struct Cacher<T>
where
    T: Fn(u32) -> u32,
{
    callculation: T,
    value: Option<u32>,
}

impl<T> Cacher<T>
where
    T: Fn(u32) -> u32,
{
    fn new(callculation: T) -> Cacher<T> {
        Cacher {
            callculation,
            value: None,
        }
    }

    fn value(&mut self, arg: u32) -> u32 {
        match self.value {
            Some(v) => v,
            None => {
                let v = (self.callculation)(arg);
                self.value = Some(v);
                v
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::Cacher;

    #[test]
    fn call_with_diffrernt_values() {
        let mut c = Cacher::new(|a| a);
        let v1 = c.value(1); // 此时闭包结果已经为1
        let v2 = c.value(2); // 再输入2返回的闭包结果依旧为2

        assert_eq!(v2, 2); // 测试不通过
    }
}