use num_bigint::BigUint;
use num_traits::{One};
use std::io;
/*
This program computes the factorial of numbers greater than 20.
Rust does not include arbitrary‑precision integers in the standard library,
but the num_bigint crate provides BigUint, which grows as needed.
*/
/*
Compute factorial using BigUint.
The algorithm multiplies numbers from 2 to n.
BigUint handles overflow internally and expands automatically.
*/
fn factorial_big(n: u64) -> BigUint {
let mut result: BigUint = One::one();
for i in 2..=n {
result *= i;
}
result
}
/*
Main entry point: read input, compute factorial, print result.
*/
fn main() {
println!("Enter a number greater than 20:");
let mut input = String::new();
io::stdin().read_line(&mut input).expect("Failed to read input");
let n: u64 = input.trim().parse().expect("Please enter a valid integer");
let result = factorial_big(n);
println!("\nFactorial of {} is:\n", n);
println!("{}", result);
}
/*
run:
Enter a number greater than 20: 25
Factorial of 25 is:
15511210043330985984000000
*/
