1 files changed, 141 insertions, 0 deletions
diff --git a/2024_rust/src/bin/day4.rs b/2024_rust/src/bin/day4.rs
new file mode 100644
index 0000000..7eb7b7a
--- /dev/null
+++ b/2024_rust/src/bin/day4.rs
@@ -0,0 +1,141 @@
+const XMAS: &str = "XMAS";
+const MAS: &str = "MAS";
+
+type Position = (usize, usize);
+type Row = Vec<Position>;
+type Rows = Vec<Row>;
+type Move = (Position, (usize, isize));
+
+struct Matrix {
+    chars: Vec<Vec<char>>,
+    limit: Position,
+}
+
+impl Matrix {
+    fn new(text: &str) -> Self {
+        let chars: Vec<Vec<char>> = text.lines().map(|row| row.chars().collect()).collect();
+        let limit = (chars.len(), chars[0].len());
+        Self { chars, limit }
+    }
+
+    fn check_iter<'a, I>(&self, it: I, needle: &str) -> bool
+    where
+        I: Iterator<Item = &'a Position>,
+    {
+        for (c, (i, j)) in needle.chars().zip(it) {
+            if self.chars[*i][*j] != c {
+                return false;
+            }
+        }
+
+        true
+    }
+
+    fn check_row(&self, row: &Row, needle: &str) -> bool {
+        self.check_iter(row.iter(), needle) || self.check_iter(row.iter().rev(), needle)
+    }
+}
+
+struct Square<'a> {
+    pos: Position,
+    size: usize,
+    matrix: &'a Matrix,
+}
+
+impl Square<'_> {
+    fn rows<M>(&self, moves: M) -> Rows
+    where
+        M: Fn(&Square) -> Vec<Move>,
+    {
+        let mut rs: Rows = vec![];
+
+        // Horizontal / vertical / diagonal
+        for (p0, (di, dj)) in moves(self) {
+            // Bounds check
+            if p0.0 + di * self.size > self.matrix.limit.0
+                || (dj > 0 && offplus(p0.1, dj * self.size as isize) > self.matrix.limit.1)
+                || p0.1 >= self.matrix.limit.1
+            {
+                continue;
+            }
+
+            let mut p_acc = p0;
+            let mut row: Row = vec![];
+            for _idx in 0..self.size {
+                row.push(p_acc);
+                p_acc = (p_acc.0 + di, offplus(p_acc.1, dj));
+            }
+            rs.push(row);
+        }
+
+        rs
+    }
+}
+
+fn offplus(x: usize, y: isize) -> usize {
+    (x as isize + y) as usize
+}
+
+fn moves_xmas(Square { pos, size, .. }: &Square) -> Vec<Move> {
+    let v = vec![
+        (*pos, (0, 1)),                       // diag from pos, top-down and left-to-right
+        (*pos, (1, 0)),                       // diag from pos, top-down and left-to-right
+        (*pos, (1, 1)),                       // diag from pos, top-down and left-to-right
+        ((pos.0, pos.1 + size - 1), (1, -1)), // the other diag, top-down and right-to-left
+    ];
+    v
+}
+
+fn p1(input: &str) -> String {
+    let matrix: Matrix = Matrix::new(input);
+    let mut result = 0;
+
+    for i in 0..matrix.limit.0 {
+        for j in 0..matrix.limit.1 {
+            let sq = Square {
+                pos: (i, j),
+                size: XMAS.len(),
+                matrix: &matrix,
+            };
+            for row in sq.rows(moves_xmas) {
+                if matrix.check_row(&row, XMAS) {
+                    result += 1;
+                }
+            }
+        }
+    }
+
+    result.to_string()
+}
+
+fn moves_mas(Square { pos, size, .. }: &Square) -> Vec<Move> {
+    let v = vec![
+        (*pos, (1, 1)),                       // diag from pos, top-down and left-to-right
+        ((pos.0, pos.1 + size - 1), (1, -1)), // the other diag, top-down and right-to-left
+    ];
+    v
+}
+
+fn p2(input: &str) -> String {
+    let matrix: Matrix = Matrix::new(input);
+    let mut result = 0;
+
+    for i in 0..=matrix.limit.0 - MAS.len() {
+        for j in 0..=matrix.limit.1 - MAS.len() {
+            let sq = Square {
+                pos: (i, j),
+                size: MAS.len(),
+                matrix: &matrix,
+            };
+            if sq.rows(moves_mas).iter().all(|r| matrix.check_row(r, MAS)) {
+                result += 1;
+            }
+        }
+    }
+
+    result.to_string()
+}
+
+fn main() {
+    aoc2024::run_day("4", p1, p2);
+}