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< h1 id = "数据布局和基本操作" > < a class = "header" href = "#数据布局和基本操作" > 数据布局和基本操作< / a > < / h1 >
< p > 对于新的链表来说，最重要的就是我们可以自由地操控列表的尾部( tail )。< / p >
< h2 id = "数据布局" > < a class = "header" href = "#数据布局" > 数据布局< / a > < / h2 >
< p > 例如以下是一个不太常见的持久化列表布局:< / p >
< pre > < code class = "language-shell" > list1 = A -> B -> C -> D
list2 = tail(list1) = B -> C -> D
list3 = push(list2, X) = X -> B -> C -> D
< / code > < / pre >
< p > 如果上面的不够清晰，我们还可以从内存角度来看：< / p >
< pre > < code class = "language-shell" > list1 -> A ---+
|
v
list2 ------> B -> C -> D
^
|
list3 -> X ---+
< / code > < / pre >
< p > 这里大家可能会看出一些端倪：节点 < code > B< / code > 被多个链表所共享，这造成了我们无法通过 < code > Box< / code > 的方式来实现，因为如果使用 < code > Box< / code > ，还存在一个问题，谁来负责清理释放？如果 drop < code > list2< / code > ，那 < code > B< / code > 节点会被清理释放吗？< / p >
< p > 函数式语言或者说其它绝大多数语言，并不存在这个问题，因为 GC 垃圾回收解千愁，但是 Rust 并没有。< / p >
< p > 好在标准库为我们提供了引用计数的数据结构: < code > Rc / Arc< / code > ，引用计数可以被认为是一种简单的 GC， ， ， ！ < / p >
< p > 不过使用 Rc 意味着我们的数据将无法被改变，因为它不具备内部可变性，关于 Rc/Arc 的详细介绍请看< a href = "https://course.rs/advance/smart-pointer/rc-arc.html" > 这里< / a > 。< / p >
< p > 下面，简单的将我们的数据结构通过 < code > Rc< / code > 来实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // in third.rs
use std::rc::Rc;
pub struct List< T> {
head: Link< T> ,
}
type Link< T> = Option< Rc< Node< T> > > ;
struct Node< T> {
elem: T,
next: Link< T> ,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 需要注意的是, < code > Rc< / code > 在 Rust 中并不是一等公民，它没有被包含在 < code > std::prelude< / code > 中，因此我们必须手动引入 < code > use std::rc::Rc< / code > (混得好失败 - , -)< / p >
< h2 id = "基本操作" > < a class = "header" href = "#基本操作" > 基本操作< / a > < / h2 >
< p > 首先，对于 List 的构造器，可以直接复制粘贴:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl< T> List< T> {
pub fn new() -> Self {
List { head: None }
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 而之前的 < code > push< / code > 和 < code > pop< / code > 已无任何意义，因为新链表是不可变的，但我们可以使用功能相似的 < code > prepend< / code > 和 < code > tail< / code > 来返回新的链表。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn prepend(& self, elem: T) -> List< T> {
List { head: Some(Rc::new(Node {
elem: elem,
next: self.head.clone(),
}))}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 大家可能会大惊失色，什么，你竟然用了 < code > clone< / code > ，不是号称高性能链表实现吗？别急，这里其实只是 < code > Rc::clone< / code > ，对于该方法而言，< code > clone< / code > 仅仅是增加引用计数，并不是复制底层的数据。虽然 < code > Rc< / code > 的性能要比 < code > Box< / code > 的引用方式低一点，但是它依然是多所有权前提下最好的解决方式或者说之一。< / p >
< p > 还有一点值得注意， < code > head< / code > 是 < code > Option< Rc< Node< T> > > < / code > 类型，那么为何不先匹配出内部的 < code > Rc< Node< T> > < / code > ，然后再 clone 呢？原因是 < code > Option< / code > 也提供了相应的 API， < / p >
< p > 运行下试试：< / p >
< pre > < code class = "language-shell" > $ cargo build
warning: field is never used: `elem`
--> src/third.rs:10:5
|
10 | elem: T,
| ^^^^^^^
|
= note: #[warn(dead_code)] on by default
warning: field is never used: `next`
--> src/third.rs:11:5
|
11 | next: Link< T> ,
| ^^^^^^^^^^^^^
< / code > < / pre >
< p > 胆战心惊的编译通过(胆战心惊? 日常基本操作，请坐下!)。< / p >
< p > 继续来实现 < code > tail< / code > ，该方法会将现有链表的首个元素移除，并返回剩余的链表:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn tail(& self) -> List< T> {
List { head: self.head.as_ref().map(|node| node.next.clone()) }
}
< span class = "boring" > }< / span > < / code > < / pre >
< pre > < code class = "language-shell" > $ cargo build
