[2025] Solution for Day 1

Author: kfarnung

2025/.gitignore

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+# Generated by Cargo
+# will have compiled files and executables
+/target/
+
+# Remove Cargo.lock from gitignore if creating an executable, leave it for libraries
+# More information here https://doc.rust-lang.org/cargo/guide/cargo-toml-vs-cargo-lock.html
+Cargo.lock
+
+# These are backup files generated by rustfmt
+**/*.rs.bk

2025/Cargo.toml

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+[package]
+name = "aoc2025"
+version = "0.1.0"
+authors = ["Kyle Farnung <[email protected]>"]
+edition = "2021"
+
+[dependencies]
+regex = "1"

2025/README.md

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+# Advent of Code 2025
+
+https://adventofcode.com/2025
+
+## Language
+
+Implemented in [Rust](https://www.rust-lang.org/).
+
+## Preparation
+
+Install
+[Rust](https://doc.rust-lang.org/cargo/getting-started/installation.html).
+
+## Running tests
+
+Run tests using `cargo`:
+
+```console
+cargo test
+```

2025/src/bin/day01.rs

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+use aoc2025::days::day01::{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

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+pub mod day01;

2025/src/days/day01.rs

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+use std::str::FromStr;
+
+pub fn part1(contents: &str) -> i64 {
+    let mut current: i64 = 50;
+    let mut count = 0;
+    for (dir, dist) in contents.lines().map(|line| parse_line(line)) {
+        match dir {
+            'L' => current = (current - dist).rem_euclid(100),
+            'R' => current = (current + dist).rem_euclid(100),
+            _ => panic!("Invalid direction"),
+        }
+
+        if current == 0 {
+            count += 1;
+        }
+    }
+    count
+}
+
+pub fn part2(contents: &str) -> i64 {
+    let mut current = 50;
+    let mut count = 0;
+    for (dir, dist) in contents.lines().map(|line| parse_line(line)) {
+        match dir {
+            'L' => {
+                count += (current - dist).div_euclid(100).abs();
+                if current == 0 && dist > 0 {
+                    // Moving left again will wrap around and count as another zero
+                    // crossing. Remove one here to avoid double-counting.
+                    count -= 1;
+                }
+
+                current = (current - dist).rem_euclid(100);
+                if current == 0 {
+                    // The division doesn't count landing on zero as a crossing,
+                    // so we need to increment the count here.
+                    count += 1;
+                }
+            }
+            'R' => {
+                count += (current + dist).div_euclid(100);
+                current = (current + dist).rem_euclid(100);
+            }
+            _ => panic!("Invalid direction"),
+        }
+    }
+    count
+}
+
+fn parse_line(line: &str) -> (char, i64) {
+    let (dir, dist) = line.split_at(1);
+    (dir.chars().next().unwrap(), i64::from_str(dist).unwrap())
+}
+
+#[cfg(test)]
+mod tests {
+    use std::vec;
+
+    use super::*;
+
+    #[test]
+    fn test_part1() {
+        let lines = vec![
+            "L68", "L30", "R48", "L5", "R60", "L55", "L1", "L99", "R14", "L82",
+        ];
+
+        assert_eq!(part1(&lines.join("\n")), 3);
+    }
+
+    #[test]
+    fn test_part2() {
+        let lines = vec![
+            "L68", "L30", "R48", "L5", "R60", "L55", "L1", "L99", "R14", "L82",
+        ];
+
+        assert_eq!(part2(&lines.join("\n")), 6);
+    }
+}

2025/src/lib.rs

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+pub mod days;

2025/tests/day01.rs

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+use aoc2025::days::day01::{part1, part2};
+
+#[test]
+fn test_part1() {
+    let content = std::include_str!("../../private/inputs/2025/day01.txt");
+    assert_eq!(part1(&content), 1118);
+}
+
+#[test]
+fn test_part2() {
+    let content = std::include_str!("../../private/inputs/2025/day01.txt");
+    assert_eq!(part2(&content), 6289);
+}