progminer zano miner fork https://github.com/hyle-team/progminer

zano/libethash/primes.c

@@ -0,0 +1,43 @@
+/* ethash: C/C++ implementation of Ethash, the Ethereum Proof of Work algorithm.
+ * Copyright 2018 Pawel Bylica.
+ * Licensed under the Apache License, Version 2.0. See the LICENSE file.
+ */
+
+#include "primes.h"
+
+/** Checks if the number is prime. Requires the number to be > 2 and odd. */
+static int is_odd_prime(int number)
+{
+    int d;
+
+    /* Check factors up to sqrt(number).
+       To avoid computing sqrt, compare d*d <= number with 64-bit precision. */
+    for (d = 3; (int64_t)d * (int64_t)d <= (int64_t)number; d += 2)
+    {
+        if (number % d == 0)
+            return 0;
+    }
+
+    return 1;
+}
+
+int ethash_find_largest_prime(int upper_bound)
+{
+    int n = upper_bound;
+
+    if (n < 2)
+        return 0;
+
+    if (n == 2)
+        return 2;
+
+    /* If even number, skip it. */
+    if (n % 2 == 0)
+        --n;
+
+    /* Test descending odd numbers. */
+    while (!is_odd_prime(n))
+        n -= 2;
+
+    return n;
+}