extern crate separator;

use separator::Separatable;

use std::io;

///! Calculate a number in the fibonnacci sequence,
///! using a lookup table for better worst-case performance.
///
/// Can calculate up to 186 using native unsigned 128 bit integers.
fn fibonacci(n: usize, table: &mut Vec<u128>) -> u128 {
    match table.get(n) {
        Some(x) => *x, // Vec<T>.get returns a Option with a reference to the value, so deref
        None => {
            let a = n - 1;
            let b = n - 2;

            let current = fibonacci(a, table) + fibonacci(b, table);

            table.insert(n, current);

            current
        }
    }
}

fn main() {
    // The lookup table for previously calculated values
    let mut table: Vec<u128> = vec![0, 1, 1, 2, 3, 5];

    println!("Fibonacci calculator.");
    println!("Any non-number will exit.");

    loop {
        println!("\nWhich index in the sequence to calculate?");

        let mut index = String::new();

        io::stdin().read_line(&mut index)
            .expect("Failed to read line");

        let index = match index.trim().parse() {
            Ok(num) => num,
            Err(_) => break, // Exit on non-number
        };

        let calculated = fibonacci(index, &mut table);

        println!("Calculating number for index: {}", index);
        println!("The Fibonnacci number at index: {} is {}", index, calculated.separated_string());
    }
}