From af97ce448c44468725ddef9f4ea2356e8dfb9146 Mon Sep 17 00:00:00 2001
From: "Peter J. Holzer" <hjp@hjp.at>
Date: Mon, 12 Jul 2021 21:09:22 +0200
Subject: [PATCH] Use bitmap instead of array

---
 eratosthenes_parallel.go | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/eratosthenes_parallel.go b/eratosthenes_parallel.go
index c57c174..b327807 100644
--- a/eratosthenes_parallel.go
+++ b/eratosthenes_parallel.go
@@ -13,16 +13,16 @@ func main() {
 		panic(err)
 	}
 	t0 := time.Now()
-	sieve := make([]bool, n)
+	sieve := make([]uint64, (n>>7)+1)
 	var found int64
 	var i int64
 	i = 2
 	found++
 	for i = 3; i < n; i += 2 {
-		if !sieve[i] {
+		if (sieve[i>>7] & (1 << ((i & 0x7F) >> 1))) == 0 {
 			found++
 			for j := i * i; j < n; j += 2 * i {
-				sieve[j] = true
+				sieve[j>>7] |= 1 << ((j & 0x7F) >> 1)
 			}
 		}
 	}