speed up prime calculations by 50%

add tool to check a prime as cmd arg

src/is_prime.rs

@@ -24,15 +24,20 @@ pub mod prime_utils {
             const ZERO: BigUint = BigUint::ZERO;
             let mut i = BigUint::one();
             let one = BigUint::one();
+            let two = &one + &one;
 
-            let sqrtnum = number.sqrt() + &one; //fake ceil function
+            if number % &two == ZERO {
+                return false;
+            }
+
+            let sqrtnum = number.sqrt() + one; //fake ceil function
 
             loop {
-                i += &one;
+                i += &two; //we begin checking 3 and then continue with every second number as every even number is divisible by 2 and we already checked that
                 if number % &i == ZERO {
                     return false;
                 }
-                if i == sqrtnum {
+                if i >= sqrtnum {
                     return true;
                 }
             }

src/main.rs

@@ -8,6 +8,7 @@ use crate::is_prime::prime_utils;
 use num::{BigUint, One};
 use once_cell::sync::Lazy;
 use std::{
+    env::args,
     io::Write,
     sync::{
         atomic::{AtomicU64, AtomicUsize},
@@ -47,8 +48,13 @@ fn try_set_thread_priority() {
 }
 
 fn write_primes(primes: &[BigUint]) {
+    let primes: Vec<String> = primes
+        .iter()
+        .map(std::string::ToString::to_string)
+        .collect();
+
     let mut file = std::fs::File::create("primes.json").unwrap();
-    file.write_all(serde_json::to_string(primes).unwrap().as_bytes())
+    file.write_all(serde_json::to_string(&primes).unwrap().as_bytes())
         .unwrap();
 }
 
@@ -90,6 +96,19 @@ fn worker(
 }
 
 fn main() {
+    let mut args = args();
+    if args.len() == 2 {
+        use std::str::FromStr;
+        let prime = args.nth(1).unwrap();
+
+        println!(
+            "Number is prime: {}",
+            is_prime::prime_utils::is_prime(&BigUint::from_str(&prime).unwrap())
+        );
+
+        return;
+    }
+
     let sys_time = SystemTime::now();
 
     let nthprime_approx: usize = to_usize(p(NF * 1.02));
@@ -132,6 +151,7 @@ fn main() {
     });
 
     let mut primes = primearc.write().unwrap();
+    primes.sort_unstable();
 
     #[allow(clippy::cast_possible_wrap)]
     let removed: i32 = if primes.len() > NU {
@@ -144,17 +164,20 @@ fn main() {
 
     let added = NU - primes.len();
     if primes.len() < NU {
-        static INC: once_cell::sync::Lazy<BigUint> = Lazy::new(BigUint::one);
+        static INC: once_cell::sync::Lazy<BigUint> = Lazy::new(|| BigUint::from(2u8));
 
         println!("[INFO] less primes than expected");
         let mut number = BigUint::from(THREADS) * BigUint::from(workloads);
+        if &number % &*INC == BigUint::ZERO {
+            number += BigUint::one();
+        }
         loop {
             number += &*INC;
             if prime_utils::is_prime(&number) {
                 primes.push(number.clone());
-            }
-            if primes.len() == NU {
-                break;
+                if primes.len() == NU {
+                    break;
+                }
             }
         }
     }