error[E0308]: mismatched types
--> src/third.rs:27:22
|
27 | List { head: self.head.as_ref().map(|node| node.next.clone()) }
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `std::rc::Rc`, found enum `std::option::Option`
|
= note: expected type `std::option::Option< std::rc::Rc< _> > `
found type `std::option::Option< std::option::Option< std::rc::Rc< _> > > `
< / code > < / pre >
< p > 看起来这里的 < code > map< / code > 多套了一层 < code > Option< / code > ，可以用 < code > and_then< / code > 替代：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn tail(& self) -> List< T> {
List { head: self.head.as_ref().and_then(|node| node.next.clone()) }
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 顺利通过编译，很棒！最后就是实现 < code > head< / code > 方法，它返回首个元素的引用，跟之前链表的 < code > peek< / code > 方法一样:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn head(& self) -> Option< & T> {
self.head.as_ref().map(|node| & node.elem )
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 好了，至此，新链表的基本操作都已经实现，最后让我们写几个测试用例来看看它们是骡子还是马：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[cfg(test)]
mod test {
use super::List;
#[test]
fn basics() {
let list = List::new();
assert_eq!(list.head(), None);
let list = list.prepend(1).prepend(2).prepend(3);
assert_eq!(list.head(), Some(& 3));
let list = list.tail();
assert_eq!(list.head(), Some(& 2));
let list = list.tail();
assert_eq!(list.head(), Some(& 1));
let list = list.tail();
assert_eq!(list.head(), None);
// Make sure empty tail works
let list = list.tail();
assert_eq!(list.head(), None);
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< pre > < code class = "language-shell" > $ cargo test
Running target/debug/lists-5c71138492ad4b4a
running 5 tests
test first::test::basics ... ok
test second::test::into_iter ... ok
test second::test::basics ... ok
test second::test::iter ... ok
test third::test::basics ... ok
test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured
< / code > < / pre >
< p > 哦对了… 我们好像忘了一个重要特性：对链表的迭代。< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub struct Iter< 'a, T> {
next: Option< & 'a Node< T> > ,
}
impl< T> List< T> {
pub fn iter(& self) -> Iter< '_, T> {
Iter { next: self.head.as_deref() }
}
}
impl< 'a, T> Iterator for Iter< 'a, T> {
type Item = & 'a T;
fn next(& mut self) -> Option< Self::Item> {
self.next.map(|node| {
self.next = node.next.as_deref();
& node.elem
})
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > #[test]
fn iter() {
let list = List::new().prepend(1).prepend(2).prepend(3);
let mut iter = list.iter();
assert_eq!(iter.next(), Some(& 3));
assert_eq!(iter.next(), Some(& 2));
assert_eq!(iter.next(), Some(& 1));
}
< span class = "boring" > }< / span > < / code > < / pre >
< pre > < code class = "language-shell" > $ cargo test
Running target/debug/lists-5c71138492ad4b4a
running 7 tests
test first::test::basics ... ok
test second::test::basics ... ok
test second::test::iter ... ok
test second::test::into_iter ... ok
test second::test::peek ... ok
test third::test::basics ... ok
test third::test::iter ... ok
test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured
< / code > < / pre >
< p > 细心的同学可能会觉得我在凑字数，这不跟之前的链表迭代实现一样一样的嘛？恭喜你答对了 ： ） < / p >
< p > 最后，给大家留个作业，你可以尝试下看能不能实现 < code > IntoIter< / code > 和 < code > IterMut< / code > ，如果实现不了请不要打我，冤有头债有主，都是 < code > Rc< / code > 惹的祸 :(< / p >
< / main >
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