From 89a7e5134c1a4670e0aee969525b8d2153b54ff9 Mon Sep 17 00:00:00 2001
From: nihonium <nihonium@rape.lol>
Date: Sat, 5 Mar 2022 13:59:53 +0300
Subject: [PATCH] eratosfen

---
 data/eratosfen.c | 92 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)
 create mode 100644 data/eratosfen.c

diff --git a/data/eratosfen.c b/data/eratosfen.c
new file mode 100644
index 0000000..ac02397
--- /dev/null
+++ b/data/eratosfen.c
@@ -0,0 +1,92 @@
+#include <stdio.h>
+#include <stdlib.h>
+#define SIZE 12500000
+
+int check_bit(char x, int pos);
+int count_bits(char x, int max_offset);
+
+int main() {
+	/*
+	Info about every number is stored into BITS of char elements, so we can have 8 * SIZE numbers. Not including numbers divided by 2, they aren't prime by design.
+	Array start: {1, 2, 3, 5, 7 ...} at sieve[0]
+	*/
+	char *sieve = (char*)calloc(SIZE, sizeof(char));
+	int N, i, j, byte, offset, in_offset, in_byte, num, byte_n;
+	int res = 0;
+	scanf("%d", &N);
+	int n = N;
+	if (N % 2) {
+		N = (N + 1) / 2 + 1;
+	}
+	else {
+		N = N / 2 + 1;
+	}
+	if (N % 8) {
+		byte_n = (int)(N / 8) + 1;
+	}
+	else {
+		byte_n = (int)(N / 8);
+	}
+#ifdef _DEBUG
+	printf("byte_n[%d], N[%d]\n", byte_n, N);
+#endif
+	/* Initialization of used numbers by 1 (bits of every char, char is 1 byte) */
+	for (i = 0; i < byte_n; ++i)
+		sieve[i] = 255;
+	
+	/* Starting from 3 */
+	for (i = 2; i * i <= N; ++i) {
+		offset = i % 8;
+		byte = (int) (i / 8);
+#ifdef _DEBUG
+//printf("Byte:%d[offset:%d]\n", byte, offset);
+		printf("Check %d[%d] - %d\n", byte, offset, check_bit(sieve[byte], offset));
+#endif
+		if (check_bit(sieve[byte], offset)) {
+			num = i + (i - 1);
+			for (j = num + 2 * num; j < n; j += 2 * num) {
+#ifdef _DEBUG
+				printf("i[%d], j[%d]. step: %d\n", i, j, 2 * num);
+#endif
+				in_offset = (int)((j + 1) / 2) % 8;
+				in_byte = (int)((int)((j + 1) / 2) / 8);
+				//in_offset = (i + j) % 8;
+				//in_byte = (int) ((i + j) / 8);
+				
+#ifdef _DEBUG
+				printf("Zeroing %d[%d]\n!=!=!\n", in_byte, in_offset);
+#endif
+				/* 128 = 1000000 */
+				sieve[in_byte] = sieve[in_byte] & ~(128 >> in_offset);
+			}
+		}
+	}
+
+
+	for (i = 0; i < byte_n - 1; ++i) {
+#ifdef _DEBUG
+		printf("Counting bits at byte %d, found %d bits\n", i, count_bits(sieve[i], 7));
+#endif
+		res += count_bits(sieve[i], 7);
+	}
+	
+	offset = (N - 1) % 8;
+
+#ifdef _DEBUG
+	printf("Counting bits at byte %d, found %d bits[final]\n", i, count_bits(sieve[i], offset));
+#endif
+	res += count_bits(sieve[i], offset);
+	printf("%d", --res);
+}
+
+int check_bit(char x, int pos) {
+	return (x >> (7 - pos)) & 1;
+}
+
+int count_bits(char x, int max_offset) {
+	int res = 0;
+	for (int i = 0; i <= max_offset; ++i) {
+		res += check_bit(x, i);
+	}
+	return res;
+}