Rust: Rust Smart Pointers

use std::rc::Rc;
use std::cell::RefCell;

fn main() {
    // Box: One person has exclusive access to a document，Destroy after reading
    let doc_box = Box::new(String::from("Project Proposal v1"));
    println!("Box Hold: {}", doc_box);
    // doc_box Automatically destroyed upon leaving the scope——No manual intervention required free

    // Rc: Shared Read-Only Access for Multiple Users
    let doc_rc = Rc::new(String::from("Shared Project Proposal"));
    let alice = Rc::clone(&doc_rc);
    let bob = Rc::clone(&doc_rc);
    println!("Reference Count: {}", Rc::strong_count(&doc_rc)); // 3

    // Rc<RefCell<T>>: Share + Variable
    let doc_shared = Rc::new(RefCell::new(String::from("Collaborative Documents")));
    let alice_view = Rc::clone(&doc_shared);
    let bob_view = Rc::clone(&doc_shared);

    // Bob Add annotations to a document（Needs to be revised）
    bob_view.borrow_mut().push_str("\nBob Comments on：Part 2 needs to be completed");
    // Alice View the documentation（Read-only）
    println!("Alice See: {}", alice_view.borrow());
    // Carol See the documentation as well（Read-only）
    println!("Carol See: {}", doc_shared.borrow());
}

MyBox It was destroyed.～
Box the value in: <b>
Recursive List: <list>
*y == <*y>
Greeting: <name>

// ============================================
// Box<T> Three Typical Uses of：Heap Allocation、Recursive Types、Deref
// ============================================

// Recursive Types：Cons list（Must use Box，Because the size is unknown at compile time）
#[derive(Debug)]
enum List {
    Cons(i32, Box<List>),
    Nil,
}

use List::{Cons, Nil};

// Custom Smart Pointers (Demo Deref and Drop)
use std::ops::Deref;

struct MyBox<T>(T);

impl<T> MyBox<T> {
    fn new(x: T) -> MyBox<T> {
        MyBox(x)
    }
}

impl<T> Deref for MyBox<T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.0  // A reference to the internal value
    }
}

impl<T> Drop for MyBox<T> {
    fn drop(&mut self) {
        // In practical applications, resources are released here（For example, closing a file、Release the network connection）
        // println!("MyBox It was destroyed.～");
    }
}

fn main() {
    // --- 1. Box Basic Heap Allocation ---
    let b = Box::new(42);
    println!("Box the value in: {}", b);  // Automatic Dereferencing

    // --- 2. Recursive Types：Cons List ---
    let list = Cons(1, Box::new(Cons(2, Box::new(Cons(3, Box::new(Nil))))));
    println!("Recursive List: {:?}", list);

    // --- 3. Custom MyBox + Deref ---
    let my_box = MyBox::new(String::from("Hello"));
    // Deref lets &MyBox<String> automatically convert to &String, then to &str
    greet(&my_box);

    // --- 4. Dereference Operator ---
    let x = 10;
    let y = MyBox::new(x);
    assert_eq!(10, *y);  // *y Equivalent to *(y.deref())
    println!("*y == {}", *y);
}

fn greet(name: &str) {
    println!("Greeting: {}", name);
}

Box the value in: 42
Recursive List: Cons(1, Cons(2, Cons(3, Nil)))
Greeting: Hello
*y == 10

Initial reference count: <Rc::strong_count(&book_c)>
Alice Reference Count After Borrowing: <Rc::strong_count(&book_c)>
Bob Reference Count After Borrowing: <Rc::strong_count(&book_c)>
Alice Book List: <*alice>
Bob Book List: <*bob>
Bob Citation Count After Returning the Book: <Rc::strong_count(&book_c)>
Alice Still reading: <*alice>

// ============================================
// Rc<T> Reference Count：Multipath Sharing of Read-Only Data
// ============================================

use std::rc::Rc;

#[derive(Debug)]
enum BookList {
    Cons(String, Rc<BookList>),
    Nil,
}

use BookList::{Cons, Nil};

fn main() {
    // --- Scene：Three books are shared by two readers ---

    // Create a Basic Reading List
    let book_c = Rc::new(Cons("Rust Programming".to_string(),
        Rc::new(Cons("Introduction to Algorithms".to_string(),
            Rc::new(Cons("Design Patterns".to_string(),
                Rc::new(Nil))))));

    println!("Initial reference count: {}", Rc::strong_count(&book_c));

