin(); x != tmp.end(); x++ )
		v->push_back( *x );

	    tmp.clear();
	}
    }
};


std::deque<int> cards;

void fill()
{
    cards.clear();
    for( int x = 0; x < 20; x++ )
	cards.push_back( x + 1 );
}

void display( std::string t )
{
    std::cout << t << "\n";
    for( std::deque<int>::iterator x = cards.begin(); x != cards.end(); x++ )
	std::cout << *x << " ";
    std::cout << "\n\n";
}

int main( int argc, char* argv[] )
{
    srand( static_cast<unsigned>( time( NULL ) ) );
    riffle r; overhand o;	

    fill(); r.shuffle( &cards, 10 ); display( "RIFFLE" );
    fill(); o.shuffle( &cards, 10 ); display( "OVERHAND" );
    fill(); std::random_shuffle( cards.begin(), cards.end() ); display( "STD SHUFFLE" );

    return 0;
}