diff --git a/2016/day-21/Cargo.toml b/2016/day-21/Cargo.toml
new file mode 100644
index 0000000..16c171d
--- /dev/null
+++ b/2016/day-21/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "day-21"
+version = "0.1.0"
+authors = ["Bert Peters <bert.ljpeters@gmail.com>"]
+
+[dependencies]
diff --git a/2016/day-21/input.txt b/2016/day-21/input.txt
new file mode 100644
index 0000000..f43fd37
--- /dev/null
+++ b/2016/day-21/input.txt
@@ -0,0 +1,100 @@
+swap position 2 with position 7
+swap letter f with letter a
+swap letter c with letter a
+rotate based on position of letter g
+rotate based on position of letter f
+rotate based on position of letter b
+swap position 3 with position 6
+swap letter e with letter c
+swap letter f with letter h
+rotate based on position of letter e
+swap letter c with letter b
+rotate right 6 steps
+reverse positions 4 through 7
+rotate based on position of letter f
+swap position 1 with position 5
+rotate left 1 step
+swap letter d with letter e
+rotate right 7 steps
+move position 0 to position 6
+swap position 2 with position 6
+swap position 2 with position 0
+swap position 0 with position 1
+rotate based on position of letter d
+rotate right 2 steps
+rotate left 4 steps
+reverse positions 0 through 2
+rotate right 2 steps
+move position 6 to position 1
+move position 1 to position 2
+reverse positions 2 through 5
+move position 2 to position 7
+rotate left 3 steps
+swap position 0 with position 1
+rotate based on position of letter g
+swap position 5 with position 0
+rotate left 1 step
+swap position 7 with position 1
+swap letter g with letter h
+rotate left 1 step
+rotate based on position of letter g
+reverse positions 1 through 7
+reverse positions 1 through 4
+reverse positions 4 through 5
+rotate left 2 steps
+swap letter f with letter d
+swap position 6 with position 3
+swap letter c with letter e
+swap letter c with letter d
+swap position 1 with position 6
+rotate based on position of letter g
+move position 4 to position 5
+swap letter g with letter h
+rotate based on position of letter h
+swap letter h with letter f
+swap position 3 with position 6
+rotate based on position of letter c
+rotate left 3 steps
+rotate based on position of letter d
+swap position 0 with position 7
+swap letter e with letter d
+move position 6 to position 7
+rotate right 5 steps
+move position 7 to position 0
+rotate left 1 step
+move position 6 to position 2
+rotate based on position of letter d
+rotate right 7 steps
+swap position 3 with position 5
+move position 1 to position 5
+rotate right 0 steps
+move position 4 to position 5
+rotate based on position of letter b
+reverse positions 2 through 4
+rotate right 3 steps
+swap letter b with letter g
+rotate based on position of letter a
+rotate right 0 steps
+move position 0 to position 6
+reverse positions 5 through 6
+rotate left 2 steps
+move position 3 to position 0
+swap letter g with letter b
+move position 6 to position 1
+rotate based on position of letter f
+move position 3 to position 2
+reverse positions 2 through 7
+swap position 0 with position 4
+swap letter e with letter b
+rotate left 4 steps
+reverse positions 0 through 4
+rotate based on position of letter a
+rotate based on position of letter b
+rotate left 6 steps
+rotate based on position of letter d
+rotate left 7 steps
+swap letter c with letter d
+rotate left 3 steps
+move position 4 to position 6
+move position 3 to position 2
+reverse positions 0 through 6
diff --git a/2016/day-21/sample.txt b/2016/day-21/sample.txt
new file mode 100644
index 0000000..865c052
--- /dev/null
+++ b/2016/day-21/sample.txt
@@ -0,0 +1,8 @@
+swap position 4 with position 0
+swap letter d with letter b
+reverse positions 0 through 4
+rotate left 1 step
+move position 1 to position 4
+move position 3 to position 0
+rotate based on position of letter b
+rotate based on position of letter d
diff --git a/2016/day-21/src/main.rs b/2016/day-21/src/main.rs
new file mode 100644
index 0000000..5559584
--- /dev/null
+++ b/2016/day-21/src/main.rs
@@ -0,0 +1,166 @@
+use std::env;
+use std::io::prelude::*;
+use std::io::BufReader;
+use std::fs::File;
+
+fn vec_str(vec: &[char]) -> String
+{
+    return vec.iter().cloned().collect();
+}
+
+fn find(password: &[char], search: &str) -> usize
+{
+    let c = search.chars().next().unwrap();
+    for (index, b) in password.iter().enumerate() {
+        if c == *b {
+            return index;
+        }
+    }
+    panic!("{} not found in {}", c, vec_str(password));
+}
+
+fn rotate_left(password: &[char], amount: usize) -> Vec<char>
+{
+    let mut new_passwd = Vec::with_capacity(password.len());
+
+    new_passwd.extend_from_slice(&password[amount..]);
+    new_passwd.extend_from_slice(&password[..amount]);
+
+    return new_passwd;
