add prime on host project

2 files changed, 40 insertions, 0 deletions

diff --git a/prime/Cargo.toml b/prime/Cargo.toml
new file mode 100644
index 0000000..799d87b
--- /dev/null
+++ b/prime/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "prime"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/prime/src/main.rs b/prime/src/main.rs
new file mode 100644
index 0000000..51b84bd
--- /dev/null
+++ b/prime/src/main.rs
@@ -0,0 +1,34 @@
+//! # Host Prime Seach
+//!
+//! Compute the next smaller prime
+//!
+
+use std::time::Instant;
+
+fn is_prime (a : u128) -> bool {
+    if a % 2 == 0 {return false};
+    for i in (3..a.isqrt()).step_by(2) {
+        if a % i == 0 {
+            return false;
+        }
+    }
+    return true;
+}
+
+fn find_next_smaller_prime(mut x : u128) -> u128 {
+    if x <= 1 {return x};
+    if x % 2 == 0 {x=x-1};
+    while !is_prime(x) {x = x - 2};
+    return x;
+}
+
+fn main() {
+    let n = 1<<56;
+    println!("What is the next prime number smaller than {}?", n);
+    let now = Instant::now();
+    let p = find_next_smaller_prime(n);
+    let duration = now.elapsed();
+    println!("Found: {}. This took {:?}.", p, duration);
+}
+
+// End of file
\ No newline at end of file