From 4c56ab2779a5028811617e6b32ff0a06c7ca4a01 Mon Sep 17 00:00:00 2001
From: ddidderr <ddidderr@paul.network>
Date: Sat, 2 Dec 2023 09:20:20 +0100
Subject: [PATCH] [feature] get primes fast with sieve of Eratosthenes

---
 src/main.rs  | 12 ++++++++++++
 src/sieve.rs | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)
 create mode 100644 src/main.rs
 create mode 100644 src/sieve.rs

diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..0002aff
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,12 @@
+mod sieve;
+
+use sieve::get_primes;
+
+const MAX_SIEVED_PRIMES: usize = 100_000_000;
+
+fn main() {
+    let primes = get_primes(MAX_SIEVED_PRIMES);
+    for prime in primes {
+        println!("{prime}");
+    }
+}
diff --git a/src/sieve.rs b/src/sieve.rs
new file mode 100644
index 0000000..d625d3a
--- /dev/null
+++ b/src/sieve.rs
@@ -0,0 +1,44 @@
+pub fn get_primes(max_nr: usize) -> Vec<u64> {
+    let mut sieve = vec![true; max_nr + 1];
+
+    let mut primes = Vec::new();
+
+    sieve[0] = false;
+    sieve[1] = false;
+
+    let mut nr = 2;
+
+    loop {
+        check_off_multiples_of_nr(nr, &mut sieve);
+
+        if !find_next_prime(&mut nr, &mut sieve) {
+            break;
+        }
+
+        primes.push(nr);
+    }
+
+    primes
+}
+
+fn find_next_prime(nr: &mut u64, sieve: &mut [bool]) -> bool {
+    if let Some((idx, _)) = sieve
+        .iter()
+        .enumerate()
+        .skip(usize::try_from(*nr).unwrap() + 1)
+        .find(|(_, val)| **val)
+    {
+        *nr = idx as u64;
+        return true;
+    }
+
+    false
+}
+
+fn check_off_multiples_of_nr(nr: u64, sieve: &mut [bool]) {
+    sieve
+        .iter_mut()
+        .skip(usize::try_from(nr * 2).unwrap())
+        .step_by(usize::try_from(nr).unwrap())
+        .for_each(|nr| *nr = false);
+}