Day 4

README.md

@@ -22,8 +22,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 1](./src/bin/01.rs) | `61.4µs` | `106.8µs` |
 | [Day 2](./src/bin/02.rs) | `220.6µs` | `404.8µs` |
 | [Day 3](./src/bin/03.rs) | `561.0µs` | `565.5µs` |
+| [Day 4](./src/bin/04.rs) | `559.4µs` | `160.7µs` |
 
-**Total: 1.92ms**
+**Total: 2.64ms**
 <!--- benchmarking table --->
 
 ---

data/examples/04.txt

@@ -0,0 +1,10 @@
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX

src/bin/04.rs

@@ -0,0 +1,128 @@
+use itertools::Itertools;
+
+advent_of_code::solution!(4);
+
+#[derive(Debug)]
+struct Pos {
+    x: isize,
+    y: isize,
+}
+
+impl Pos {
+
+    fn offset(&self, offset: &(isize, isize)) -> Pos {
+        Pos { x: self.x + offset.0, y: self.y + offset.1 }
+    }
+
+    fn is_within(&self, dimensions: &(isize, isize)) -> bool {
+        (0..dimensions.0).contains(&self.x) && (0..dimensions.1).contains(&self.y)
+    }
+
+}
+
+impl From<(usize, usize)> for Pos {
+    fn from(value: (usize, usize)) -> Self {
+        Pos { x: value.0 as isize, y: value.1 as isize }
+    }
+}
+
+const SEQUENCE: &[char; 4] = &['X', 'M', 'A', 'S'];
+const DIRECTIONS: [(isize, isize); 8] = [
+    (-1, -1),
+    (-1, 0),
+    (-1, 1),
+    (0, -1),
+    (0, 1),
+    (1, -1),
+    (1, 0),
+    (1, 1),
+];
+
+fn get_char(matrix: &[Vec<char>], pos: &Pos) -> char {
+    matrix[pos.y as usize][pos.x as usize]
+}
+
+pub fn part_one(input: &str) -> Option<u32> {
+    Some(search_xmas(input))
+}
+
+fn search_xmas(input: &str) -> u32 {
+    let matrix = input.lines().map(|line| line.chars().collect_vec()).collect_vec();
+    let dimensions = (matrix[0].len() as isize, matrix.len() as isize);
+    (0..dimensions.0)
+        .cartesian_product(0..dimensions.1)
+        .map(|(x, y)| Pos { x, y })
+        .map(|pos| count_xmas_seq(&matrix, &dimensions, &pos, &DIRECTIONS, 0))
+        .sum()
+}
+
+fn count_xmas_seq(matrix: &[Vec<char>], dimensions: &(isize, isize), pos: &Pos, directions: &[(isize, isize)], seq_index: usize) -> u32 {
+    let seq_char = SEQUENCE[seq_index];
+    if get_char(matrix, pos) != seq_char {
+        return 0;
+    }
+    if seq_index + 1 >= SEQUENCE.len() {
+        return 1;
+    }
+    let mut matches = 0;
+    for direction in directions {
+        let offset_pos = pos.offset(direction);
+        if !offset_pos.is_within(dimensions) {
+            continue;
+        }
+        matches += count_xmas_seq(matrix, dimensions, &offset_pos, &[*direction], seq_index + 1)
+    }
+    matches
+}
+
+pub fn part_two(input: &str) -> Option<u32> {
+    Some(search_cross_mas(input))
+}
+
+fn search_cross_mas(input: &str) -> u32 {
+    let matrix = input.lines().map(|line| line.chars().collect_vec()).collect_vec();
+    let dimensions = (matrix[0].len() as isize, matrix.len() as isize);
+    (0..dimensions.0)
+        .cartesian_product(0..dimensions.1)
+        .map(|(x, y)| Pos { x, y })
+        .filter(|pos| is_cross_mas(&matrix, &dimensions, pos))
+        .count() as u32
+}
+
+fn is_cross_mas(matrix: &[Vec<char>], dimensions: &(isize, isize), pos: &Pos) -> bool {
+    if !(1..(dimensions.0 - 1)).contains(&pos.x) || !(1..(dimensions.1 - 1)).contains(&pos.y) {
+        return false
+    }
+    let center = get_char(matrix, pos);
+    if center != 'A' {
+        return false;
+    }
+
+    let top_left = get_char(matrix, &pos.offset(&(-1, -1)));
+    let top_right = get_char(matrix, &pos.offset(&(-1, 1)));
+    let bottom_left = get_char(matrix, &pos.offset(&(1, -1)));
+    let bottom_right = get_char(matrix, &pos.offset(&(1, 1)));
+
+    let diagonal1 = [top_left, center, bottom_right];
+    let diagonal2 = [bottom_left, center, top_right];
+
+    (diagonal1 == ['M', 'A', 'S'] || diagonal1 == ['S', 'A', 'M'])
+        && (diagonal2 == ['M', 'A', 'S'] || diagonal2 == ['S', 'A', 'M'])
+}
+
+#[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(18));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(9));
+    }
+}