Optimize algorithm.

src/main.rs

@@ -16,7 +16,6 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-use std::collections::VecDeque;
 use std::fs::File;
 use std::io::{BufRead, BufReader};
 use std::path::PathBuf;
@@ -38,24 +37,24 @@ struct Opt {
 	jobs:u64,
 }
 
+const SEGMENT_SIZE:usize = 0x40000000;
+
 fn main() {
 	let opts = Opt::from_args();
 
-	let mut prime_list = VecDeque::<u64>::new();
-	let mut arr = Vec::new();
-	for _ in 0..=opts.num {
-		arr.push(false);
-	}
+	let mut prime_list = Vec::<usize>::new();
+	let mut arr = vec![false; SEGMENT_SIZE];
 
 	if opts.import.is_some() {
 		let in_file = File::open(opts.import.unwrap()).unwrap();
 		let reader = BufReader::new(in_file);
 		for p in reader.lines().into_iter() {
-			prime_list.push_back(p.unwrap().parse().unwrap());
+			prime_list.push(p.unwrap().parse().unwrap());
 		}
 	}
 
-	if opts.num < 2 {
+	let limit = (opts.num * 5) as usize;
+	if opts.num < 2 || limit > SEGMENT_SIZE {
 		eprintln!("Invalid value for num: {}", opts.num);
 		process::exit(1);
 	}
@@ -66,21 +65,28 @@ fn main() {
 		arr[3] = true;
 	}
 
-	for x in 1..=(f64::sqrt(opts.num as f64) as u64 + 1) {
-		for y in 1..=(f64::sqrt(opts.num as f64) as u64 + 1) {
-			let n1 = ((4 * x * x) + (y * y)) as usize;
-			if n1 <= (opts.num as usize) && (n1 % 12 == 1 || n1 % 12 == 5) {
+	let sqrt_of_num = f64::sqrt(opts.num as f64) as u64;
+	for x in 1..=sqrt_of_num {
+		let xx4 = 4 * x * x;
+		let xx3 = 3 * x * x;
+		for y in 1..=sqrt_of_num {
+			let yy = y * y;
+
+			let n1 = (xx4 + yy) as usize;
+			if n1 % 12 == 1 || n1 % 12 == 5 {
 				arr[n1] = !arr[n1];
 			}
 
-			let n2 = ((3 * x * x) + (y * y)) as usize;
-			if n2 <= (opts.num as usize) && n2 % 12 == 7 {
+			let n2 = (xx3 + yy) as usize;
+			if n2 % 12 == 7 {
 				arr[n2] = !arr[n2];
 			}
 
-			let n3 = ((3 * x * x) - (y * y)) as usize;
-			if x > y && n3 <= (opts.num as usize) && n3 % 12 == 11 {
-				arr[n3] = !arr[n3];
+			if x > y {
+				let n3 = (xx3 - yy) as usize;
+				if n3 % 12 == 11 {
+					arr[n3] = !arr[n3];
+				}
 			}
 		}
 	}
@@ -102,14 +108,14 @@ fn main() {
 			if opts.verbose && !opts.test {
 				println!("{}", i);
 			}
-			prime_list.push_back(i as u64);
+			prime_list.push(i);
 		}
 	}
 
 	if !opts.verbose && !opts.test {
-		println!("{}", prime_list.back().unwrap());
+		println!("{}", prime_list.last().unwrap());
 	} else if opts.test {
-		if *prime_list.back().unwrap() == opts.num {
+		if *prime_list.last().unwrap() == (opts.num as usize) {
 			if opts.verbose {
 				println!("{} is prime", opts.num);
 			}