Task: Ludic numbers
C: #include <stdio.h>
#include <stdlib.h>

typedef unsigned uint;
typedef struct { uint i, v; } filt_t;


uint* ludic(uint min_len, uint min_val, uint *len)
{
	uint cap, i, v, active = 1, nf = 0;
	filt_t *f = calloc(cap = 2, sizeof(*f));
	f[1].i = 4;

	for (v = 1; ; ++v) {
		for (i = 1; i < active && --f[i].i; i++);

		if (i < active)
			f[i].i = f[i].v;
		else if (nf == f[i].i)
			f[i].i = f[i].v, ++active;  
		else {
			if (nf >= cap)
				f = realloc(f, sizeof(*f) * (cap*=2));
			f[nf] = (filt_t){ v + nf, v };
			if (++nf >= min_len && v >= min_val) break;
		}
	}

	
	
	uint *x = (void*) f;
	for (i = 0; i < nf; i++) x[i] = f[i].v;
	x = realloc(x, sizeof(*x) * nf);

	*len = nf;
	return x;
}

int find(uint *a, uint v)
{
	uint i;
	for (i = 0; a[i] <= v; i++)
		if (v == a[i]) return 1;
	return 0;
}

int main(void)
{
	uint len, i, *x = ludic(2005, 1000, &len);

	printf("First 25:");
	for (i = 0; i < 25; i++) printf(" %u", x[i]);
	putchar('\n');

	for (i = 0; x[i] <= 1000; i++);
	printf("Ludics below 1000: %u\n", i);

	printf("Ludic 2000 to 2005:");
	for (i = 2000; i <= 2005; i++) printf(" %u", x[i - 1]);
	putchar('\n');

	printf("Triples below 250:");
	for (i = 0; x[i] + 6 <= 250; i++)
		if (find(x, x[i] + 2) && find(x, x[i] + 6))
			printf(" (%u %u %u)", x[i], x[i] + 2, x[i] + 6);

	putchar('\n');

	free(x);
	return 0;
}

C#: using System;
using System.Linq;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        Console.WriteLine("First 25 ludic numbers:");
        Console.WriteLine(string.Join(", ", LudicNumbers(150).Take(25)));
        Console.WriteLine();
        
        Console.WriteLine($"There are {LudicNumbers(1001).Count()} ludic numbers below 1000");
        Console.WriteLine();
        
        foreach (var ludic in LudicNumbers(22000).Skip(1999).Take(6)
                .Select((n, i) => $"#{i+2000} = {n}")) {
            Console.WriteLine(ludic);
        }
        Console.WriteLine();
        
        Console.WriteLine("Triplets below 250:");
        var queue = new Queue<int>(5);
        foreach (int x in LudicNumbers(255)) {
            if (queue.Count == 5) queue.Dequeue();
            queue.Enqueue(x);
            if (x - 6 < 250 && queue.Contains(x - 6) && queue.Contains(x - 4)) {
                Console.WriteLine($"{x-6}, {x-4}, {x}");
            }
        }
    }
    
    public static IEnumerable<int> LudicNumbers(int limit) {
        yield return 1;
        
        
        
        Entry[] values = Enumerable.Range(0, limit + 1).Select(n => new Entry(n)).ToArray();
        for (int i = 2; i < limit; i = values[i].Next) {
            yield return values[i].N;
            int start = i;
            while (start < limit) {
                Unlink(values, start);
                for (int step = 0; step < i && start < limit; step++)
                    start = values[start].Next;
            }
        }
    }
    
    static void Unlink(Entry[] values, int index) {
        values[values[index].Prev].Next = values[index].Next;
        values[values[index].Next].Prev = values[index].Prev;
    }
    
}

struct Entry
{
    public Entry(int n) : this() {
        N = n;
        Prev = n - 1;
        Next = n + 1;
    }
    
    public int N { get; }
    public int Prev { get; set; }
    public int Next { get; set; }
}