kat.rs

//! Kat Spiers has yet again bullied me into writing code.
//! Today we worked on type-level arithmetic.

use std::ops::{Add, Sub, Mul};

#[derive(Debug, Clone, PartialEq)]
enum Kat {
    Kat(Box<Kat>),
    Hryn,
}

impl Kat {
    fn from_size(size: usize) -> Self {
        match size {
            0 => Kat::Hryn,
            n => Kat::Kat(Kat::from_size(n - 1).into()),
        }
    }
    
    fn inc(self) -> Self {
        Kat::Kat(Box::new(self))
    }
    
    fn dec(self) -> Self {
        match self {
            Kat::Kat(kat) => *kat,
            Kat::Hryn => self,
        }
    }
    
    fn size(&self) -> usize {
        match self {
            Kat::Hryn => 0,
            Kat::Kat(kat) => 1 + kat.size(),
        }
    }
}

impl From<usize> for Kat {
    fn from(size: usize) -> Self {
        Kat::from_size(size)
    }
}

impl Add for Kat {
    type Output = Self;
    
    fn add(self, rhs: Self) -> Self {
        match (self, rhs) {
            (Kat::Hryn, rhs) => rhs,
            (lhs, Kat::Hryn) => lhs,
            (lhs, rhs) => lhs.inc() + rhs.dec(),
        }
    }
}

impl Sub for Kat {
    type Output = Self;
    
    fn sub(self, rhs: Self) -> Self {
        match (self, rhs) {
            (Kat::Hryn, _rhs) => Kat::Hryn,
            (lhs, Kat::Hryn) => lhs,
            (lhs, rhs) => lhs.inc() - rhs.dec(),
        }
    }
}

impl Mul for Kat {
    type Output = Self;
    
    fn mul(self, rhs: Self) -> Self {
        match (self, rhs) {
            (Kat::Hryn, _rhs) => Kat::Hryn,
            (_lhs, Kat::Hryn) => Kat::Hryn,
            (Kat::Kat(inner), rhs) if matches!(*inner, Kat::Hryn) => rhs,
            (lhs, Kat::Kat(inner)) if matches!(*inner, Kat::Hryn)  => lhs,
            (lhs, rhs) => lhs.clone() + lhs * rhs.dec(),
        }
    }
}

fn main() {
    let eight = Kat::from(5) + Kat::from(3);
    let also_eight = Kat::from(6) + Kat::from(2);
    println!("{} = {}", eight.size(), also_eight.size());
    println!("8 = 8? {}", eight == also_eight);
    println!("6 * 2 = {}", (Kat::from(6) * Kat::from(2)).size());
}