**Problem: **What is the highest prime factor of *n*? In this case: *n* = 600851475143

**Q:** What is the simplest way to find the highest prime factor of *n*?

**A: **Try dividing *n *by every number from 2 (the smallest prime) to *n – 1*. If the remainder of the division is zero, check the number for primeness and keep a record of the highest value.

**Q: **How do we reduce the search space?

**A:** Several ways based on what we know about prime numbers.

**A1: **Skip even numbers, since an even number (other than 2) cannot be prime and therefore cannot be a prime factor. This reduces the search space by 50%.

**A2: **Only check potential factors from *n – 1* to *floor(sqrt(n))*. After that we will have checked all of the factors because any factor *f < sqrt(n)* will have a corresponding factor *f’ > sqrt(n)*.

Reducing the search space by skipping even numbers and numbers greater than the square root of *n *reduces the number of potential factors to check in this case from 600851475141 (*n – 2*) to 387573 (*sqrt(n) / 2*).

**Q: **How do we tell if a factor is prime?

**A: **Divide it by all of the prime numbers below the factor. Any number which is composite (i.e. not prime) will have at least one prime factor of its own. Conversely, if a number has no prime factors, it is prime.

**Q: **How do we generate a list of all prime numbers below a given factor?

**A: **Use the Sieve of Eratosthenes. As with reducing the search space, we only need to generate a list of all prime numbers to *floor(sqrt(factor)). *Note that this method will not scale to extremely large numbers, as building the sieve would require too much memory.

**Solution:**

#include <stdio.h> #include <stdlib.h> #include <stdbool.h> #include <inttypes.h> #include <math.h> #include <glib.h> GSList* prime_numbers(uint32_t max_prime_candidate) { const int8_t SIEVE_UNKNOWN = -1; const int8_t SIEVE_NOT_PRIME = 0; const int8_t SIEVE_PRIME = 1; const uint8_t FIRST_PRIME = 2; uint32_t current_prime = 0; uint32_t primes_found = 0; int8_t* sieve = (int8_t*) calloc(max_prime_candidate + 1, sizeof(int8_t)); // Set the status of all numbers to unknown for (uint32_t i = 0; i <= max_prime_candidate; i++) { sieve[i] = SIEVE_UNKNOWN; } sieve[FIRST_PRIME] = SIEVE_PRIME; current_prime = FIRST_PRIME; primes_found = 1; for (uint32_t i = FIRST_PRIME; current_prime != 0 && i <= max_prime_candidate; i++) { uint32_t next_prime = 0; // assume this prime will be the last uint32_t step = current_prime; // Mark all multiples of current prime as non-prime for (uint32_t j = current_prime + step; j <= max_prime_candidate; j += step) { sieve[j] = SIEVE_NOT_PRIME; } // Find the next unknown element after the current prime - this is the next prime for (uint32_t k = current_prime + 1; next_prime == 0 && k <= max_prime_candidate; k++) { if (sieve[k] == SIEVE_UNKNOWN) { sieve[k] = SIEVE_PRIME; next_prime = k; primes_found++; } } // Set current prime to be the next prime. If there is no next prime, this // will end the search current_prime = next_prime; } // Create a linked list to return to the caller GSList* primes = NULL; for (uint32_t i = FIRST_PRIME; i <= max_prime_candidate; i++) { if (sieve[i] == SIEVE_PRIME) { primes = g_slist_append(primes, GUINT_TO_POINTER(i)); } } free(sieve); return primes; } gint compare_int(gconstpointer a, gconstpointer b) { uint32_t a_comp = GPOINTER_TO_UINT(a); uint32_t b_comp = GPOINTER_TO_UINT(b); if (a_comp < b_comp) { return -1; } else if (a_comp == b_comp) { return 0; } else { return 1; } } int main(void) { const uint64_t FACTORISE_TARGET = 600851475143; // Highest possible factor at the start is the square root of the target uint32_t highest_possible_factor = floor(sqrt(FACTORISE_TARGET)); // Build list of factors from 2 to the highest possible factor GSList* factors = NULL; for (uint32_t candidate_factor = 2; candidate_factor < highest_possible_factor; candidate_factor++) { printf("Factorising %lu with %u\n", FACTORISE_TARGET, candidate_factor); if (FACTORISE_TARGET % candidate_factor == 0) { uint32_t other_factor = FACTORISE_TARGET / candidate_factor; factors = g_slist_append(factors, GUINT_TO_POINTER(candidate_factor)); factors = g_slist_append(factors, GUINT_TO_POINTER(other_factor)); // Because we check candidate factors from the lowest value, the other // factor is potentially the highest possible factor if (highest_possible_factor > other_factor) { highest_possible_factor = other_factor; } } } // If we have found no factors, the target is prime and therefore has no // highest prime factor if (factors == NULL) { fprintf(stderr, "No prime factors found\n"); g_slist_free(factors); return EXIT_FAILURE; } // Sort factors factors = g_slist_sort(factors, &compare_int); // Print list of factors for (GSList* current_factor = factors; current_factor != NULL; current_factor = current_factor->next) { printf("Factor: %u\n", GPOINTER_TO_UINT(current_factor->data)); } // Which of the factors are prime? First build an array of primes up to the // square root of the highest factor. // The alternative is to build an array of primes up to the highest factor, as // that would allow us to do a lookup instead of multiple divisions, however // that would also increase the memory requirements for the array by an order // of highest_factor (which could be >1m). uint32_t highest_factor = GPOINTER_TO_UINT(g_slist_last(factors)->data); uint32_t max_prime_candidate = floor(sqrt(highest_factor)); GSList* primes = prime_numbers(max_prime_candidate); for (GSList* current_prime = primes; current_prime != NULL; current_prime = current_prime->next) { printf("Prime: %u\n", GPOINTER_TO_UINT(current_prime->data)); } uint32_t highest_prime_factor = 0; // Check each factor for primeness by dividing it by our primes. All composite // numbers have at least one prime factor. for (GSList* current_factor = factors; current_factor != NULL; current_factor = current_factor->next) { // Assume a target is prime unless we successively factorise it uint32_t target_factor = GPOINTER_TO_UINT(current_factor->data); bool is_prime = true; printf("Checking %u for primeness\n", target_factor); for (GSList* current_prime = primes; is_prime && current_prime != NULL && GPOINTER_TO_UINT(current_prime->data) < target_factor; current_prime = current_prime->next) { if (target_factor % GPOINTER_TO_UINT(current_prime->data) == 0) { is_prime = false; printf("%u is not prime, is divisible by: %u\n", target_factor, GPOINTER_TO_UINT(current_prime->data)); } } if (is_prime) { printf("%u is prime\n", target_factor); highest_prime_factor = target_factor; } } // We do not have to check for highest prime factor being 0 because all // composite numbers have at least one prime factor printf("Highest prime factor of %lu is %u\n", FACTORISE_TARGET, highest_prime_factor); g_slist_free(primes); g_slist_free(factors); return EXIT_SUCCESS; }