Solve 04

justfile

@@ -27,3 +27,12 @@ begin: _folders
 # Makes sure that folders exist
 _folders:
     mkdir -p input src/bin
+
+# Creates all input files. Use this to start using the repo.
+init: _folders
+    #!/usr/bin/env bash
+    for i in $(seq -w 1 25);
+    do
+        touch input/$i
+        touch input/$i-test
+    done

src/bin/04.rs

@@ -0,0 +1,100 @@
+use aoc::*;
+use glam::IVec2;
+
+const INPUT: &str = include_str!("../../input/04");
+
+fn main() {
+    assert_example!(part1, "04-test", 18);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "04-test", 9);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(input: &str) -> usize {
+    let field = parse(input);
+    count_over_field(&field, count_xmas_at)
+}
+
+fn part2(input: &str) -> usize {
+    let field = parse(input);
+    count_over_field(&field, check_x_mas)
+}
+
+fn count_over_field(field: &Field, count_at: fn(IVec2, &Field) -> usize) -> usize {
+    let mut found = 0;
+    for y in 0..field.data.len() {
+        for x in 0..field.data[y].len() {
+            let start = IVec2::new(x as i32, y as i32);
+            found += count_at(start, field);
+        }
+    }
+    found
+}
+
+fn check_x_mas(start: IVec2, field: &Field) -> usize {
+    if field.get(start) != 'A' {
+        return 0;
+    }
+
+    let up1 =
+        field.get(start + IVec2::new(-1, -1)) == 'M' && field.get(start + IVec2::new(1, 1)) == 'S';
+    let up2 =
+        field.get(start + IVec2::new(-1, -1)) == 'S' && field.get(start + IVec2::new(1, 1)) == 'M';
+    let up = up1 || up2;
+
+    let down1 =
+        field.get(start + IVec2::new(1, -1)) == 'M' && field.get(start + IVec2::new(-1, 1)) == 'S';
+    let down2 =
+        field.get(start + IVec2::new(1, -1)) == 'S' && field.get(start + IVec2::new(-1, 1)) == 'M';
+    let down = down1 || down2;
+
+    if up && down {
+        1
+    } else {
+        0
+    }
+}
+
+fn count_xmas_at(start: IVec2, field: &Field) -> usize {
+    DIRECTIONS
+        .iter()
+        .filter(|&&dir| check_xmas(start, dir, field))
+        .count()
+}
+
+fn check_xmas(start: IVec2, direction: IVec2, field: &Field) -> bool {
+    field.get(start) == 'X'
+        && field.get(start + direction) == 'M'
+        && field.get(start + direction * 2) == 'A'
+        && field.get(start + direction * 3) == 'S'
+}
+
+const DIRECTIONS: [IVec2; 8] = [
+    IVec2::new(1, 0),
+    IVec2::new(1, 1),
+    IVec2::new(0, 1),
+    IVec2::new(-1, 1),
+    IVec2::new(-1, 0),
+    IVec2::new(-1, -1),
+    IVec2::new(0, -1),
+    IVec2::new(1, -1),
+];
+
+struct Field {
+    data: Vec<Vec<char>>,
+}
+
+impl Field {
+    fn get(&self, at: IVec2) -> char {
+        self.data
+            .get(at.y as usize)
+            .and_then(|row| row.get(at.x as usize))
+            .copied()
+            .unwrap_or('.')
+    }
+}
+
+fn parse(input: &str) -> Field {
+    let data = input.lines().map(|l| l.chars().collect()).collect();
+    Field { data }
+}