diff --git a/2016/rust/Cargo.lock b/2016/rust/Cargo.lock
index 5abecf0..5a18724 100644
--- a/2016/rust/Cargo.lock
+++ b/2016/rust/Cargo.lock
@@ -1,4 +1,161 @@
 [root]
 name = "advent_of_code_2016"
 version = "0.1.0"
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diff --git a/2016/rust/Cargo.toml b/2016/rust/Cargo.toml
index 5f59c0b..6f439f1 100644
--- a/2016/rust/Cargo.toml
+++ b/2016/rust/Cargo.toml
@@ -4,3 +4,5 @@ version = "0.1.0"
 authors = ["Alpha Chen <alpha.chen@gmail.com>"]
 
 [dependencies]
+error-chain = "0.7"
+regex = "0.1"
diff --git a/2016/rust/src/day_04.rs b/2016/rust/src/day_04.rs
new file mode 100644
index 0000000..121f9b6
--- /dev/null
+++ b/2016/rust/src/day_04.rs
@@ -0,0 +1,84 @@
+use std::cmp::Ordering;
+use std::collections::HashMap;
+
+use errors::*;
+use regex::Regex;
+
+pub fn solve(input: &str) -> Result<String> {
+  let rooms: Vec<(Room, String)> = input.lines()
+    .map(|line| {
+      let re = Regex::new(r"(?P<room>.+)\[(?P<checksum>.+)\]").unwrap();
+      let caps = re.captures(line).ok_or("")?;
+      let room = caps.name("room").unwrap();
+      let checksum = caps.name("checksum").unwrap();
+
+      Room::new(room).map(|room| (room, checksum.into()))
+    })
+    .collect::<Result<_>>()?;
+  let real_rooms = rooms.iter()
+    .filter(|&&(ref room, ref checksum)| room.checksum() == *checksum);
+  let sector_ids = real_rooms.map(|&(ref room, _)| room.sector_id);
+  let sum: usize = sector_ids.sum();
+
+  Ok(sum.to_string())
+}
+
+#[test]
+fn test_solve() {}
+
+struct Room {
+  encrypted_name: String,
+  sector_id: usize,
+}
+
+impl Room {
+  fn new(s: &str) -> Result<Self> {
+    let re = Regex::new(r"(?P<name>.+)-(?P<id>\d+)").unwrap();
+    let caps = re.captures(s).ok_or("")?;
+    let name = caps.name("name").unwrap();
+    let id = caps.name("id").unwrap().parse().chain_err(|| "")?;
+    Ok(Room {
+      encrypted_name: name.into(),
+      sector_id: id,
+    })
+  }
+
+  fn checksum(&self) -> String {
+    let mut counts = HashMap::new();
+    for c in self.encrypted_name.chars().filter(|c| c.is_alphabetic()) {
+      *counts.entry(c).or_insert(0) += 1;
+    }
+
+    let mut counts: Vec<_> = counts.iter().collect();
+    counts.sort_by(|a, b| {
+      match b.1.cmp(a.1) {
+        Ordering::Equal => a.0.cmp(b.0),
+        x => x,
+      }
+    });
+
+    counts.iter().take(5).map(|x| *x.0).collect()
+  }
+}
+
+#[test]
+fn test_new() {
+  let room = Room::new("aaaaa-bbb-z-y-x-123").unwrap();
+  assert_eq!(room.encrypted_name, "aaaaa-bbb-z-y-x".to_string());
+  assert_eq!(room.sector_id, 123);
+}
+
+#[test]
+fn test_checksum() {
+  let room = Room::new("aaaaa-bbb-z-y-x-123").unwrap();
+  assert_eq!(room.checksum(), "abxyz".to_string());
+
+  let room = Room::new("a-b-c-d-e-f-g-h-987").unwrap();
+  assert_eq!(room.checksum(), "abcde".to_string());
+
+  let room = Room::new("not-a-real-room-404").unwrap();
+  assert_eq!(room.checksum(), "oarel".to_string());
+
+  let room = Room::new("totally-real-room-200").unwrap();
+  assert_ne!(room.checksum(), "decoy".to_string());
+}
diff --git a/2016/rust/src/errors.rs b/2016/rust/src/errors.rs
new file mode 100644
index 0000000..0c6d43c
--- /dev/null
+++ b/2016/rust/src/errors.rs
@@ -0,0 +1 @@
+error_chain!{}
diff --git a/2016/rust/src/lib.rs b/2016/rust/src/lib.rs
index 3160911..2d76c5b 100644
--- a/2016/rust/src/lib.rs
+++ b/2016/rust/src/lib.rs
@@ -1,3 +1,11 @@
+#![recursion_limit = "1024"]
+
+#[macro_use]
+extern crate error_chain;
+extern crate regex;
+
+mod errors;
 pub mod day_01;
 pub mod day_02;
 pub mod day_03;
+pub mod day_04;
diff --git a/2016/rust/src/main.rs b/2016/rust/src/main.rs
index bd7710f..512d3d0 100644
--- a/2016/rust/src/main.rs
+++ b/2016/rust/src/main.rs
@@ -8,5 +8,5 @@ fn main() {
   let mut input = String::new();
   io::stdin().read_to_string(&mut input).ok();
 
-  println!("{}", day_03::solve(&input));
+  println!("{}", day_04::solve(&input).unwrap());
 }