rust でオイラープロジェクト２

まずは逐次型で書いた場合。

fn create_fibonacci_sequence(n: i32) -> Vec<i32> {
    let mut a = 0;
    let mut b = 1;
    let mut fib = a + b;
    let mut sequence = vec![a, b];

    while fib < n {
        a = b;
        b = fib;
        fib = a + b;
        sequence.push(fib);
    }
    sequence
}

pub fn solver(n: i32) -> i32 {
    create_fibonacci_sequence(n)
        .iter()
        .filter(|&x| x % 2 == 0)
        .sum()
}

つぎに宣言型で書いた場合。

struct Fibonacci {
    a: i32,
    b: i32,
}

impl Fibonacci {
    fn new() -> Self {
        Self { a: 0, b: 1 }
    }
}

impl Iterator for Fibonacci {
    type Item = i32;
    fn next(&mut self) -> Option<Self::Item> {
        let next = self.b + self.a;
        self.a = self.b;
        self.b = next;
        Some(next)
    }
}

pub fn solver(n: i32) -> i32 {
    Fibonacci::new()
        .take_while(|&x| x < n)
        .filter(|&x| x % 2 == 0)
        .sum()
}