    // Alice Borrowed the reading list（Rc::clone Increase the reference count only，Do not deep-copy data）
    let alice = Rc::clone(&book_c);
    println!("Alice Reference Count After Borrowing: {}", Rc::strong_count(&book_c));

    {
        // Bob I also borrowed the reading list（In the inner scope）
        let bob = Rc::clone(&book_c);
        println!("Bob Reference Count After Borrowing: {}", Rc::strong_count(&book_c));

        // Both of them can read it.
        println!("Alice Book List: {:?}", *alice);
        println!("Bob Book List: {:?}", *bob);
    }  // Bob Out of scope，Decrease in reference count

    println!("Bob Citation Count After Returning the Book: {}", Rc::strong_count(&book_c));

    // Alice Still reading
    println!("Alice Still reading: {:?}", *alice);

    // All Rc After they have all gone out of scope，Only then is the data truly destroyed
    // Rc::strong_count == 0 When triggered drop
}

Initial reference count: 1
Alice Reference Count After Borrowing: 2
Bob Reference Count After Borrowing: 3
Alice Book List: Cons("Rust Programming", Cons("Introduction to Algorithms", Cons("Design Patterns", Nil)))
Bob Book List: Cons("Rust Programming", Cons("Introduction to Algorithms", Cons("Design Patterns", Nil)))
Bob Citation Count After Returning the Book: 2
Alice Still reading: Cons("Rust Programming", Cons("Introduction to Algorithms", Cons("Design Patterns", Nil)))

Log #<i + 1>: <msg>
<ref1[0]>
RefCell The presentation is over.

// ============================================
// RefCell<T>：Runtime Borrowing Checks + Internal Variability
// ============================================

use std::cell::RefCell;

// Simulate a "Log Entry" Messenger
// External code can only obtain immutable references, but the log needs to be modified internally.
trait Messenger {
    fn send(&self, msg: &str);
}

// Logger：For internal use only RefCell Store Messages
struct Logger {
    // Even if Logger Inherently immutable，messages It can still be edited
    messages: RefCell<Vec<String>>,
}

impl Logger {
    fn new() -> Logger {
        Logger {
            messages: RefCell::new(Vec::new()),
        }
    }
}

impl Messenger for Logger {
    fn send(&self, msg: &str) {
        // borrow_mut() gets a mutable reference — even if self is &self
        self.messages.borrow_mut().push(msg.to_string());
    }
}

fn main() {
    let logger = Logger::new();

    // Calling via an immutable reference send——But it has actually been modified internally.
    logger.send("User Login");
    logger.send("Click the button");
    logger.send("Data submitted successfully");

    // borrow() Get an immutable reference
    let msgs = logger.messages.borrow();
    for (i, msg) in msgs.iter().enumerate() {
        println!("Log #{}: {}", i + 1, msg);
    }

    // --- Examples of Runtime Borrowing Violations（Cancel the Commentary Session panic）---
    // let mut_ref = logger.messages.borrow_mut();
    // let ref1 = logger.messages.borrow();    // panic! Both mutable and immutable references
    // println!("{}", ref1[0]);

    println!("RefCell The presentation is over.");
}

Log #1: User Login
Log #2: Click the button
Log #3: Data submitted successfully
RefCell The presentation is over.

Current reference count: <Rc::strong_count(&board)>
=== Whiteboard Content ===
<board.read()>
Alice Length of content viewed: <alice_board.read().len()>
Bob Length of content viewed: <bob_board.read().len()>
Carol Length of content viewed: <carol_board.read().len()>

// ============================================
// Rc<RefCell<T>> Combination Mode：Shared Ownership + Internal Variability
// ============================================

use std::rc::Rc;
use std::cell::RefCell;

// Simulate a"Shared Whiteboard"——Team members can write and read
#[derive(Debug)]
struct Whiteboard {
    content: RefCell<String>,
}

impl Whiteboard {
    fn new() -> Whiteboard {
        Whiteboard {
            content: RefCell::new(String::new()),
        }
    }

    fn write(&self, text: &str) {
        let mut content = self.content.borrow_mut();
        content.push_str(text);
        content.push('\n');
    }

    fn read(&self) -> String {
        self.content.borrow().clone()
    }
}

fn main() {
    // Create a shared whiteboard, wrapped with Rc so it can be held by multiple people
    let board = Rc::new(Whiteboard::new());

