adventofcode2024/day_07/src/equation.rs

mod operator_set;

use std::collections::HashSet;

use operator_set::{Op, create_permutations};

#[derive(Debug)]
pub struct Equation {
    answer: i64,
    components: Vec<i64>,
}

impl Equation {
    /// Create a new Equation from a string like:
    ///
    /// Answer: components
    /// 190: 10 19
    pub fn new(equation_str: &str) -> Result<Equation, String> {
        // Split answer and components
        let split_equation: Vec<_> = equation_str.split(":").collect();
        if split_equation.len() != 2 {
            return Err(format!("Invalid equation string: {}", equation_str));
        }
        let answer = match split_equation[0].parse() {
            Ok(a) => a,
            Err(_) => {
                return Err(format!(
                    "Error parsing answer {} in equation: {}",
                    split_equation[0], equation_str
                ));
            }
        };
        // Parse components
        let components = split_equation[1]
            .split_whitespace()
            .map(|c| match c.parse::<i64>() {
                Ok(a) => Ok(a),
                Err(_) => Err(format!(
                    "Error parsing component {} in equation: {}",
                    c, equation_str
                )),
            })
            .collect::<Result<Vec<i64>, String>>()?;
        Ok(Equation { answer, components })
    }

    pub fn answer(&self) -> i64 {
        self.answer
    }

    pub fn components(&self) -> &Vec<i64> {
        &self.components
    }

    fn compute_answer(&self, operators: &[Op]) -> i64 {
        let mut accumulator = self.components[0];

        for (i, op) in operators.iter().enumerate() {
            match op {
                Op::Multiply => accumulator *= self.components[i + 1],
                Op::Add => accumulator += self.components[i + 1],
            }
        }

        accumulator
    }

    pub fn solvable_multiply_add(&self) -> bool {
        let op_combinations = create_permutations(
            HashSet::from([Op::Add, Op::Multiply]),
            self.components.len() - 1,
        );

        for combination in op_combinations {
            if self.answer == self.compute_answer(&combination) {
                return true;
            }
        }
        false
    }
}