Task: Intersecting number wheels
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct Wheel {
    char *seq;
    int len;
    int pos;
};

struct Wheel *create(char *seq) {
    struct Wheel *w = malloc(sizeof(struct Wheel));
    if (w == NULL) {
        return NULL;
    }

    w->seq = seq;
    w->len = strlen(seq);
    w->pos = 0;

    return w;
}

char cycle(struct Wheel *w) {
    char c = w->seq[w->pos];
    w->pos = (w->pos + 1) % w->len;
    return c;
}

struct Map {
    struct Wheel *v;
    struct Map *next;
    char k;
};

struct Map *insert(char k, struct Wheel *v, struct Map *head) {
    struct Map *m = malloc(sizeof(struct Map));
    if (m == NULL) {
        return NULL;
    }

    m->k = k;
    m->v = v;
    m->next = head;

    return m;
}

struct Wheel *find(char k, struct Map *m) {
    struct Map *ptr = m;

    while (ptr != NULL) {
        if (ptr->k == k) {
            return ptr->v;
        }
        ptr = ptr->next;
    }

    return NULL;
}

void printOne(char k, struct Map *m) {
    struct Wheel *w = find(k, m);
    char c;

    if (w == NULL) {
        printf("Missing the wheel for: %c\n", k);
        exit(1);
    }

    c = cycle(w);
    if ('0' <= c && c <= '9') {
        printf(" %c", c);
    } else {
        printOne(c, m);
    }
}

void exec(char start, struct Map *m) {
    struct Wheel *w;
    int i;

    if (m == NULL) {
        printf("Unable to proceed.");
        return;
    }

    for (i = 0; i < 20; i++) {
        printOne(start, m);
    }
    printf("\n");
}

void group1() {
    struct Wheel *a = create("123");

    struct Map *m = insert('A', a, NULL);

    exec('A', m);
}

void group2() {
    struct Wheel *a = create("1B2");
    struct Wheel *b = create("34");

    struct Map *m = insert('A', a, NULL);
    m = insert('B', b, m);

    exec('A', m);
}

void group3() {
    struct Wheel *a = create("1DD");
    struct Wheel *d = create("678");

    struct Map *m = insert('A', a, NULL);
    m = insert('D', d, m);

    exec('A', m);
}

void group4() {
    struct Wheel *a = create("1BC");
    struct Wheel *b = create("34");
    struct Wheel *c = create("5B");

    struct Map *m = insert('A', a, NULL);
    m = insert('B', b, m);
    m = insert('C', c, m);

    exec('A', m);
}

int main() {
    group1();
    group2();
    group3();
    group4();

    return 0;
}

Java: package intersectingNumberWheels;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.IntStream;

public class WheelController {
	private static final String IS_NUMBER = "[0-9]";
	private static final int TWENTY = 20;
	private static Map<String, WheelModel> wheelMap;

	public static void advance(String wheel) {
		WheelModel w = wheelMap.get(wheel);
		if (w.list.get(w.position).matches(IS_NUMBER)) {
			w.printThePosition();
			w.advanceThePosition();
		} else {
			String wheelName = w.list.get(w.position);
			advance(wheelName);
			w.advanceThePosition();
		}
	}

	public static void run() {
		System.out.println(wheelMap);
		IntStream.rangeClosed(1, TWENTY).forEach(i -> advance("A"));
		System.out.println();
		wheelMap.clear();
	}

	public static void main(String[] args) {
		wheelMap = new HashMap<>();
		wheelMap.put("A", new WheelModel("A", "1", "2", "3"));
		run();
		wheelMap.put("A", new WheelModel("A", "1", "B", "2"));
		wheelMap.put("B", new WheelModel("B", "3", "4"));
		run();
		wheelMap.put("A", new WheelModel("A", "1", "D", "D"));
		wheelMap.put("D", new WheelModel("D", "6", "7", "8"));
		run();
		wheelMap.put("A", new WheelModel("A", "1", "B", "C"));
		wheelMap.put("B", new WheelModel("B", "3", "4"));
		wheelMap.put("C", new WheelModel("C", "5", "B"));
		run();
	}

}

class WheelModel {
	String name;
	List<String> list;
	int position;
	int endPosition;
	private static final int INITIAL = 0;

	public WheelModel(String name, String... values) {
		super();

		this.name = name.toUpperCase();
		this.list = new ArrayList<>();
		for (String value : values) {
			list.add(value);
		}
		this.position = INITIAL;
		this.endPosition = this.list.size() - 1;
	}

	@Override
	public String toString() {
		return list.toString();
	}

	public void advanceThePosition() {
		if (this.position == this.endPosition) {
			this.position = INITIAL;
		} else {
			this.position++;
		}
	}

	public void printThePosition() {
		System.out.print(" " + this.list.get(position));
	}
}