    // Alice and Bob both hold references to the whiteboard
    let alice_board = Rc::clone(&board);
    let bob_board = Rc::clone(&board);
    let carol_board = Rc::clone(&board);

    println!("Current reference count: {}", Rc::strong_count(&board));  // 4

    // Alice Write on the whiteboard
    alice_board.write("Alice: Today's Discussion Rust Smart Pointers");
    // Bob Additional Information
    bob_board.write("Bob: I'll explain the difference between Box and Rc");
    // Carol Write as well
    carol_board.write("Carol: RefCell Runtime borrowing checks are important");

    // Everyone can view the full content（Because they share the same RefCell）
    println!("=== Whiteboard Content ===");
    println!("{}", board.read());

    // Verify that all references point to the same data
    println!("Alice Length of content viewed: {}", alice_board.read().len());
    println!("Bob Length of content viewed: {}", bob_board.read().len());
    println!("Carol Length of content viewed: {}", carol_board.read().len());

    // board When leaving the scope，Reference Count Reset to Zero，Whiteboard Automatic Destruction
}

Current reference count: 4
=== Whiteboard Content ===
Alice: Today's Discussion Rust Smart Pointers
Bob: I'll explain the difference between Box and Rc
Carol: RefCell Runtime borrowing checks are important

Alice Length of content viewed: 73
Bob Length of content viewed: 73
Carol Length of content viewed: 73

=== Graph Structure ===
<node.borrow()>

=== Nodes Reachable from Starting Point A ===
A's first neighbor: <neighbor.borrow().value>
Take it a step further from that neighbor: <second_hop>

=== Strong reference counting ===
A's Rc reference count: <Rc::strong_count(&a)>
After clone, A's Rc reference count: <Rc::strong_count(&a)>
After drop, A's Rc reference count: <Rc::strong_count(&a)>

// ============================================
// Comprehensive Example：Rc<RefCell<T>> Graph Node Implementation
// ============================================

use std::rc::{Rc, Weak};
use std::cell::RefCell;
use std::fmt;

struct Node {
    value: String,
    neighbors: Vec<Weak<RefCell<Node>>>,
}

impl Node {
    fn new(value: &str) -> Rc<RefCell<Node>> {
        Rc::new(RefCell::new(Node {
            value: value.to_string(),
            neighbors: Vec::new(),
        }))
    }

    fn add_neighbor(node: &Rc<RefCell<Node>>, neighbor: &Rc<RefCell<Node>>) {
        node.borrow_mut().neighbors.push(Rc::downgrade(neighbor));
    }
}

impl fmt::Debug for Node {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let neighbor_names: Vec<String> = self.neighbors.iter()
            .filter_map(|w| w.upgrade())
            .map(|n| n.borrow().value.clone())
            .collect();
        write!(f, "{} -> {:?}", self.value, neighbor_names)
    }
}

fn main() {
    let a = Node::new("A");
    let b = Node::new("B");
    let c = Node::new("C");
    let d = Node::new("D");

    Node::add_neighbor(&a, &b);
    Node::add_neighbor(&a, &c);
    Node::add_neighbor(&b, &c);
    Node::add_neighbor(&b, &d);
    Node::add_neighbor(&c, &d);

    println!("=== Graph Structure ===");
    for node in [&a, &b, &c, &d] {
        println!("{:?}", node.borrow());
    }

    println!("\n=== Nodes Reachable from Starting Point A ===");
    if let Some(neighbor) = a.borrow().neighbors.first().and_then(|w| w.upgrade()) {
        println!("A's first neighbor: {}", neighbor.borrow().value);
        let second_hop: Vec<String> = neighbor.borrow().neighbors.iter()
            .filter_map(|w| w.upgrade())
            .map(|n| n.borrow().value.clone())
            .collect();
        println!("Take it a step further from that neighbor: {:?}", second_hop);
    }

    println!("\n=== Strong reference counting ===");
    println!("A's Rc reference count: {}", Rc::strong_count(&a));
    let a_clone = Rc::clone(&a);
    println!("After clone, A's Rc reference count: {}", Rc::strong_count(&a));
    drop(a_clone);
    println!("After drop, A's Rc reference count: {}", Rc::strong_count(&a));
}

=== Graph Structure ===
A -> ["B", "C"]
B -> ["C", "D"]
C -> ["D"]
D -> []

=== Nodes Reachable from Starting Point A ===
A's first neighbor: B
Take it a step further from that neighbor: ["C", "D"]

=== Strong reference counting ===
A's Rc reference count: 1
After clone, A's Rc reference count: 2
After drop, A's Rc reference count: 1