2024 day 4

Author: ictrobot

crates/year2024/examples/day04_example0.txt

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

crates/year2024/src/day04.rs

@@ -0,0 +1,88 @@
+use utils::grid;
+use utils::prelude::*;
+
+/// Counting matches in a word search.
+#[derive(Clone, Debug)]
+pub struct Day04 {
+    rows: usize,
+    cols: usize,
+    grid: Vec<u8>,
+}
+
+impl Day04 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let (rows, cols, grid) = grid::from_str_padded(input, 3, b'\0', |c| match c {
+            b'X' | b'M' | b'A' | b'S' => Some(c),
+            _ => None,
+        })?;
+        Ok(Self { rows, cols, grid })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u32 {
+        let offsets = [
+            self.cols as isize,
+            -(self.cols as isize),
+            1,
+            -1,
+            (self.cols as isize) + 1,
+            (self.cols as isize) - 1,
+            -(self.cols as isize) + 1,
+            -(self.cols as isize) - 1,
+        ];
+
+        let mut count = 0;
+        for r in 3..self.rows - 3 {
+            for c in 3..self.cols - 3 {
+                let i = r * self.cols + c;
+                if self.grid[i] != b'X' {
+                    continue;
+                }
+
+                for o in offsets {
+                    if self.grid[i.wrapping_add_signed(o)] == b'M'
+                        && self.grid[i.wrapping_add_signed(o * 2)] == b'A'
+                        && self.grid[i.wrapping_add_signed(o * 3)] == b'S'
+                    {
+                        count += 1;
+                    }
+                }
+            }
+        }
+        count
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> u32 {
+        let mut count = 0;
+        for r in 4..self.rows - 4 {
+            for c in 4..self.cols - 4 {
+                let i = r * self.cols + c;
+                if self.grid[i] != b'A' {
+                    continue;
+                }
+
+                let (nw, ne, sw, se) = (
+                    self.grid[i.wrapping_add_signed(-(self.cols as isize) - 1)],
+                    self.grid[i.wrapping_add_signed(-(self.cols as isize) + 1)],
+                    self.grid[i.wrapping_add_signed((self.cols as isize) - 1)],
+                    self.grid[i.wrapping_add_signed((self.cols as isize) + 1)],
+                );
+
+                // Given each variable is one of (b'\0', b'X', b'M', b'A', b'S') this is
+                // equivalent to and slightly faster than
+                //  ((nw == b'M' && se == b'S') || (nw == b'S' && se == b'M'))
+                //      && ((ne == b'M' && sw == b'S') || (ne == b'S' && sw == b'M'))
+                // As no other pair XORed equals b'M' ^ b'S'
+                if (nw ^ se) == (b'M' ^ b'S') && (ne ^ sw) == (b'M' ^ b'S') {
+                    count += 1;
+                }
+            }
+        }
+        count
+    }
+}
+
+examples!(Day04 -> (u32, u32) [
+    {file: "day04_example0.txt", part1: 18, part2: 9},
+]);

crates/year2024/src/lib.rs

@@ -5,4 +5,5 @@ utils::year!(2024 => year2024, ${
     1 => day01::Day01,
     2 => day02::Day02,
     3 => day03::Day03,
+    4 => day04::Day04,
 });