1 files changed, 134 insertions, 0 deletions
diff --git a/2024_rust/src/day4.rs b/2024_rust/src/day4.rs
new file mode 100644
index 0000000..1ebe67b
--- /dev/null
+++ b/2024_rust/src/day4.rs
@@ -0,0 +1,134 @@
+const XMAS: &str = "XMAS";
+const MAS: &str = "MAS";
+
+type Matrix = Vec<Vec<char>>;
+type Position = (usize, usize);
+type RowXmas = [Position; XMAS.len()];
+type RowMas = [Position; MAS.len()];
+
+fn check_xmas<'a, I>(m: &Matrix, it: I) -> bool
+where
+    I: Iterator<Item = &'a Position>,
+{
+    for (c, (i, j)) in XMAS.chars().zip(it) {
+        if m[*i][*j] != c {
+            return false;
+        }
+    }
+
+    true
+}
+
+fn offplus(x: usize, y: isize) -> usize {
+    (x as isize + y) as usize
+}
+
+fn rows_latdiag_from(p: Position, max_x: usize, max_y: usize) -> Vec<RowXmas> {
+    let mut rs: Vec<RowXmas> = vec![];
+
+    // Horizontal / vertical / diagonal
+    for (p0, (di, dj)) in [
+        (p, (0, 1)),                                   // horizontal, left-to-right
+        (p, (1, 0)),                                   // vertical, top-down
+        (p, (1, 1)),                                   // diag from p, top-down and left-to-right
+        ((p.0, p.1 + XMAS.len() - 1), (1, -1isize)), // the other diag, top-down and right-to-left
+    ] {
+        // Bounds check
+        let max_off = XMAS.len();
+        if p0.0 + di * max_off > max_x
+            || (dj > 0 && offplus(p0.1, dj * max_off as isize) > max_y)
+            || p0.1 >= max_y
+        {
+            continue;
+        }
+
+        let mut p_acc = p0;
+        let mut ps = [(0, 0); XMAS.len()];
+        for idx in 0..ps.len() {
+            ps[idx].0 = p_acc.0;
+            ps[idx].1 = p_acc.1;
+            p_acc = (p_acc.0 + di, offplus(p_acc.1, dj));
+        }
+        rs.push(ps);
+    }
+
+    rs
+}
+
+pub fn p1(input: &str) -> String {
+    let m: Matrix = input.lines().map(|row| row.chars().collect()).collect();
+    let mut result = 0;
+
+    for i in 0..m.len() {
+        for j in 0..m[i].len() {
+            let p = (i, j);
+            for row in rows_latdiag_from(p, m.len(), m[i].len()) {
+                if check_xmas(&m, row.iter()) || check_xmas(&m, row.iter().rev()) {
+                    result += 1;
+                }
+            }
+        }
+    }
+
+    result.to_string()
+}
+
+fn check_mas<'a, I>(m: &Matrix, it: I) -> bool
+where
+    I: Iterator<Item = &'a Position>,
+{
+    for (c, (i, j)) in MAS.chars().zip(it) {
+        if m[*i][*j] != c {
+            return false;
+        }
+    }
+
+    true
+}
+
+fn rows_diag_from(p: Position, max_x: usize, max_y: usize) -> Vec<RowMas> {
+    let mut rs: Vec<RowMas> = vec![];
+
+    // Horizontal / vertical / diagonal
+    for (p0, (di, dj)) in [
+        (p, (1, 1)),                                   // diag from p, top-down and left-to-right
+        ((p.0, p.1 + MAS.len() - 1), (1, -1isize)), // the other diag, top-down and right-to-left
+    ] {
+        // Bounds check
+        let max_off = MAS.len();
+        if p0.0 + di * max_off > max_x
+            || (dj > 0 && offplus(p0.1, dj * max_off as isize) > max_y)
+            || p0.1 >= max_y
+        {
+            continue;
+        }
+
+        let mut p_acc = p0;
+        let mut ps = [(0, 0); MAS.len()];
+        for idx in 0..ps.len() {
+            ps[idx].0 = p_acc.0;
+            ps[idx].1 = p_acc.1;
+            p_acc = (p_acc.0 + di, offplus(p_acc.1, dj));
+        }
+        rs.push(ps);
+    }
+
+    rs
+}
+
+pub fn p2(input: &str) -> String {
+    let m: Matrix = input.lines().map(|row| row.chars().collect()).collect();
+    let mut result = 0;
+
+    for i in 0..=m.len() - MAS.len() {
+        for j in 0..=m[i].len() - MAS.len() {
+            let p = (i, j);
+            let rows = rows_diag_from(p, m.len(), m[i].len());
+            if rows.iter().all(|r| check_mas(&m, r.iter()) || check_mas(&m, r.iter().rev())) {
+                result += 1;
+            }
+        }
+    }
+
+    result.to_string()
+}