[2025] Solution for Day 3

Author: kfarnung

2025/src/bin/day03.rs

@@ -0,0 +1,16 @@
+use aoc2025::days::day03::{part1, part2};
+use std::env;
+use std::fs;
+use std::process;
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    if args.len() < 2 {
+        println!("Missing required arguments");
+        process::exit(1);
+    }
+
+    let contents = fs::read_to_string(&args[1]).expect("Something went wrong");
+    println!("Part 1: {}", part1(&contents));
+    println!("Part 2: {}", part2(&contents));
+}

2025/src/days.rs

@@ -1,2 +1,3 @@
 pub mod day01;
 pub mod day02;
+pub mod day03;

2025/src/days/day03.rs

@@ -0,0 +1,84 @@
+use std::vec;
+
+pub fn part1(contents: &str) -> i64 {
+    calculate_joltage(contents, 2)
+}
+
+pub fn part2(contents: &str) -> i64 {
+    calculate_joltage(contents, 12)
+}
+
+fn calculate_joltage(contents: &str, digits: usize) -> i64 {
+    let mut sum = 0;
+    for l in contents.lines() {
+        let nums: Vec<i64> = l.chars().map(|c| c.to_digit(10).unwrap() as i64).collect();
+        let mut indices = vec![];
+        find_max_recursive(&nums, 0, nums.len(), &mut indices, digits);
+        indices.sort();
+
+        sum += indices
+            .iter()
+            .map(|e| nums[*e])
+            .reduce(|acc, e| acc * 10 + e)
+            .unwrap();
+    }
+    sum
+}
+
+fn find_max_recursive(
+    nums: &[i64],
+    start: usize,
+    end: usize,
+    indices: &mut Vec<usize>,
+    digits: usize,
+) {
+    if start == end || indices.len() == digits {
+        return;
+    }
+
+    let max_index = find_max_index(nums, start, end);
+    indices.push(max_index);
+    find_max_recursive(nums, max_index + 1, end, indices, digits);
+    find_max_recursive(nums, start, max_index, indices, digits);
+}
+
+fn find_max_index(nums: &[i64], start: usize, end: usize) -> usize {
+    let mut max = -1;
+    let mut max_index: usize = 0;
+    for (i, n) in nums[start..end].iter().enumerate() {
+        if *n > max {
+            max = *n;
+            max_index = start + i;
+        }
+    }
+    max_index
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part1() {
+        let input = vec![
+            "987654321111111",
+            "811111111111119",
+            "234234234234278",
+            "818181911112111",
+        ];
+
+        assert_eq!(part1(&input.join("\n")), 357);
+    }
+
+    #[test]
+    fn test_part2() {
+        let input = vec![
+            "987654321111111",
+            "811111111111119",
+            "234234234234278",
+            "818181911112111",
+        ];
+
+        assert_eq!(part2(&input.join("\n")), 3121910778619);
+    }
+}

2025/tests/day03.rs

@@ -0,0 +1,13 @@
+use aoc2025::days::day03::{part1, part2};
+
+#[test]
+fn test_part1() {
+    let content = std::include_str!("../../private/inputs/2025/day03.txt");
+    assert_eq!(part1(&content), 16854);
+}
+
+#[test]
+fn test_part2() {
+    let content = std::include_str!("../../private/inputs/2025/day03.txt");
+    assert_eq!(part2(&content), 167526011932478);
+}