From 992756faa6604099257aab8f04fcba45910cb6a4 Mon Sep 17 00:00:00 2001
From: uku <hi@uku.moe>
Date: Wed, 4 Dec 2024 21:57:40 +0100
Subject: [PATCH] feat(day4): init

---
 src/solutions/day_04.rs | 127 ++++++++++++++++++++++++++++++++++++++++
 src/solutions/mod.rs    |   8 ++-
 2 files changed, 134 insertions(+), 1 deletion(-)
 create mode 100644 src/solutions/day_04.rs

diff --git a/src/solutions/day_04.rs b/src/solutions/day_04.rs
new file mode 100644
index 0000000..37cd359
--- /dev/null
+++ b/src/solutions/day_04.rs
@@ -0,0 +1,127 @@
+use itertools::Itertools;
+
+use crate::common::{Answer, Solution};
+
+pub struct Day04;
+
+impl Solution for Day04 {
+    fn name(&self) -> &'static str {
+        "Ceres Search"
+    }
+
+    fn part_a(&self, input: &str) -> Answer {
+        let horizontal = input.lines().map(xmas_count).sum::<usize>();
+        let vertical = rotate_90(input).map(xmas_count).sum::<usize>();
+        let diag_45 = rotate_45(input).map(xmas_count).sum::<usize>();
+        let diag_135 = rotate_45(&rotate_90(input).join("\n"))
+            .map(xmas_count)
+            .sum::<usize>();
+
+        let sum = horizontal + vertical + diag_45 + diag_135;
+
+        Answer::Number(sum as u64)
+    }
+
+    fn part_b(&self, input: &str) -> Answer {
+        let input_90 = rotate_90(input).join("\n");
+        let input_180 = rotate_90(&input_90).join("\n");
+        let input_270 = rotate_90(&input_180).join("\n");
+
+        let sum = cross_mas_count(input)
+            + cross_mas_count(&input_90)
+            + cross_mas_count(&input_180)
+            + cross_mas_count(&input_270);
+
+        Answer::Number(sum as u64)
+    }
+}
+
+fn rotate_90(input: &str) -> impl Iterator<Item = String> + use<'_> {
+    let col_count = input.lines().next().unwrap().chars().count();
+
+    (0..col_count)
+        .rev() // this is important otherwise 135 rotation doesn't work
+        .map(|i| input.lines().map(|l| l.chars().nth(i).unwrap()).join(""))
+}
+
+fn rotate_45(input: &str) -> impl Iterator<Item = String> + use<'_> {
+    let line_count = input.lines().count();
+    let col_count = input.lines().next().unwrap().chars().count();
+
+    let cols = (0..col_count).map(|i| traverse_diagonal(input, (0, i)));
+    let lines = (1..line_count).map(|i| traverse_diagonal(input, (i, 0)));
+
+    cols.chain(lines)
+}
+
+fn traverse_diagonal(input: &str, start: (usize, usize)) -> String {
+    (start.0..)
+        .zip(start.1..)
+        .map_while(|(x, y)| input.lines().nth(x).and_then(|l| l.chars().nth(y)))
+        .collect()
+}
+
+fn xmas_count(line: impl AsRef<str>) -> usize {
+    line.as_ref().matches("XMAS").count() + line.as_ref().matches("SAMX").count()
+}
+
+fn cross_mas_count(input: &str) -> usize {
+    let mut sum = 0;
+
+    for (x, line) in input.lines().enumerate() {
+        for (y, _) in line.match_indices('A') {
+            let prev_line = x.checked_sub(1).and_then(|n| input.lines().nth(n));
+            let next_line = input.lines().nth(x + 1);
+
+            if let (Some(prev_line), Some(next_line)) = (prev_line, next_line) {
+                let prev_y = y.checked_sub(1);
+                let next_y = y + 1;
+
+                let prev_prev = prev_y.and_then(|n| prev_line.chars().nth(n));
+                let prev_next = prev_line.chars().nth(next_y);
+
+                let next_prev = prev_y.and_then(|n| next_line.chars().nth(n));
+                let next_next = next_line.chars().nth(next_y);
+
+                if let (Some('M'), Some('M'), Some('S'), Some('S')) =
+                    (prev_prev, prev_next, next_prev, next_next)
+                {
+                    sum += 1;
+                }
+            }
+        }
+    }
+
+    sum
+}
+
+#[cfg(test)]
+mod test {
+    use super::Day04;
+    use crate::common::Solution;
+
+    use indoc::indoc;
+
+    const INPUT: &str = indoc! {"
+        MMMSXXMASM
+        MSAMXMSMSA
+        AMXSXMAAMM
+        MSAMASMSMX
+        XMASAMXAMM
+        XXAMMXXAMA
+        SMSMSASXSS
+        SAXAMASAAA
+        MAMMMXMMMM
+        MXMXAXMASX
+    "};
+
+    #[test]
+    fn part_a() {
+        assert_eq!(Day04.part_a(INPUT), 18.into());
+    }
+
+    #[test]
+    fn part_b() {
+        assert_eq!(Day04.part_b(INPUT), 9.into());
+    }
+}
diff --git a/src/solutions/mod.rs b/src/solutions/mod.rs
index f8c8bdb..e27f88f 100644
--- a/src/solutions/mod.rs
+++ b/src/solutions/mod.rs
@@ -3,5 +3,11 @@ use crate::common::Solution;
 mod day_01;
 mod day_02;
 mod day_03;
+mod day_04;
 
-pub const SOLUTIONS: &[&dyn Solution] = &[&day_01::Day01, &day_02::Day02, &day_03::Day03];
+pub const SOLUTIONS: &[&dyn Solution] = &[
+    &day_01::Day01,
+    &day_02::Day02,
+    &day_03::Day03,
+    &day_04::Day04,
+];