Add C solution

1 files changed, 97 insertions, 0 deletions

diff --git a/challenge-015/paulo-custodio/c/ch-1.c b/challenge-015/paulo-custodio/c/ch-1.c
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+/*
+Challenge 015
+
+Task #1
+Write a script to generate first 10 strong and weak prime numbers.
+
+  For example, the nth prime number is represented by p(n).
+
+  p(1) = 2
+  p(2) = 3
+  p(3) = 5
+  p(4) = 7
+  p(5) = 11
+
+  Strong Prime number p(n) when p(n) > [ p(n-1) + p(n+1) ] / 2
+  Weak   Prime number p(n) when p(n) < [ p(n-1) + p(n+1) ] / 2
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+
+void* check_mem(void* p) {
+    if (!p) {
+        fputs("Out of memory", stderr);
+        exit(EXIT_FAILURE);
+    }
+    return p;
+}
+
+bool is_prime(int n) {
+    if (n <= 1)
+        return false;
+    if (n <= 3)
+        return true;
+    if ((n % 2) == 0 || (n % 3) == 0)
+        return false;
+    for (int i = 5; i * i <= n; i += 6)
+        if ((n % i) == 0 || (n % (i + 2)) == 0)
+            return false;
+    return true;
+}
+
+int next_prime(int n) {
+    if (n <= 1)
+        return 2;
+    do {
+        n++;
+    } while (!is_prime(n));
+    return n;
+}
+
+int* strong_weak_primes(int N, bool want_strong) {
+    int* primes = check_mem(malloc(N * sizeof(int)));
+    int p1 = 2, p2 = 3, p3 = 5;
+    for (int i = 0; i < N; ) {
+        float avg = ((float)p1+p3)/2;
+        if (want_strong) {
+            if (p2 > avg)
+                primes[i++] = p2;
+        }
+        else {
+            if ((float)p2 < avg)
+                primes[i++] = p2;
+        }
+
+        // next prime
+        p1 = p2; p2 = p3; p3 = next_prime(p2);
+    }
+    return primes;
+}
+
+void print_list(int* nums, int N) {
+    const char* sep = "";
+    for (int i = 0; i < N; i++) {
+        printf("%s%d", sep, nums[i]);
+        sep = ", ";
+    }
+}
+
+int main (int argc, char* argv[]) {
+    int N = 10;
+    if (argc == 2)
+        N = atoi(argv[1]);
+
+    int* primes = strong_weak_primes(N, true);
+    printf("Strong Prime: ");
+    print_list(primes, N);
+    printf("\n");
+    free(primes);
+
+    primes = strong_weak_primes(N, false);
+    printf("Weak Prime:   ");
+    print_list(primes, N);
+    printf("\n");
+    free(primes);
+}