initial import of the C runtime

src/runtime/c/gu/prime.c

@@ -0,0 +1,154 @@
+#include <gu/defs.h>
+#include <gu/assert.h>
+
+static const uint32_t gu_prime_wheel_mask = 0UL
+	| 1 << 1
+	| 1 << 7
+	| 1 << 11
+	| 1 << 13
+	| 1 << 17
+	| 1 << 19
+	| 1 << 23
+	| 1 << 29;
+
+static bool
+gu_prime_wheel(int i)
+{
+	gu_assert(i >= 0 && i < 30);
+	return !!(gu_prime_wheel_mask & (1 << i));
+}
+
+static const uint32_t gu_small_prime_mask = 0UL
+	| 1 << 2
+	| 1 << 3 
+	| 1 << 5
+	| 1 << 7
+	| 1 << 11
+	| 1 << 13
+	| 1 << 17
+	| 1 << 19
+	| 1 << 23
+	| 1 << 29
+	| 1U << 31;
+
+static bool
+gu_is_wheel_prime(int u)
+{
+	gu_assert(u > 30 && u % 2 != 0 && u % 3 != 0 && u % 5 != 0);
+	int d = 0;
+	int i = 7;
+	goto start;
+	while (d * d <= u) {
+		for (i = 1; i <= 29; i+=2) {
+		start:
+			if (gu_prime_wheel(i) && u % (d + i) == 0) {
+				return false;
+			}
+		}
+		d += 30;
+	}
+	return true;
+}
+
+int
+gu_prime_inf(int i)
+{
+	if (i < 2) {
+		return 0;
+	} else if (i < 32) {
+		while (!(gu_small_prime_mask & (1 << i))) {
+			i--;
+		}
+		return i;
+	}
+
+	int d = (i - 1) | 1;
+	int r = d % 30;
+
+	while (!gu_prime_wheel(r) || !gu_is_wheel_prime(d)) {
+		d -= 2;
+		r -= 2;
+		if (r < 0) {
+			r += 30;
+		} 
+	}
+	return d;
+}
+
+int
+gu_prime_sup(int i)
+{
+	if (i <= 2) {
+		return 2;
+	} else if (i < 32) {
+		while (!(gu_small_prime_mask & (1 << i))) {
+			i++;
+		}
+		return i;
+	}
+
+	int d = i | 1;
+	int r = d % 30;
+
+	while (!gu_prime_wheel(r) || !gu_is_wheel_prime(d)) {
+		d += 2;
+		r += 2;
+		if (r > 30) {
+			r -= 30;
+		}
+	}
+	return d;
+}
+
+bool
+gu_is_prime(int i)
+{
+	if (i < 2) {
+		return false;
+	} else if (i < 30) {
+		return !!(gu_small_prime_mask & (1 << i));
+	} else if (!gu_prime_wheel(i % 30)) {
+		return false;
+	} else {
+		return gu_is_wheel_prime(i);
+	}
+}
+
+bool
+gu_is_twin_prime(int i)
+{
+	return gu_is_prime(i) && gu_is_prime(i - 2);
+}
+
+int
+gu_twin_prime_inf(int i)
+{
+	while (true) {
+		i = gu_prime_inf(i);
+		if (i == 0) {
+			return 0;
+		} else if (gu_is_prime(i - 2)) {
+			return i;
+		}
+		i = i - 4;
+	}
+	return i;
+}
+
+int
+gu_twin_prime_sup(int i)
+{
+	if (i <= 5) {
+		return 5;
+	}
+	i = i - 2;
+	while (true) {
+		i = gu_prime_sup(i);
+		if (gu_is_prime(i + 2)) {
+			return i + 2;
+		}
+		i = i + 4;
+	}
+	return i;
+}
+