Data commit

Task/Prime-conspiracy/C/prime-conspiracy.c

@@ -0,0 +1,121 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stdio.h>
+
+typedef unsigned char byte;
+
+struct Transition {
+    byte a, b;
+    unsigned int c;
+} transitions[100];
+
+void init() {
+    int i, j;
+    for (i = 0; i < 10; i++) {
+        for (j = 0; j < 10; j++) {
+            int idx = i * 10 + j;
+            transitions[idx].a = i;
+            transitions[idx].b = j;
+            transitions[idx].c = 0;
+        }
+    }
+}
+
+void record(int prev, int curr) {
+    byte pd = prev % 10;
+    byte cd = curr % 10;
+    int i;
+
+    for (i = 0; i < 100; i++) {
+        int z = 0;
+        if (transitions[i].a == pd) {
+            int t = 0;
+            if (transitions[i].b == cd) {
+                transitions[i].c++;
+                break;
+            }
+        }
+    }
+}
+
+void printTransitions(int limit, int last_prime) {
+    int i;
+
+    printf("%d primes, last prime considered: %d\n", limit, last_prime);
+
+    for (i = 0; i < 100; i++) {
+        if (transitions[i].c > 0) {
+            printf("%d->%d  count: %5d  frequency: %.2f\n", transitions[i].a, transitions[i].b, transitions[i].c, 100.0 * transitions[i].c / limit);
+        }
+    }
+}
+
+bool isPrime(int n) {
+    int s, t, a1, a2;
+
+    if (n % 2 == 0) return n == 2;
+    if (n % 3 == 0) return n == 3;
+    if (n % 5 == 0) return n == 5;
+    if (n % 7 == 0) return n == 7;
+    if (n % 11 == 0) return n == 11;
+    if (n % 13 == 0) return n == 13;
+    if (n % 17 == 0) return n == 17;
+    if (n % 19 == 0) return n == 19;
+
+    // assuming that addition is faster then multiplication
+    t = 23;
+    a1 = 96;
+    a2 = 216;
+    s = t * t;
+    while (s <= n) {
+        if (n % t == 0) return false;
+
+        // first increment
+        s += a1;
+        t += 2;
+        a1 += 24;
+        assert(t * t == s);
+
+        if (s <= n) {
+            if (n % t == 0) return false;
+
+            // second increment
+            s += a2;
+            t += 4;
+            a2 += 48;
+            assert(t * t == s);
+        }
+    }
+
+    return true;
+}
+
+#define LIMIT 1000000
+int main() {
+    int last_prime = 3, n = 5, count = 2;
+
+    init();
+    record(2, 3);
+
+    while (count < LIMIT) {
+        if (isPrime(n)) {
+            record(last_prime, n);
+            last_prime = n;
+            count++;
+        }
+        n += 2;
+
+        if (count < LIMIT) {
+            if (isPrime(n)) {
+                record(last_prime, n);
+                last_prime = n;
+                count++;
+            }
+            n += 4;
+        }
+    }
+
+    printTransitions(LIMIT, last_prime);
+
+    return 0;
+}