如何以高效的方式绕过 "cannot borrow `*self` as mutable more than once at a time" 以进行基于代理的模拟？

Question

我想在 Rust 中实现基于代理的模拟，但我运行进入了借用检查器。

代理应该生活在一个可变的网格中，在每个单元格中携带一个状态。每个代理都带有一些可变状态。通常，我会实现一组代理，例如作为从网格位置到代理的 HashMap。在模拟步骤中，我将遍历所有代理，然后根据代理自身的状态、该位置的网格状态以及附近其他代理的状态更新代理的状态。

虚构的示例可能如下所示：

use std::collections::HashMap;

struct Agent { // each agent carries some state
    id: i64,
    state: i32,
}

struct CellState { // some state of a grid cell
    state: i64,
}

struct Chart {
    agents: HashMap<usize, Agent>,
    grid: Vec<CellState>,
}

impl Chart {
    fn new(size: usize) -> Chart {
        let mut agents = HashMap::new(); // generate hash and populate with 2 agents
        agents.insert(10, Agent { id: 1, state: 1 });
        agents.insert(11, Agent { id: 2, state: 0 });
        let mut grid: Vec<CellState> = Vec::with_capacity(size);

        Chart {
            agents: agents,
            grid: grid,
        }
    }

    fn do_stuff(&mut self, agent: &mut Agent) {
        // here we want to update the state of agent,
        // based on the state of other agents in the grid
    }

    fn step_agents(&mut self) {
        for (_, agent) in &mut self.agents {
            self.do_stuff(agent);
        }
    }
}

fn main() {
    let mut ch = Chart::new(128);
    ch.step_agents();
}

此代码产生错误

error[E0499]: cannot borrow `*self` as mutable more than once at a time
  --> src/main.rs:37:13
   |
36 |         for (_, agent) in &mut self.agents {
   |                           ----------------
   |                           |
   |                           first mutable borrow occurs here
   |                           first borrow later used here
37 |             self.do_stuff(agent);
   |             ^^^^ second mutable borrow occurs here

我了解错误以及 Rust 编译器出现问题的原因。我不明白的是如何以高效的方式规避这个问题。

如果我不可变地借用对代理的引用，我就不能更新它的状态。在一个真实的例子中，代理会携带相当多的状态，因此克隆并不便宜。

实现这个的惯用 Rust 方法是什么？

Answer 1

拥有多个独占引用，在本例中指向同一个 HashMap，确实违反了借用检查器的约束。

鉴于 Agent 显然复制起来并不便宜，我想你可能想研究一下用 std::cell::RefCell 包装 Agent 以动态借用这些值。

这是一个简单的例子：

use std::cell::RefCell;
use std::collections::HashMap;

#[derive(Debug, PartialEq)]
struct Agent {
    id: i64,
    state: i32,
}

struct CellState {
    state: i64,
}

struct Chart {
    agents: HashMap<usize, RefCell<Agent>>,
    grid: Vec<CellState>,
}

impl Chart {
    fn new(size: usize) -> Self {
        let mut agents = HashMap::new();
        agents.insert(1, RefCell::new(Agent { id: 1, state: 0 }));
        agents.insert(2, RefCell::new(Agent { id: 2, state: 1 }));

        let mut grid: Vec<CellState> = Vec::with_capacity(size);

        Self { agents, grid }
    }

    fn do_stuff(&self, agent: &RefCell<Agent>) {
        for other in self.agents.values().filter(|&other| agent != other) {
            if other.borrow().state == 1 {
                agent.borrow_mut().state += 1;
            }
        }
    }

    fn step_agents(&self) {
        for agent in self.agents.values() {
            self.do_stuff(agent);
        }
    }
}

fn main() {
    let mut chart = Chart::new(128);
    chart.step_agents();
    
    for agent in chart.agents.values() {
        println!("{:?}", agent);
    }
}

由于 HashMap 以任意顺序访问，上面的可以然后 return:

RefCell { value: Agent { id: 1, state: 1 } }
RefCell { value: Agent { id: 2, state: 1 } }

如何以高效的方式绕过 "cannot borrow `*self` as mutable more than once at a time" 以进行基于代理的模拟？

How to circumvent "cannot borrow `*self` as mutable more than once at a time" in a performant way for agent based simulation?

rust

borrow-checker