solve problem 50

Author: tca19

problem_026-050/problem_050.c

@@ -0,0 +1,72 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+#define N 1000000
+
+uint8_t sieve[N];
+
+/* init_sieve: initialize the array to test if a number is prime */
+void init_sieve(void)
+{
+	int32_t i, j;
+
+	for (i = 0; i < N; ++i)
+		sieve[i] = 1;
+	sieve[0] = sieve[1] = 0;
+
+	for (i = 2; i < N; ++i)
+		if (sieve[i])
+			for (j = 2*i; j < N; j += i)
+				sieve[j] = 0;
+}
+
+/* find_longest_prime_sum: find the prime number that can be written with the
+ * longest sum of consecutive primes */
+int32_t find_longest_prime_sum(void)
+{
+	int32_t i, j, n_primes, longest_length, longest_sum, sum, *primes;
+
+	/* count number of primes */
+	n_primes = 0;
+	for (i = 0; i < N; ++i)
+		if (sieve[i])
+			++n_primes;
+
+	/* regroup primes into a single array */
+	primes = malloc(sizeof *primes * n_primes);
+	for (i = j = 0; i < N; ++i)
+		if (sieve[i])
+			primes[j++] = i;
+
+	longest_sum = longest_length = 0;
+	for (i = 0; i < n_primes; ++i)
+	{
+		sum = 0;
+		for (j = i; j < n_primes; ++j)
+		{
+			sum += primes[j];
+
+			if (sum > N)
+				break;
+
+			/* current sum is a prime and longer than current known
+			 * longest sequence. Save its statistics. */
+			if (sieve[sum] && j - i > longest_length)
+			{
+				longest_length = j - i;
+				longest_sum = sum;
+			}
+		}
+	}
+
+	return longest_sum;
+}
+
+int32_t main(void)
+{
+	init_sieve();
+	printf("Problem 50: %d\n", find_longest_prime_sum());
+
+	return 0;
+}