advent-of-code-2025-cursor/src/bin/03.rs

advent_of_code::solution!(3);

/// Find the maximum 2-digit number by selecting exactly 2 batteries from a bank
fn max_joltage_2(bank: &[u8]) -> u64 {
    let mut max = 0u64;

    // For each position as the tens digit
    for i in 0..bank.len() - 1 {
        let tens = bank[i] as u64;
        // Find max digit after position i
        let units = *bank[i + 1..].iter().max().unwrap() as u64;
        let joltage = tens * 10 + units;
        max = max.max(joltage);
    }

    max
}

pub fn part_one(input: &str) -> Option<u64> {
    let mut total = 0u64;

    for line in input.lines() {
        let line = line.trim();
        if line.is_empty() {
            continue;
        }

        // Convert line to digits
        let digits: Vec<u8> = line.chars().map(|c| c.to_digit(10).unwrap() as u8).collect();
        total += max_joltage_2(&digits);
    }

    Some(total)
}

/// Find the maximum k-digit number by selecting exactly k batteries from a bank
fn max_joltage_k(bank: &[u8], k: usize) -> u64 {
    let n = bank.len();
    let mut result = 0u64;
    let mut start = 0;

    // Greedily pick each of the k digits
    for remaining in (1..=k).rev() {
        // We can pick from [start, n - remaining] inclusive
        let end = n - remaining;
        
        // Find the FIRST position of the max digit in this range
        let mut best_pos = 0;
        let mut best_digit = bank[start];
        for (i, &d) in bank[start..=end].iter().enumerate() {
            if d > best_digit {
                best_digit = d;
                best_pos = i;
            }
        }
        
        result = result * 10 + best_digit as u64;
        start = start + best_pos + 1;
    }

    result
}

pub fn part_two(input: &str) -> Option<u64> {
    let mut total = 0u64;

    for line in input.lines() {
        let line = line.trim();
        if line.is_empty() {
            continue;
        }

        let digits: Vec<u8> = line.chars().map(|c| c.to_digit(10).unwrap() as u8).collect();
        total += max_joltage_k(&digits, 12);
    }

    Some(total)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_part_one() {
        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
        assert_eq!(result, Some(357));
    }

    #[test]
    fn test_part_two() {
        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
        assert_eq!(result, Some(3121910778619));
    }
}