too-many-lists/unsafe-queue/layout2

[giscus[bot]]

too-many-lists/unsafe-queue/layout2

https://course.rs/too-many-lists/unsafe-queue/layout2.html

[compdzwio]

数据布局1的解决方案，其中IterMut的实现很丑陋以至于用数据布局2的方案会简单得多。

use std::ptr;

pub struct List<T> {
    head: Link<T>,
    tail: *mut Node<T>
}

type Link<T> = Option<Box<Node<T>>>;

struct Node<T> {
    elem: T,
    next: Link<T>
}


pub struct IntoIter<T>(List<T>);

pub struct Iter<'a, T> {
    next: Option<&'a Node<T>>,
}

pub struct IterMut<'a, T> {
    next: Option<&'a mut Node<T>>,
    tail: *mut Node<T>
}

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None, tail: ptr::null_mut() }
    }

    pub fn push(&mut self, elem: T) {
        let mut new_tail = Some(Box::new(Node {
            elem: elem,
            next: None,
        }));

        let raw_tail: *mut _ = &mut * new_tail.as_deref_mut().unwrap();

        if !self.tail.is_null() {
            unsafe { (*self.tail).next = new_tail; }
        } else {
            self.head = new_tail;
        }
        self.tail = raw_tail;
    }

    pub fn pop(&mut self) -> Option<T> {
        self.head.take().map(|head| {
            let head = *head;
            self.head = head.next;

            if self.head.is_none() {
                self.tail = ptr::null_mut();
            }
            head.elem
        })
    }

    pub fn peek(&self) -> Option<&T> {
        self.head.as_ref().map(|node| &node.elem)
    }

    pub fn peek_mut(&mut self) -> Option<&mut T> {
        self.head.as_mut().map(|node| &mut node.elem)
    }

    pub fn into_iter(self) -> IntoIter<T> {
        IntoIter(self)
    }

    pub fn iter(&self) -> Iter<'_, T> {
        Iter { next: self.head.as_deref() }
    }

    pub fn iter_mut(&mut self) -> IterMut<'_, T> {
        IterMut { next: self.head.as_deref_mut(), tail: self.tail }
    }
}

impl<T> Drop for List<T> {
    fn drop(&mut self) {
        while let Some(_) = self.pop() { }
    }
}

impl<T> Iterator for IntoIter<T> {
    type Item = T;
    fn next(&mut self) -> Option<Self::Item> {
        self.0.pop()
    }
}

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
        })
    }
}

impl<'a, T> Iterator for IterMut<'a, T> {
    type Item = &'a mut T;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.take().map(|node| {
            if let Some(_) = node.next {  // 要不是这个实现很恶心就可以用数据布局1的方案了。
                if node.next.as_deref().unwrap() as * const Node<T> == self.tail {
                    unsafe { self.next = Some(&mut (*self.tail)); }
                } else {
                    self.next = node.next.as_deref_mut();
                }
            }
            &mut node.elem
        })
    }
}

#[cfg(test)]
mod test {
    use super::List;
    #[test]
    fn basics() {
        let mut list = List::new();

        // Check empty list behaves right
        assert_eq!(list.pop(), None);

        // Populate list
        list.push(1);
        list.push(2);
        list.push(3);

        // Check normal removal
        assert_eq!(list.pop(), Some(1));
        assert_eq!(list.pop(), Some(2));

        // Push some more just to make sure nothing's corrupted
        list.push(4);
        list.push(5);

        // Check normal removal
        assert_eq!(list.pop(), Some(3));
        assert_eq!(list.pop(), Some(4));

        // Check exhaustion
        assert_eq!(list.pop(), Some(5));
        assert_eq!(list.pop(), None);

        // Check the exhaustion case fixed the pointer right
        list.push(6);
        list.push(7);

        // Check normal removal
        assert_eq!(list.pop(), Some(6));
        assert_eq!(list.pop(), Some(7));
        assert_eq!(list.pop(), None);
    }

    #[test]
    fn into_iter() {
        let mut list = List::new();
        list.push(1); list.push(2); list.push(3);

        let mut iter = list.into_iter();
        assert_eq!(iter.next(), Some(1));
        assert_eq!(iter.next(), Some(2));
        assert_eq!(iter.next(), Some(3));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn iter() {
        let mut list = List::new();
        list.push(1); list.push(2); list.push(3);

        let mut iter = list.iter();
        assert_eq!(iter.next(), Some(&1));
        assert_eq!(iter.next(), Some(&2));
        assert_eq!(iter.next(), Some(&3));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn iter_mut() {
        let mut list = List::new();
        list.push(1); list.push(2); list.push(3);

        let mut iter = list.iter_mut();
        assert_eq!(iter.next(), Some(&mut 1));
        assert_eq!(iter.next(), Some(&mut 2));
        assert_eq!(iter.next(), Some(&mut 3));
        assert_eq!(iter.next(), None);
    }

    #[test]
    fn miri_food() {
        let mut list = List::new();

        list.push(1);
        list.push(2);
        list.push(3);

        assert!(list.pop() == Some(1));
        list.push(4);
        assert!(list.pop() == Some(2));
        list.push(5);

        assert!(list.peek() == Some(&3));
        list.push(6);
        list.peek_mut().map(|x| *x *= 10);
        assert!(list.peek() == Some(&30));
        assert!(list.pop() == Some(30));

        for elem in list.iter_mut() {
            *elem *= 100;
        }

        let mut iter = list.iter();
        assert_eq!(iter.next(), Some(&400));
        assert_eq!(iter.next(), Some(&500));
        assert_eq!(iter.next(), Some(&600));
        assert_eq!(iter.next(), None);
        assert_eq!(iter.next(), None);

        assert!(list.pop() == Some(400));
        list.peek_mut().map(|x| *x *= 10);
        assert!(list.peek() == Some(&5000));
        list.push(7);

        // Drop it on the ground and let the dtor exercise itself
    }
}

[licq]

pub struct List {
head: Link,
tail: *mut Node,
}

type Link = *mut Node; // 嘀，新的好人卡，请查收

在这段代码里，head和tail类型已经相同了，完全可以都用Link来作为类型，也更一致一些。

[meloalright]

确实