Tower of hanoi solver in rust.

hanoi.rs

/***
Tower of Hanoi solver.

https://en.wikipedia.org/wiki/Tower_of_Hanoi

Example :

```
$ rustc hanoi.rs
$ ./hanoi 3
║─  ║   ║   ║
║── ║   ║   ║
║───║   ║   ║

║   ║   ║   ║
║── ║   ║   ║
║───║   ║─  ║

║   ║   ║   ║
║   ║   ║   ║
║───║── ║─  ║

║   ║   ║   ║
║   ║─  ║   ║
║───║── ║   ║

║   ║   ║   ║
║   ║─  ║   ║
║   ║── ║───║

║   ║   ║   ║
║   ║   ║   ║
║─  ║── ║───║

║   ║   ║   ║
║   ║   ║── ║
║─  ║   ║───║

║   ║   ║─  ║
║   ║   ║── ║
║   ║   ║───║
```

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=f265350ecd5aa7511235e2d122695725

***/

use std::fmt::{Display, Formatter};

#[derive(Debug)]
struct Hanoi {
    disks: [Vec<u32>; 3],
    size: u32
}

impl Hanoi {
    pub fn new(num_disks: u32) -> Self {
        Hanoi {
            size: num_disks,
            disks: [
                (1..=num_disks).rev().collect(),
                vec![],
                vec![]
            ]
        }
    }
    
    fn move_disk(&mut self, from: usize, to: usize){
        let d = self.disks[from].pop().unwrap();
        self.disks[to].push(d);
        println!("{}", self);
    }
    
    fn solve_at(&mut self, n: u32, from: usize, middle: usize, to: usize){
        if n>0 {
            self.solve_at(n-1, from, to, middle);
            self.move_disk(from, to);
            self.solve_at(n-1, middle, from, to);
        }
    }
    
    pub fn solve(&mut self) {
        println!("{}", self);
        self.solve_at(self.size, 0, 1, 2);
    }
}

impl Display for Hanoi {
    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
        let s = self.size as usize;
        for i in (0..s).rev() {
            for j in 0..(3*s) {
                if j%s==0 {write!(f, "║")?;}
                write!(f, "{}",
                    match self.disks[j/s].get(i) {
                        Some(&n) if (n as usize)>(j%s) => '─',
                        _                              => ' '
                    }
                )?;
            }
            write!(f, "║\n")?;
        }
        Ok(())
    }
}

fn main(){
    let n = std::env::args().nth(1).and_then(|s| s.parse().ok()).unwrap_or(4);
    Hanoi::new(n).solve();
}