Use bitmap instead of array

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)
 			}
 		}
 	}