2.rs

/*
Problem 2 - Even Fibonacci numbers

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.
*/

use rayon::prelude::*;

fn fibonacci(upper_bound: usize) -> Vec<usize> {
    // F_0 = 1, F_1 = 2
    let mut sequence = vec![1, 2];

    // F_n = F_(n - 1) + F_(n - 2)
    while sequence[sequence.len() - 1] < upper_bound {
        let new_value = sequence[sequence.len() - 1] + sequence[sequence.len() - 2];
        sequence.push(new_value);
    }

    return sequence;
}

pub fn main() {
    let sequence = fibonacci(4000000);
    let even = sequence.par_iter().filter(|n| *n % 2 == 0);
    let sum: usize = even.sum();

    print!(
        "The sum of the first 4000000 even fibonacci numbers is {}",
        sum
    );
}