Task: N-smooth numbers
C: #include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <gmp.h>

void* xmalloc(size_t n) {
    void* ptr = malloc(n);
    if (ptr == NULL) {
        fprintf(stderr, "Out of memory\n");
        exit(1);
    }
    return ptr;
}

void* xrealloc(void* p, size_t n) {
    void* ptr = realloc(p, n);
    if (ptr == NULL) {
        fprintf(stderr, "Out of memory\n");
        exit(1);
    }
    return ptr;
}

bool is_prime(uint32_t n) {
    if (n == 2)
        return true;
    if (n < 2 || n % 2 == 0)
        return false;
    for (uint32_t p = 3; p * p <= n; p += 2) {
        if (n % p == 0)
            return false;
    }
    return true;
}



uint32_t find_primes(uint32_t from, uint32_t to, uint32_t** primes) {
    uint32_t count = 0, buffer_length = 16;
    uint32_t* buffer = xmalloc(sizeof(uint32_t) * buffer_length);
    for (uint32_t p = from; p <= to; ++p) {
        if (is_prime(p)) {
            if (count >= buffer_length) {
                uint32_t new_length = buffer_length * 2;
                if (new_length < count + 1)
                    new_length = count + 1;
                buffer = xrealloc(buffer, sizeof(uint32_t) * new_length);
                buffer_length = new_length;
            }
            buffer[count++] = p;
        }
    }
    *primes = buffer;
    return count;
}

void free_numbers(mpz_t* numbers, size_t count) {
    for (size_t i = 0; i < count; ++i)
        mpz_clear(numbers[i]);
    free(numbers);
}


mpz_t* find_nsmooth_numbers(uint32_t n, uint32_t count) {
    uint32_t* primes = NULL;
    uint32_t num_primes = find_primes(2, n, &primes);
    mpz_t* numbers = xmalloc(sizeof(mpz_t) * count);
    mpz_t* queue = xmalloc(sizeof(mpz_t) * num_primes);
    uint32_t* index = xmalloc(sizeof(uint32_t) * num_primes);
    for (uint32_t i = 0; i < num_primes; ++i) {
        index[i] = 0;
        mpz_init_set_ui(queue[i], primes[i]);
    }
    for (uint32_t i = 0; i < count; ++i)
        mpz_init(numbers[i]);
    mpz_set_ui(numbers[0], 1);
    for (uint32_t i = 1; i < count; ++i) {
        for (uint32_t p = 0; p < num_primes; ++p) {
            if (mpz_cmp(queue[p], numbers[i - 1]) == 0)
                mpz_mul_ui(queue[p], numbers[++index[p]], primes[p]);
        }
        uint32_t min_index = 0;
        for (uint32_t p = 1; p < num_primes; ++p) {
            if (mpz_cmp(queue[min_index], queue[p]) > 0)
                min_index = p;
        }
        mpz_set(numbers[i], queue[min_index]);
    }
    free_numbers(queue, num_primes);
    free(primes);
    free(index);
    return numbers;
}

void print_nsmooth_numbers(uint32_t n, uint32_t begin, uint32_t count) {
    uint32_t num = begin + count;
    mpz_t* numbers = find_nsmooth_numbers(n, num);
    printf("%u: ", n);
    mpz_out_str(stdout, 10, numbers[begin]);
    for (uint32_t i = 1; i < count; ++i) {
        printf(", ");
        mpz_out_str(stdout, 10, numbers[begin + i]);
    }
    printf("\n");
    free_numbers(numbers, num);
}

int main() {
    printf("First 25 n-smooth numbers for n = 2 -> 29:\n");
    for (uint32_t n = 2; n <= 29; ++n) {
        if (is_prime(n))
            print_nsmooth_numbers(n, 0, 25);
    }
    printf("\n3 n-smooth numbers starting from 3000th for n = 3 -> 29:\n");
    for (uint32_t n = 3; n <= 29; ++n) {
        if (is_prime(n))
            print_nsmooth_numbers(n, 2999, 3);
    }
    printf("\n20 n-smooth numbers starting from 30,000th for n = 503 -> 521:\n");
    for (uint32_t n = 503; n <= 521; ++n) {
        if (is_prime(n))
            print_nsmooth_numbers(n, 29999, 20);
    }
    return 0;
}

