Task: Card shuffles
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

void init() {
    srand((unsigned int)time(NULL));
}

int random(int low, int high) {
    int diff, val;

    diff = high - low;
    if (diff == 0) {
        return low;
    }

    val = rand() % diff;
    return val + low;
}

void initDeck(int *deck, const int size) {
    int i;
    for (i = 0; i < size; ++i) {
        *deck++ = i + 1;
    }
}

void writeDeck(const int *deck, const int size) {
    int i;

    printf("[");
    if (size > 0) {
        printf("%d", *deck++);
    }
    for (i = 1; i < size; ++i) {
        printf(", %d", *deck++);
    }
    printf("]");
}

void riffleShuffle(int * const deck, const int size, int flips) {
    int n, cutPoint, nlp, lp, rp, bound;
    int *nl;

    nl = (int *)malloc(size * sizeof(int));

    for (n = 0; n < flips; ++n) {
        cutPoint = size / 2;
        if (random(0, 2) > 0) {
            cutPoint = cutPoint + random(0, size / 10);
        } else {
            cutPoint = cutPoint - random(0, size / 10);
        }

        nlp = 0;
        lp = 0;
        rp = cutPoint;

        while (lp < cutPoint && rp < size) {
            
            bound = (cutPoint - lp) * 50 / (size - rp);
            if (random(0, 50) >= bound) {
                nl[nlp++] = deck[rp++];
            } else {
                nl[nlp++] = deck[lp++];
            }
        }
        while (lp < cutPoint) {
            nl[nlp++] = deck[lp++];
        }
        while (rp < size) {
            nl[nlp++] = deck[rp++];
        }

        memcpy(deck, nl, size * sizeof(int));
    }

    free(nl);
}

void overhandShuffle(int * const mainHand, const int size, int passes) {
    int n, cutSize, mp, op, tp, i;
    int *otherHand, *temp;

    otherHand = (int *)malloc(size * sizeof(int));
    temp = (int *)malloc(size * sizeof(int));

    for (n = 0; n < passes; ++n) {
        mp = 0;
        op = 0;
        tp = 0;

        while (mp < size) {
            cutSize = random(0, size / 5) + 1;

            
            for (i = 0; i < cutSize && mp < size; ++i) {
                temp[tp++] = mainHand[mp++];
            }

            
            if (random(0, 10) >= 1) {
                

                
                for (i = op - 1; i >= 0; --i) {
                    otherHand[i + tp] = otherHand[i];
                }

                
                memcpy(otherHand, temp, tp * sizeof(int));
                op += tp;
                tp = 0;
            } else {
                
                for (i = 0; i < tp; ++i, ++op) {
                    otherHand[op] = temp[i];
                }
                tp = 0;
            }
        }

        
        memcpy(mainHand, otherHand, size * sizeof(int));
    }

    free(otherHand);
    free(temp);
}

#define SIZE 20
int main() {
    int deck[SIZE];

    init();

    printf("Riffle shuffle\n");
    initDeck(deck, SIZE);
    writeDeck(deck, SIZE);
    printf("\n");
    riffleShuffle(deck, SIZE, 10);
    writeDeck(deck, SIZE);
    printf("\n\n");

    printf("Riffle shuffle\n");
    initDeck(deck, SIZE);
    writeDeck(deck, SIZE);
    printf("\n");
    riffleShuffle(deck, SIZE, 1);
    writeDeck(deck, SIZE);
    printf("\n\n");

    printf("Overhand shuffle\n");
    initDeck(deck, SIZE);
    writeDeck(deck, SIZE);
    printf("\n");
    overhandShuffle(deck, SIZE, 10);
    writeDeck(deck, SIZE);
    printf("\n\n");

    printf("Overhand shuffle\n");
    initDeck(deck, SIZE);
    writeDeck(deck, SIZE);
    printf("\n");
    overhandShuffle(deck, SIZE, 1);
    writeDeck(deck, SIZE);
    printf("\n\n");

    return 0;
}

Java: import java.util.Arrays;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.Random;

public class CardShuffles{

	private static final Random rand = new Random();

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

		newList.addAll(list);

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

			
			List<T> left = new LinkedList<T>();
			left.addAll(newList.subList(0, cutPoint));
			List<T> right = new LinkedList<T>();
			right.addAll(newList.subList(cutPoint, newList.size()));

			newList.clear();

			while(left.size() > 0 && right.size() > 0){
				
				
				
				if(rand.nextDouble() >= ((double)left.size() / right.size()) / 2){
					newList.add(right.remove(0));
				}else{
					newList.add(left.remove(0));
				}
			}

			
			if(left.size() > 0) newList.addAll(left);
			if(right.size() > 0) newList.addAll(right);
		}
		return newList;
	}

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

		mainHand.addAll(list);
		for(int n = 0; n < passes; n++){
			LinkedList<T> otherHand = new LinkedList<T>();

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

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

				
				for(int i = 0; i < cutSize && mainHand.size() > 0; i++){
					temp.add(mainHand.remove());
				}

				
				if(rand.nextDouble()  >= 0.1){
					
					otherHand.addAll(0, temp);
				}else{
					
					otherHand.addAll(temp);
				}
			}

			
			mainHand = otherHand;
		}		
		return mainHand;
	}

	public static void main(String[] args){
		List<Integer> list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
		System.out.println(list);
		list = riffleShuffle(list, 10);
		System.out.println(list + "\n");

                list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
		System.out.println(list);
		list = riffleShuffle(list, 1);
		System.out.println(list + "\n");

		list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
		System.out.println(list);
		list = overhandShuffle(list, 10);
		System.out.println(list + "\n");

                list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
		System.out.println(list);
		list = overhandShuffle(list, 1);
		System.out.println(list + "\n");

		list = Arrays.asList(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20);
		System.out.println(list);
		Collections.shuffle(list);
		System.out.println(list + "\n");
	}
}