diff --git a/src/days.rs b/src/days.rs
index 0958165..21b6c99 100644
--- a/src/days.rs
+++ b/src/days.rs
@@ -1,7 +1,7 @@
 pub mod d1;
 pub mod d2;
 pub mod d3;
-// pub mod d4;
+pub mod d4;
 // pub mod d5;
 // pub mod d6;
 // pub mod d7;
diff --git a/src/days/d4.rs b/src/days/d4.rs
new file mode 100644
index 0000000..850f52a
--- /dev/null
+++ b/src/days/d4.rs
@@ -0,0 +1,86 @@
+use crate::prelude::*;
+
+pub type T = (String, M<char, usize>, Vec<char>, u32);
+pub type I = Vec<T>;
+pub type O = u32;
+
+fn _parse (data: &str) -> I {
+    data.lines().map(|line| {
+        let mut parts = line.split('-').collect_vec();
+        let last = parts.pop().unwrap();
+        let rest = parts.join(" ");
+        let m = parts.into_iter().map(|p| p.chars()).flatten().counts();
+
+        let (number, checksum) = last.split_once('[').unwrap();
+
+        let number = number.parse().unwrap();
+        let checksum = checksum.trim_matches(']').chars().collect();
+
+        (rest, m, checksum, number)
+    }).collect()
+}
+
+fn _solve <'a> (data: &'a I) -> impl Iterator<Item = (&'a String, &'a u32)> {
+    data.into_iter().filter_map(|(st, m, checksum, num)| {
+        let mut m = m.into_iter().collect_vec();
+
+        m.sort_by(|a, b| match b.1.cmp(&a.1) {
+            Or::Equal => a.0.cmp(&b.0),
+            e => e
+        });
+
+        if checksum.into_iter().zip(m.into_iter()).all(|(&l, (&r, _))| l == r) {
+            Some((st, num))
+        } else {
+            None
+        }
+    })
+}
+
+fn _silver (data: &I) -> O {
+    _solve(data).map(|(_, num)| num).sum()
+}
+
+fn _gold (data: &I) -> O {
+    *_solve(data).find(|(s, num)| {
+
+        let mut s = s.chars().collect_vec();
+
+        for c in s.iter_mut() {
+            match *c {
+                ' ' => (),
+                a => {
+                    *c = ((((a as u8 - b'a') as u32 + *num) % 26) + b'a' as u32) as u8 as char;
+                }
+            }
+        }
+        let s = s.into_iter().collect::<String>();
+        s.contains("north")
+    }).unwrap().1
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn read () -> I {
+        let data = inc!(4);
+        _parse(data)
+    }
+
+    #[test]
+    fn silver () {
+        let data = read();
+        let ans = _silver(&data);
+
+        assert_eq!(ans, 278221)
+    }
+
+    #[test]
+    fn gold () {
+        let data = read();
+        let ans = _gold(&data);
+
+        assert_eq!(ans, 267)
+    }
+}