58, 76, 97, 104

1 files changed, 45 insertions, 0 deletions

diff --git a/euler58.c b/euler58.c
new file mode 100644
index 0000000..ac46073
--- /dev/null
+++ b/euler58.c
@@ -0,0 +1,45 @@
+#include <stdio.h>
+
+/*
+ *   delta = 4n+4           delta = 4n+2
+ *                 5     3
+ *                   [1]
+ *                 7     9
+ *   delta = 4n+6           delta = 4n+8
+ */
+
+int isprime(int n)
+{
+	if (n == 1 || n % 2 == 0)
+		return 0;
+
+	for (int i = 3; i * i <= n; i += 2) {
+		if (n % i == 0)
+			return 0;
+	}
+
+	return 1;
+}
+
+int main()
+{
+	int ndiag = 5;
+	int nprime = 3;
+	int s[4] = {3, 5, 7, 9};
+	int n = 2;
+
+	while (nprime * 10 > ndiag) {
+		s[0] += n * 4 + 2;
+		s[1] += n * 4 + 4;
+		s[2] += n * 4 + 6;
+		s[3] += n * 4 + 8;
+		for (int i = 0; i < 4; i++) {
+			if (isprime(s[i]))
+				nprime++;
+		}
+		ndiag += 4;
+		n += 2;
+	}
+	int sidelen = n + 1;
+	printf("%d\n", sidelen);
+}