Problem 27

27.c

@@ -0,0 +1,98 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+bool isPrime (int n) {
+    switch (n) {
+        case 1:
+        case 9:
+            return false;
+        case 2:
+        case 3:
+        case 5:
+        case 7:
+            return true;
+        default:
+            break;
+    }
+
+    switch (n % 10) {
+        case 0:
+        case 2:
+        case 4:
+        case 5:
+        case 6:
+        case 8:
+            return false;
+        default:
+            if ((n + 1) % 2 != 0)
+                return false;
+            for (int i = 3; i * i <= n; i += 2) {
+                if (n % i == 0)
+                    return false;
+            }
+            return true;
+    }
+}
+
+bool* primeTable;
+
+void findPrimes (int n) {
+    primeTable = malloc (sizeof (bool) * (n + 1));
+    for (int i = 0; i <= n; i++)
+        primeTable[i] = isPrime (i);
+}
+
+int numOfPrimes (int a, int b) {
+    for (int n = 0; n < b; n++) {
+        int val = n * n + a * n + b;
+        if (val < 0)
+            val = -val;
+        if (primeTable[val] == false)
+            return n;
+    }
+}
+
+int main (int argc, char* argv[]) {
+    int maxVal = 999 * 999 + 999 * 999 + 1000;
+    findPrimes (maxVal);
+
+    int bestA = 0;
+    int bestB = 0;
+    int bestNum = 0;
+    for (int a = 0; a < 1000; a++) {
+        for (int b = 0; b <= 1000; b++) {
+            if (primeTable[b] == true) {
+                int n = numOfPrimes (a, b);
+                if (n > bestNum) {
+                    bestNum = n - 1;
+                    bestA = a;
+                    bestB = b;
+                }
+
+                n = numOfPrimes (a, -b);
+                if (n > bestNum) {
+                    bestNum = n;
+                    bestA = a;
+                    bestB = -b;
+                }
+
+                n = numOfPrimes (-a, b);
+                if (n > bestNum) {
+                    bestNum = n;
+                    bestA = -a;
+                    bestB = b;
+                }
+
+                n = numOfPrimes (-a, -b);
+                if (n > bestNum) {
+                    bestNum = n;
+                    bestA = -a;
+                    bestB = -b;
+                }
+            }
+        }
+    }
+
+    printf ("a: %d, b: %d, n: %d, a*b: %d", bestA, bestB, bestNum, bestA * bestB);
+}