Day 4：自訂迭代器

pub trait Iterator {
    type Item;                                    // 產出元素的型別
    fn next(&mut self) -> Option<Self::Item>;     // 唯一必實作的方法
    // 其他方法（map、filter 等）都是從 next 推導的預設實作
}

struct Counter {
    count: u32,
    max: u32,
}

impl Counter {
    fn new(max: u32) -> Counter {
        Counter { count: 0, max }
    }
}

impl Iterator for Counter {
    type Item = u32;

    fn next(&mut self) -> Option<Self::Item> {
        if self.count < self.max {
            self.count += 1;
            Some(self.count)
        } else {
            None
        }
    }
}

fn main() {
    let counter = Counter::new(5);
    let v: Vec<_> = counter.collect();   // [1, 2, 3, 4, 5]
    println!("{:?}", v);
}

struct Fibonacci {
    a: u64,
    b: u64,
}

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

impl Iterator for Fibonacci {
    type Item = u64;
    fn next(&mut self) -> Option<Self::Item> {
        let current = self.a;
        let next = self.a + self.b;
        self.a = self.b;
        self.b = next;
        Some(current)   // 0, 1, 1, 2, 3, 5...
    }
}

// 1. 用 take(10) 取前 10 個費式數
// 2. 用 sum() 加總前 10 個