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//! # 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
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