CON-3623 Correct and improve previousprime (#171)

* fix(previousprime): improve solution and subject

* feat(rust): bump rust to 1.89

---------

Co-authored-by: Pedro Ferreira <pedro.ferreira@01talent.com>

Author: pedrodesu

Dockerfile

@@ -1,6 +1,6 @@
 ### Test Image specific config
 
-FROM rust:1.86-slim-bookworm
+FROM rust:1.89-slim-bookworm
 
 RUN apt-get update
 RUN apt-get -y install pkg-config libssl-dev moreutils

solutions/previousprime/Cargo.toml

@@ -2,7 +2,7 @@
 name = "previousprime"
 version = "0.1.0"
 authors = ["OGordoo <marcosafonso8@hotmail.com>"]
-edition = "2021"
+edition = "2024"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 

solutions/previousprime/src/lib.rs

@@ -1,33 +1,33 @@
-pub fn prev_prime(nbr: u64) -> u64 {
-    if nbr <= 2 {
-        return 0;
-    }
-
-    let mut min_number = 2;
+use std::iter;
 
-    for i in 2..nbr {
-        if is_prime(i) {
-            min_number = i;
-        }
+#[inline]
+pub fn prev_prime(nbr: usize) -> usize {
+    if nbr > 2 {
+        iter::once(2)
+            .chain((3..nbr).step_by(2))
+            .rfind(|&n| is_prime(n))
+            .unwrap_or_default()
+    } else {
+        Default::default()
     }
-
-    min_number
 }
 
-fn is_prime(nbr: u64) -> bool {
-    if nbr > 0 {
-        if nbr <= 1 {
-            return false;
-        }
-        let mut d = 2;
-        while d * d <= nbr {
-            if nbr % d == 0 {
-                return false;
-            }
-            d += 1;
-        }
+fn is_prime(n: usize) -> bool {
+    if n < 2 {
+        return false;
+    }
+    if n <= 3 {
         return true;
-    } else {
+    }
+    if n.is_multiple_of(2) || n.is_multiple_of(3) {
         return false;
     }
+    let mut d = 5;
+    while d * d <= n {
+        if n.is_multiple_of(d) || n.is_multiple_of(d + 2) {
+            return false;
+        }
+        d += 6;
+    }
+    true
 }

tests/previousprime_test/Cargo.toml

@@ -2,9 +2,9 @@
 name = "previousprime_test"
 version = "0.1.0"
 authors = ["OGordoo <marcosafonso8@hotmail.com>"]
-edition = "2021"
+edition = "2024"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-previousprime = {path = "../../solutions/previousprime"}
+previousprime = {path = "../../solutions/previousprime"}

tests/previousprime_test/src/lib.rs

@@ -0,0 +1,12 @@
+use previousprime::*;
+
+#[test]
+fn prev_prime_test() {
+    assert_eq!(0, prev_prime(0));
+    assert_eq!(0, prev_prime(2));
+    assert_eq!(2, prev_prime(3));
+    assert_eq!(3, prev_prime(5));
+    assert_eq!(31, prev_prime(34));
+    assert_eq!(631, prev_prime(633));
+    assert_eq!(478139, prev_prime(478152));
+}