+}
+
+fn reverse_range(password: &mut[char], pos_1: usize, pos_2: usize)
+{
+    password[pos_1..pos_2 + 1].reverse();
+}
+
+fn swap(password: &mut[char], by: &str, id_1: &str, id_2: &str)
+{
+    let pos_1: usize;
+    let pos_2: usize;
+    match by {
+        "position" => {
+            pos_1 = id_1.parse().unwrap();
+            pos_2 = id_2.parse().unwrap();
+        },
+        "letter" => {
+            pos_1 = find(password, id_1);
+            pos_2 = find(password, id_2);
+        },
+        _ => panic!("Can't swap {}", by),
+    }
+
+    password.swap(pos_1, pos_2);
+}
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    let f = File::open(&args[1]).expect("Could not open file");
+    let reader = BufReader::new(f);
+
+    let mut password: Vec<char> = "abcdefgh".chars().collect();
+    let mut rules = Vec::new();
+
+    for line in reader.lines() {
+        let contents = line.unwrap();
+        let parts: Vec<&str> = contents.split(" ").collect();
+        rules.push(contents.clone());
+
+        match parts[0] {
+            "swap" => {
+                swap(&mut password, &parts[1], &parts[2], &parts[5]);
+            },
+            "rotate" => {
+                let amount = match parts[1] {
+                    "left" => parts[2].parse().unwrap(),
+                    "right" => password.len() - parts[2].parse::<usize>().unwrap(),
+                    "based" => {
+                        let pos = find(&password, parts[6]);
+
+                        2 * password.len() - if pos >= 4 { pos + 2 } else { pos + 1 }
+                    },
+
+                    _ => panic!("Cannot rotate by {}", parts[2]),
+                } % password.len();
+                password = rotate_left(&password, amount);
+            },
+
+            "reverse" => {
+                let pos_1: usize = parts[2].parse().unwrap();
+                let pos_2: usize = parts[4].parse().unwrap();
+                reverse_range(&mut password, pos_1, pos_2);
+            },
+            "move" => {
+                let pos_1: usize = parts[2].parse().unwrap();
+                let pos_2: usize = parts[5].parse().unwrap();
+
+                let c = password.remove(pos_1);
+                password.insert(pos_2, c);
+            },
+            _ => panic!("Don't understand {}", parts[0]),
+        }
+    }
+
+    println!("Final password is {}", vec_str(&password));
+    let mut password: Vec<char> = "fbgdceah".chars().collect();
+
+    for line in rules.iter().rev() {
+        let parts: Vec<&str> = line.split(" ").collect();
+
+        match parts[0] {
+            "swap" => {
+                swap(&mut password, &parts[1], &parts[2], &parts[5]);
+            },
+            "rotate" => {
+                // invert regular rotations, and
+                let amount = match parts[1] {
+                    "right" => parts[2].parse().unwrap(),
+                    "left" => password.len() - parts[2].parse::<usize>().unwrap(),
+                    "based" => {
+                        let pos = find(&password, parts[6]);
+
+                        let mut original = usize::max_value();
+                        if pos % 2 == 1 {
+                            // original pos < 4
+                            for x in 0..4 {
+                                if (2 * x + 1) % password.len() == pos {
+                                    original = x + 1;
+                                    break;
+                                }
+                            }
+                        } else {
+                            // original pos >= 4
+                            for x in 4..password.len() {
+                                if (2 * x + 2) % password.len() == pos {
+                                    original = x + 2;
+                                }
+                            }
+                        }
+
+                        original
+                    },
+
+                    _ => panic!("Cannot rotate by {}", parts[2]),
+                } % password.len();
+                password = rotate_left(&password, amount);
+            },
+
+            "reverse" => {
+                let pos_1: usize = parts[2].parse().unwrap();
+                let pos_2: usize = parts[4].parse().unwrap();
+                reverse_range(&mut password, pos_1, pos_2);
+            },
+            "move" => {
+                let pos_1: usize = parts[2].parse().unwrap();
+                let pos_2: usize = parts[5].parse().unwrap();
+
+                let c = password.remove(pos_2);
+                password.insert(pos_1, c);
+            },
+            _ => panic!("Don't understand {}", parts[0]),
+        }
+    }
+
+    println!("Reversed password is {}", vec_str(&password));
+
+}