diff --git a/day-04/Cargo.lock b/day-04/Cargo.lock
new file mode 100644
index 0000000..7b7b254
--- /dev/null
+++ b/day-04/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day-04"
+version = "0.1.0"
diff --git a/day-04/Cargo.toml b/day-04/Cargo.toml
new file mode 100644
index 0000000..4fb02de
--- /dev/null
+++ b/day-04/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "day-04"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
diff --git a/day-04/src/main.rs b/day-04/src/main.rs
new file mode 100644
index 0000000..3818cf8
--- /dev/null
+++ b/day-04/src/main.rs
@@ -0,0 +1,133 @@
+const INPUT: &str = include_str!(concat!(env!("CARGO_MANIFEST_DIR"), "/input.txt"));
+
+fn main() {
+
+    let mut data = Vec::new();
+    let mut cols = None;
+
+    for line in INPUT.lines() {
+        let mut count = 0;
+        for ch in line.bytes() {
+            data.push(ch);
+            count += 1;
+        }
+        if let Some(cols) = cols {
+            assert_eq!(cols, count);
+        } else {
+            cols = Some(count);
+        }
+    }
+
+    let data = Data { data, cols: cols.unwrap(), };
+
+    let mut total = 0;
+    for row in 0..data.rows() as isize {
+        for col in 0..data.cols as isize {
+            total += xmas(&data, row, col);
+        }
+    }
+
+    println!("Part 1: {}", total);
+
+    let mut total = 0;
+    for row in 0..data.rows() as isize {
+        for col in 0..data.cols as isize {
+            total += u32::from(x_mas(&data, row, col));
+        }
+    }
+    println!("Part 2: {}", total);
+}
+
+struct Data {
+    data: Vec<u8>,
+    cols: usize,
+}
+
+impl Data {
+    fn get(&self, row: isize, col: isize) -> Option<u8> {
+        if row >= 0 && col >= 0 {
+            let row = row as usize;
+            let col = col as usize;
+            self.data
+                // From row start until end of array
+                .get(row * self.cols..)
+                // Just the desired row
+                .and_then(|x| x.get(..self.cols))
+                // The desired column
+                .and_then(|x| x.get(col))
+                .copied()
+        } else {
+            None
+        }
+    }
+
+    fn rows(&self) -> usize {
+        assert_eq!(self.data.len() % self.cols, 0);
+        self.data.len() / self.cols
+    }
+}
+
+fn x_mas(
+    data: &Data,
+    row: isize,
+    col: isize,
+) -> bool {
+    if data.get(row, col) == Some(b'A') {
+        let br = data.get(row + 1, col + 1);
+        let bl = data.get(row + 1, col - 1);
+        let tl = data.get(row - 1, col - 1);
+        let tr = data.get(row - 1, col + 1);
+
+        if let Some((((bl, br), tl), tr)) =  bl.zip(br).zip(tl).zip(tr) {
+               br == bl && tr == tl && (
+                 br == b'S' && tl == b'M' || br == b'M' && tl == b'S'
+               )
+            || br == tr && bl == tl && (
+                 br == b'S' && tl == b'M' || br == b'M' && tl == b'S'
+               )
+        } else {
+            false
+        }
+    } else {
+        false
+    }
+}
+
+fn xmas(
+    data: &Data,
+    row: isize,
+    col: isize,
+) -> u32 {
+    let mut total = 0;
+    for direction in [
+        ( 0,  1),
+        ( 1,  0),
+        ( 0, -1),
+        (-1,  0),
+        ( 1,  1),
+        (-1, -1),
+        (-1,  1),
+        ( 1, -1),
+    ] {
+        total += u32::from(xmas_one_direction(data, row, col, direction));
+    }
+    total
+}
+
+fn xmas_one_direction(
+    data: &Data,
+    mut row: isize,
+    mut col: isize,
+    direction: (isize, isize)
+) -> bool {
+    for ch in [b'X', b'M', b'A', b'S'] {
+        if data.get(row, col) != Some(ch) {
+            return false;
+        }
+
+        row += direction.0;
+        col += direction.1;
+    }
+
+    return true;
+}