Data commit

Task/Truncatable-primes/C/truncatable-primes-1.c

@@ -0,0 +1,63 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define MAX_PRIME 1000000
+char *primes;
+int n_primes;
+
+/*  Sieve. If we were to handle 10^9 range, use bit field. Regardless,
+ *  if a large amount of prime numbers need to be tested, sieve is fast.
+ */
+void init_primes()
+{
+	int j;
+	primes = malloc(sizeof(char) * MAX_PRIME);
+	memset(primes, 1, MAX_PRIME);
+	primes[0] = primes[1] = 0;
+	int i = 2;
+	while (i * i < MAX_PRIME) {
+		for (j = i * 2; j < MAX_PRIME; j += i)
+			primes[j] = 0;
+		while (++i < MAX_PRIME && !primes[i]);
+	}
+}
+
+int left_trunc(int n)
+{
+	int tens = 1;
+	while (tens < n) tens *= 10;
+
+	while (n) {
+		if (!primes[n]) return 0;
+		tens /= 10;
+		if (n < tens) return 0;
+		n %= tens;
+	}
+	return 1;
+}
+
+int right_trunc(int n)
+{
+	while (n) {
+		if (!primes[n]) return 0;
+		n /= 10;
+	}
+	return 1;
+}
+
+int main()
+{
+	int n;
+	int max_left = 0, max_right = 0;
+	init_primes();
+
+	for (n = MAX_PRIME - 1; !max_left;  n -= 2)
+		if (left_trunc(n)) max_left = n;
+
+	for (n = MAX_PRIME - 1; !max_right; n -= 2)
+		if (right_trunc(n)) max_right = n;
+
+	printf("Left: %d; right: %d\n", max_left, max_right);
+	return 0;
+}

Task/Truncatable-primes/C/truncatable-primes-2.c

@@ -0,0 +1 @@
+Left: 998443; right: 739399

Task/Truncatable-primes/C/truncatable-primes-3.c

@@ -0,0 +1,47 @@
+#include <stdio.h>
+
+#define MAXN 1000000
+int maxl, maxr;
+
+int is_prime(int n)
+{
+	int p;
+	if (n % 3 == 0) return 0;
+
+	for (p = 6; p * p <= n; p += 6)
+		if (!(n % (p + 1) && n % (p + 5)))
+			return 0;
+	return 1;
+}
+
+void left(int n, int tens)
+{
+	int i, nn;
+
+	if (n > maxl) maxl = n;
+	if (n < MAXN / 10)
+		for (tens *= 10, i = 1; i < 10; i++)
+			if (is_prime(nn = i * tens + n))
+				left(nn, tens);
+}
+
+void right(int n)
+{
+	int i, nn;
+	static int d[] = {1,3,7,9};
+
+	if (n > maxr) maxr = n;
+	if (n < MAXN / 10)
+		for (i = 1; i < 4; i++)
+			if (is_prime(nn = n * 10 + d[i])) right(nn);
+}
+
+int main(void)
+{
+	left(3, 1); left(7, 1);
+	right(3); right(5); right(7);
+
+	printf("%d %d\n", maxl, maxr);
+
+	return 0;
+}