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

namespace NSmooth {
    class Program {
        static readonly List<BigInteger> primes = new List<BigInteger>();
        static readonly List<int> smallPrimes = new List<int>();

        static Program() {
            primes.Add(2);
            smallPrimes.Add(2);

            BigInteger i = 3;
            while (i <= 521) {
                if (IsPrime(i)) {
                    primes.Add(i);
                    if (i <= 29) {
                        smallPrimes.Add((int)i);
                    }
                }
                i += 2;
            }
        }

        static bool IsPrime(BigInteger value) {
            if (value < 2) return false;

            if (value % 2 == 0) return value == 2;
            if (value % 3 == 0) return value == 3;

            if (value % 5 == 0) return value == 5;
            if (value % 7 == 0) return value == 7;

            if (value % 11 == 0) return value == 11;
            if (value % 13 == 0) return value == 13;

            if (value % 17 == 0) return value == 17;
            if (value % 19 == 0) return value == 19;

            if (value % 23 == 0) return value == 23;

            BigInteger t = 29;
            while (t * t < value) {
                if (value % t == 0) return false;
                value += 2;

                if (value % t == 0) return false;
                value += 4;
            }

            return true;
        }

        static List<BigInteger> NSmooth(int n, int size) {
            if (n < 2 || n > 521) {
                throw new ArgumentOutOfRangeException("n");
            }
            if (size < 1) {
                throw new ArgumentOutOfRangeException("size");
            }

            BigInteger bn = n;
            bool ok = false;
            foreach (var prime in primes) {
                if (bn == prime) {
                    ok = true;
                    break;
                }
            }
            if (!ok) {
                throw new ArgumentException("must be a prime number", "n");
            }

            BigInteger[] ns = new BigInteger[size];
            ns[0] = 1;
            for (int i = 1; i < size; i++) {
                ns[i] = 0;
            }

            List<BigInteger> next = new List<BigInteger>();
            foreach (var prime in primes) {
                if (prime > bn) {
                    break;
                }
                next.Add(prime);
            }

            int[] indices = new int[next.Count];
            for (int i = 0; i < indices.Length; i++) {
                indices[i] = 0;
            }
            for (int m = 1; m < size; m++) {
                ns[m] = next.Min();
                for (int i = 0; i < indices.Length; i++) {
                    if (ns[m] == next[i]) {
                        indices[i]++;
                        next[i] = primes[i] * ns[indices[i]];
                    }
                }
            }

            return ns.ToList();
        }

        static void Println<T>(IEnumerable<T> nums) {
            Console.Write('[');

            var it = nums.GetEnumerator();
            if (it.MoveNext()) {
                Console.Write(it.Current);
            }
            while (it.MoveNext()) {
                Console.Write(", ");
                Console.Write(it.Current);
            }

            Console.WriteLine(']');
        }

        static void Main() {
            foreach (var i in smallPrimes) {
                Console.WriteLine("The first {0}-smooth numbers are:", i);
                Println(NSmooth(i, 25));
                Console.WriteLine();
            }
            foreach (var i in smallPrimes.Skip(1)) {
                Console.WriteLine("The 3,000 to 3,202 {0}-smooth numbers are:", i);
                Println(NSmooth(i, 3_002).Skip(2_999));
                Console.WriteLine();
            }
            foreach (var i in new int[] { 503, 509, 521 }) {
                Console.WriteLine("The 30,000 to 3,019 {0}-smooth numbers are:", i);
                Println(NSmooth(i, 30_019).Skip(29_999));
                Console.WriteLine();
            }
        }
    }
}