Solve Day 3: Lobby

src/bin/03.rs

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+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));
+    }
+}