Countdown

.gitignore

/target

Cargo.lock

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Cargo.toml

[package]
name = "countdown"
version = "0.1.0"
authors = ["Surma <surma@surma.dev>"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
rand = "0.7.3"
itertools = "0.9.0"

[[bin]]
name = "countdown"
path = "main.rs"

main.rs

use std::collections::HashMap;
use std::string::ToString;

use itertools::Itertools;
use rand::prelude::*;

#[derive(Clone, Debug)]
enum Op {
    Plus,
    Minus,
    Mul,
    Div,
}

impl ToString for Op {
    fn to_string(&self) -> String {
        match self {
            Op::Plus => "+".to_string(),
            Op::Minus => "-".to_string(),
            Op::Mul => "*".to_string(),
            Op::Div => "/".to_string(),
        }
    }
}

#[derive(Clone, Debug)]
enum Expr {
    Number(isize),
    Expr(Op, Box<Expr>, Box<Expr>),
}

impl ToString for Expr {
    fn to_string(&self) -> String {
        match self {
            Expr::Number(v) => format!("{}", v),
            Expr::Expr(op, left, right) => format!(
                "({}{}{})",
                left.to_string(),
                op.to_string(),
                right.to_string()
            ),
        }
    }
}

fn all_possible_values_in_order(numbers: &[isize]) -> HashMap<isize, Expr> {
    let mut result = HashMap::new();
    if numbers.len() == 1 {
        let number = numbers[0];
        result.insert(number, Expr::Number(number));
        return result;
    }

    for i in 1..numbers.len() {
        let (left, right) = numbers.split_at(i);
        let left_values = all_possible_values_in_order(left);
        let right_values = all_possible_values_in_order(right);
        for (left_value, left_expr) in &left_values {
            for (right_value, right_expr) in &right_values {
                result.insert(
                    left_value + right_value,
                    Expr::Expr(
                        Op::Plus,
                        left_expr.clone().into(),
                        right_expr.clone().into(),
                    ),
                );
                result.insert(
                    left_value - right_value,
                    Expr::Expr(
                        Op::Minus,
                        left_expr.clone().into(),
                        right_expr.clone().into(),
                    ),
                );
                result.insert(
                    left_value * right_value,
                    Expr::Expr(Op::Mul, left_expr.clone().into(), right_expr.clone().into()),
                );
                if *right_value != 0 && left_value % right_value == 0 {
                    result.insert(
                        left_value / right_value,
                        Expr::Expr(Op::Div, left_expr.clone().into(), right_expr.clone().into()),
                    );
                }
            }
        }
    }
    result
}

fn all_possible_values(numbers: &[isize]) -> HashMap<isize, Expr> {
    let mut result = HashMap::new();
    for permutation in numbers.iter().copied().permutations(numbers.len()) {
        let values = all_possible_values_in_order(&permutation);
        for (value, expr) in &values {
            result.insert(*value, expr.clone());
        }
    }
    result
}

fn main() {
    let mut rng = thread_rng();
    let numbers = vec![6, 9, 25, 50, 75, 100];
    let goal: isize = rng.gen_range(101, 990);
    println!("Number: {:?}", numbers);
    println!("Goal: {}", goal);
    let values = all_possible_values(&numbers);
    if values.contains_key(&goal) {
        println!("Solved: {}", values.get(&goal).unwrap().to_string());
    } else {
        println!("Impossible");
    }
}