From 5ee76502fad62d40ca07050cd6106303b83f1d51 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nicol=C3=A1s=20Ortega=20Froysa?= <nicolas@ortegas.org>
Date: Wed, 10 Dec 2025 11:46:22 +0100
Subject: [PATCH] Switch to (segmented) sieve of Eratosthenes.

---
 src/main.rs   | 50 ++++++++++++++++++++++++++------------
 src/worker.rs | 67 +++++++++++++++------------------------------------
 2 files changed, 54 insertions(+), 63 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 6c4a903..b1aef50 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -16,9 +16,9 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-//use std::fs::File;
-//use std::io::{BufRead, BufReader};
-//use std::path::PathBuf;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+use std::path::PathBuf;
 use std::process;
 use structopt::StructOpt;
 
@@ -29,8 +29,8 @@ mod worker;
 struct Opt {
 	#[structopt(short, long, help = "Print all found primes")]
 	verbose:bool,
-	//#[structopt(short, long, name = "FILE", help = "Import prime numbers from FILE")]
-	//import:Option<PathBuf>,
+	#[structopt(short, long, name = "FILE", help = "Import prime numbers from FILE")]
+	import:Option<PathBuf>,
 	#[structopt(short, long, help = "Test if num is prime instead of generation")]
 	test:bool,
 	#[structopt(help = "Max of the prime to generate or number to test for primality")]
@@ -39,23 +39,47 @@ struct Opt {
 	//jobs:u64,
 }
 
+const SEGMENT_SIZE:usize = 0x100000000;
+
 fn main() {
 	let opts = Opt::from_args();
+	let mut prime_list = Vec::new();
 
-	/*if opts.import.is_some() {
+	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(p.unwrap().parse().unwrap());
 		}
-	}*/
+	}
 
 	if opts.num < 2 {
 		eprintln!("Invalid value for num: {}", opts.num);
 		process::exit(1);
 	}
 
-	let prime_list = worker::work_segment(0, opts.num);
+	let mut start:usize = if prime_list.is_empty() {
+		2
+	} else {
+		*prime_list.last().unwrap() as usize
+	};
+	while start < opts.num {
+		let end = if start + SEGMENT_SIZE < opts.num {
+			start + SEGMENT_SIZE
+		} else {
+			opts.num + 1
+		};
+		let mut new_primes = worker::work_segment(&prime_list, start, end);
+
+		if opts.verbose {
+			for p in &new_primes {
+				println!("{}", *p);
+			}
+		}
+		prime_list.append(&mut new_primes);
+
+		start += SEGMENT_SIZE;
+	}
 
 	if opts.test {
 		if *prime_list.last().unwrap() == (opts.num as u64) {
@@ -69,13 +93,7 @@ fn main() {
 			}
 			process::exit(1);
 		}
-	} else {
-		if !opts.verbose {
-			println!("{}", prime_list.last().unwrap());
-		} else {
-			for p in prime_list {
-				println!("{}", p);
-			}
-		}
+	} else if !opts.verbose {
+		println!("{}", prime_list.last().unwrap());
 	}
 }
diff --git a/src/worker.rs b/src/worker.rs
index 1cd0420..033925c 100644
--- a/src/worker.rs
+++ b/src/worker.rs
@@ -20,62 +20,35 @@
  * @brief Work on a segment.
  *
  * @param start:usize Beginning of the segment (inclusive).
- * @param end:usize End of the segment (inclusive).
+ * @param end:usize End of the segment (exclusive).
  *
  * @return List of primes found in segment.
  */
-pub fn work_segment(start:usize, end:usize) -> Vec<u64> {
-	let mut found_primes = Vec::<u64>::new();
-	let mut arr = vec![false; end - start + 1];
+pub fn work_segment(known_primes:&Vec<u64>, start:usize, end:usize) -> Vec<u64> {
+	let mut sieve = vec![true; end - start];
+	let mut found_primes = Vec::new();
 
-	if start < 2 && end > 2 {
-		arr[2] = true;
-	}
-	if start < 3 && end > 3 {
-		arr[3] = true;
-	}
-
-	let sqrt_of_num = f64::sqrt(end as f64) as usize;
-	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;
-			if n1 <= end && (n1 % 12 == 1 || n1 % 12 == 5) {
-				arr[n1] = !arr[n1];
-			}
-
-			let n2 = xx3 + yy;
-			if n2 <= end && n2 % 12 == 7 {
-				arr[n2] = !arr[n2];
-			}
-
-			if x > y {
-				let n3 = xx3 - yy;
-				if n3 <= end && n3 % 12 == 11 {
-					arr[n3] = !arr[n3];
-				}
+	for p in known_primes {
+		let prime = *p as usize;
+		let mut mult = prime * prime;
+		while mult < end {
+			if mult > start {
+				sieve[mult - start] = false;
 			}
+			mult += prime;
 		}
 	}
 
-	for i in 5..=sqrt_of_num {
-		if !arr[i] {
-			continue;
-		}
+	for i in 0..(end - start) {
+		if sieve[i] {
+			let prime = i + start;
+			found_primes.push(prime as u64);
 
-		let mut j = i * i;
-		while j <= end {
-			arr[j] = false;
-			j += i * i;
-		}
-	}
-
-	for i in 2..=end {
-		if arr[i] {
-			found_primes.push(i as u64);
+			let mut mult = prime * prime;
+			while mult < end {
+				sieve[mult - start] = false;
+				mult += prime;
+			}
 		}
 	}