use std::fmt::Display;

fn main() {
    // {
    //     let r;
    //     {
    //         let x = 5;
    //         r = &x; // 报错
    //     }
    //     // 当使用r的时候，r引用的x已经离开作用域失效
    //     println!("r: {}", r);
    // }

    // 修改方案，被引用的x必须生命周期比引用者r长
    let x = 5;
    let r = &x;
    println!("r: {}", r);

    fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
        if x.len() > y.len() {
            x
        } else {
            y
        }
    }

    let string1 = String::from("aaaa");
    let string2 = "sss";

    let result = longest(string1.as_str(), string2);

    println!("the longest string is {}", result);

    // 悬垂引用例子
    // fn dailing_reference<'a>() -> &'a str {
    //     let result = String::from("vvv");
    //     result.as_str()
    // }

    // struct生命周期标注
    struct ImportantExcerpt<'a> {
        part: &'a str,
    }

    let novel = String::from("call me ishmael. Some years age ...");

    let first_sentence = novel.split('.').next().expect("Could not found a '.'");

    let i = ImportantExcerpt {
        part: first_sentence,
    };

    // 生命周期的省略
    fn first_word(s: &str) -> &str {
        let bytes = s.as_bytes();
        for (i, &item) in bytes.iter().enumerate() {
            if item == b' ' {
                return &s[0..i];
            }
        }
        &s[..]
    }

    // 泛型参数类型、trait bound、生命周期
    fn longest_with_an_announcement<'a, T>(x: &'a str, y: &'a str, ann: T) -> &'a str
    where
        T: Display,
    {
        println!("Announcement! {}", ann);
        if x.len() > y.len() {
            x
        } else {
            y
        }
    }
}