day 4

Author: johnterickson

Diff for: 2024/src/day4.rs

@@ -0,0 +1,178 @@
+use aoc_runner_derive::{aoc, aoc_generator};
+
+/*
+
+--- Day 4: Ceres Search ---
+"Looks like the Chief's not here. Next!" One of The Historians pulls out a device and pushes the only button on it. After a brief flash, you recognize the interior of the Ceres monitoring station!
+
+As the search for the Chief continues, a small Elf who lives on the station tugs on your shirt; she'd like to know if you could help her with her word search (your puzzle input). She only has to find one word: XMAS.
+
+This word search allows words to be horizontal, vertical, diagonal, written backwards, or even overlapping other words. It's a little unusual, though, as you don't merely need to find one instance of XMAS - you need to find all of them. Here are a few ways XMAS might appear, where irrelevant characters have been replaced with .:
+
+
+..X...
+.SAMX.
+.A..A.
+XMAS.S
+.X....
+The actual word search will be full of letters instead. For example:
+
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX
+In this word search, XMAS occurs a total of 18 times; here's the same word search again, but where letters not involved in any XMAS have been replaced with .:
+
+....XXMAS.
+.SAMXMS...
+...S..A...
+..A.A.MS.X
+XMASAMX.MM
+X.....XA.A
+S.S.S.S.SS
+.A.A.A.A.A
+..M.M.M.MM
+.X.X.XMASX
+Take a look at the little Elf's word search. How many times does XMAS appear?
+
+ */
+
+struct Grid(Vec<Vec<char>>);
+
+impl Grid {
+    fn get(&self, r: usize, c: usize) -> Option<char> {
+        self.0.get(r).and_then(|row| row.get(c)).copied()
+    }
+
+    fn get_offset(&self, r: usize, r_offset: isize, c: usize, c_offset: isize) -> Option<char> {
+        let r: isize = r.try_into().ok()?;
+        let r: usize = (r + r_offset).try_into().ok()?;
+        let c: isize = c.try_into().ok()?;
+        let c: usize = (c + c_offset).try_into().ok()?;
+        self.get(r, c)
+    }
+}   
+
+
+#[aoc_generator(day4)]
+fn parse_input(input: &str) -> Grid {
+    let input = input.trim();
+    Grid(input.lines().map(|l| l.trim().chars().collect()).collect())
+}
+
+const WORD: [char; 4] = ['X', 'M', 'A', 'S'];
+const DIRS: [(isize, isize); 8] = [
+    (0, 1),
+    (1, 0),
+    (1, 1),
+    (1, -1),
+    (0, -1),
+    (-1, 0),
+    (-1, 1),
+    (-1, -1),
+];
+
+#[aoc(day4, part1)]
+fn part1(grid: &Grid) -> u32 {
+    let mut count = 0;
+    for (r, row) in grid.0.iter().enumerate() {
+        for (c, ch) in row.iter().enumerate() {
+            if *ch == WORD[0] {
+                'dir: 
+                for (dr, dc) in DIRS.iter() {
+                    for i in 1..WORD.len() {
+                        if grid.get_offset(r, i as isize * dr, c, i as isize * dc) != Some(WORD[i]) {
+                            continue 'dir;
+                        }
+                    }
+
+                    count += 1;
+                }
+            }
+        }
+    }
+
+    count
+}
+
+#[aoc(day4, part2)]
+fn part2(grid: &Grid) -> u32 {
+    let mut count = 0;
+    for (r, row) in grid.0.iter().enumerate() {
+        for (c, ch) in row.iter().enumerate() {
+            if *ch != 'A' {
+                continue;
+            }
+
+            let up_left = grid.get_offset(r, -1, c, -1);
+            let up_right = grid.get_offset(r, -1, c, 1);
+            let down_left = grid.get_offset(r, 1, c, -1);
+            let down_right = grid.get_offset(r, 1, c, 1);
+
+            let up_left_diag = 
+                (up_left == Some('M') && down_right == Some('S')) ||
+                (up_left == Some('S') && down_right == Some('M'));
+
+            let down_left_diag = 
+                (down_left == Some('M') && up_right == Some('S')) ||
+                (down_left == Some('S') && up_right == Some('M'));
+
+            if up_left_diag && down_left_diag {
+                count += 1;
+            }
+        }
+    }
+    count
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example() {
+        let input = parse_input(
+            r#"
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX
+        "#
+            .trim(),
+        );
+        assert_eq!(part1(&input), 18);
+    }
+
+    #[test]
+    fn part2_example() {
+        let input = parse_input(
+            r#"
+.M.S......
+..A..MSMS.
+.M.S.MAA..
+..A.ASMSM.
+.M.S.M....
+..........
+S.S.S.S.S.
+.A.A.A.A..
+M.M.M.M.M.
+..........
+        "#
+            .trim(),
+        );
+        assert_eq!(part2(&input), 9);
+    }
+}

Diff for: 2024/src/lib.rs

@@ -3,6 +3,7 @@ use aoc_runner_derive::aoc_lib;
 mod day1;
 mod day2;
 mod day3;
+mod day4;
 
 aoc_lib! {
     year = 2024, extra_alternatives = ["fnv"]