init

src/main.rs

@@ -0,0 +1,45 @@
+use std::io::{self, Write};
+use num_bigint::BigUint;
+
+fn log_2(x: BigUint) -> u64 {
+    x.bits() - 1
+}
+
+fn is_prime(number: BigUint) -> bool {
+    let a = log_2(number.clone()+1u8);
+    if BigUint::from(2u8).pow(a as u32)-BigUint::from(1u8) != number {
+        eprintln!("only supports numbers of type (2^x)-1");
+        std::process::exit(1);
+    }
+
+    let mut last = BigUint::from(4u8)%number.clone();
+    let two = BigUint::from(2u8);
+    for _i in 2..a {
+        last = (last.clone()*last-two.clone())%number.clone();
+    }
+    if last == BigUint::from(0u8) {
+        true
+    } else {
+        false
+    }
+}
+
+fn main() {
+    let mut buffer = String::new();
+    let stdin = io::stdin();
+    print!("please enter the number to check: ");
+    io::stdout().flush().expect("could not flush");
+    stdin.read_line(&mut buffer).expect("could not read from stdin");
+    buffer = buffer.trim().to_owned();
+    
+    let number = BigUint::parse_bytes(&buffer.as_bytes(), 10).expect("expected number");
+
+    // number = 2^x - 1
+    // x = log2(number + 1)
+
+    if is_prime(number.clone()) {
+        println!("{number} is a prime");
+    } else {
+        println!("{number} is not a prime");
+    }
+}