AdventOfCode2025/src/solutions/day03.rs

use super::Solution;

pub struct Day03 {}

impl Solution for Day03 {
    fn part1(
        &self,
        input: &mut Vec<String>,
    ) -> Result<Box<dyn std::fmt::Display>, Box<dyn std::error::Error>> {
        let battery_banks: Vec<Vec<u8>> = self.collect_banks(input);

        let batteries = 2;
        let mut total: u64 = 0;
        for bank in battery_banks {
            total += self.calculate_joltage(batteries, &bank);
        }

        Ok(Box::new(total))
    }

    fn part2(
        &self,
        input: &mut Vec<String>,
    ) -> Result<Box<dyn std::fmt::Display>, Box<dyn std::error::Error>> {
        let battery_banks: Vec<Vec<u8>> = self.collect_banks(input);

        let batteries = 12;
        let mut total: u64 = 0;
        for bank in battery_banks {
            total += self.calculate_joltage(batteries, &bank);
        }

        Ok(Box::new(total))
    }

    fn get_day(&self) -> u8 {
        3
    }
}

impl Day03 {
    /// Collect battery banks from input
    ///
    /// # Arguments
    /// * `arr` - Vec of strings representing battery banks
    ///
    /// # Returns
    /// * `Vec<Vec<u8>>` - Vector of battery banks as vectors of u8s
    fn collect_banks(&self, arr: &Vec<String>) -> Vec<Vec<u8>> {
        let mut battery_banks: Vec<Vec<u8>> = vec![];

        // Maps Battery banks into vec of u8s
        for bank in arr {
            let n = bank
                .as_str()
                .chars()
                .filter_map(|c| c.to_digit(10))
                .map(|d| d as u8)
                .collect();

            battery_banks.push(n);
        }
        battery_banks
    }

    /// Scan a slice of u8s to find the max int,
    /// (including its location) within the slice
    fn get_max(&self, arr: &[u8]) -> (u8, usize) {
        let mut max: u8 = 0;
        let mut max_idx: usize = 0;
        for (idx, i) in arr.iter().enumerate() {
            if *i > max {
                max = *i;
                max_idx = idx;
            }
        }
        (max, max_idx)
    }

    /// Calculate the joltage given a number of batteries and a bank
    ///
    /// # Arguments
    /// * `num_batteries` - Number of batteries to consider
    /// * `bank` - Slice of u8s representing the battery bank
    ///
    /// # Returns
    /// * `u64` - The calculated joltage
    fn calculate_joltage(&self, num_batteries: u8, bank: &[u8]) -> u64 {
        // Find the highest val in the avilable bank slice
        // leaving enough room for the remaining batteries
        let (max_val, max_idx) = self.get_max(&bank[0..=(bank.len() - (num_batteries as usize))]);

        // Base case: only one battery left to place
        if num_batteries == 1 {
            return max_val as u64;
        }

        // Otherwise, place the battery and recurse
        (max_val as u64) * 10u64.pow((num_batteries - 1) as u32)
            + self.calculate_joltage(num_batteries - 1, &bank[max_idx + 1..bank.len()])
    }
}

/// Test from puzzle input

#[cfg(test)]

mod test {
    use super::*;
    use crate::*;

    #[test]
    fn part1() {
        let challenge = day03::Day03 {};

        //Complete the Challenge
        let answer = challenge
            .part1(
                utils::get_input(challenge.get_day(), utils::InputType::Test1)
                    .unwrap()
                    .as_mut(),
            )
            .unwrap()
            .to_string();

        assert_eq!(answer, "357");
    }

    #[test]
    fn part2() {
        let challenge = day03::Day03 {};

        //Complete the Challenge
        let answer = challenge
            .part2(
                utils::get_input(challenge.get_day(), utils::InputType::Test2)
                    .unwrap()
                    .as_mut(),
            )
            .unwrap()
            .to_string();

        assert_eq!(answer, "3121910778619");
    }
}