struct VectorProducer {
    pub pool: Vec<Vec<i32>>,
}

impl VectorProducer {
    fn new() -> VectorProducer {
        VectorProducer{pool: Vec::new()}
    }
    fn get(&mut self) -> Vec<i32> {
        self.pool.pop().unwrap_or(Vec::new())
    }
    fn put(&mut self, v: Vec<i32>) {
        self.pool.push(v);
    }
}

impl Iterator for VectorProducer {
    type Item = Vec<i32>;
    fn next(&mut self) -> Option<Self::Item> {
        let mut buffer = self.get();
        if buffer.len() == 0 {
            for i in 0..10 {
                buffer.push(i);
            }
        }
        Some(buffer)
    }
}

fn main() {
    println!("Hello, world!");
    let mut vp = VectorProducer::new();
    for v in vp {
        println!("{:?}",v);
        vp.put(v);
    }
}