diff --git a/day12/Cargo.lock b/day12/Cargo.lock
new file mode 100644
index 0000000..f9a061e
--- /dev/null
+++ b/day12/Cargo.lock
@@ -0,0 +1,143 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "aho-corasick"
+version = "0.7.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "memchr 2.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "autocfg"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "day12"
+version = "0.1.0"
+dependencies = [
+ "lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "regex 1.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "lazy_static"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "memchr"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "num"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "num-bigint 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-complex 0.2.4 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-integer 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-iter 0.1.40 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-rational 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-bigint"
+version = "0.2.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-integer 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-complex"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-integer"
+version = "0.1.42"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-iter"
+version = "0.1.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-integer 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-rational"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-bigint 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-integer 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)",
+ "num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "num-traits"
+version = "0.2.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "regex"
+version = "1.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "aho-corasick 0.7.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "memchr 2.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "regex-syntax 0.6.13 (registry+https://github.com/rust-lang/crates.io-index)",
+ "thread_local 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.6.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
+[[package]]
+name = "thread_local"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[metadata]
+"checksum aho-corasick 0.7.6 (registry+https://github.com/rust-lang/crates.io-index)" = "58fb5e95d83b38284460a5fda7d6470aa0b8844d283a0b614b8535e880800d2d"
+"checksum autocfg 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "f8aac770f1885fd7e387acedd76065302551364496e46b3dd00860b2f8359b9d"
+"checksum lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
+"checksum memchr 2.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "3197e20c7edb283f87c071ddfc7a2cca8f8e0b888c242959846a6fce03c72223"
+"checksum num 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "b8536030f9fea7127f841b45bb6243b27255787fb4eb83958aa1ef9d2fdc0c36"
+"checksum num-bigint 0.2.5 (registry+https://github.com/rust-lang/crates.io-index)" = "f6f115de20ad793e857f76da2563ff4a09fbcfd6fe93cca0c5d996ab5f3ee38d"
+"checksum num-complex 0.2.4 (registry+https://github.com/rust-lang/crates.io-index)" = "b6b19411a9719e753aff12e5187b74d60d3dc449ec3f4dc21e3989c3f554bc95"
+"checksum num-integer 0.1.42 (registry+https://github.com/rust-lang/crates.io-index)" = "3f6ea62e9d81a77cd3ee9a2a5b9b609447857f3d358704331e4ef39eb247fcba"
+"checksum num-iter 0.1.40 (registry+https://github.com/rust-lang/crates.io-index)" = "dfb0800a0291891dd9f4fe7bd9c19384f98f7fbe0cd0f39a2c6b88b9868bbc00"
+"checksum num-rational 0.2.3 (registry+https://github.com/rust-lang/crates.io-index)" = "da4dc79f9e6c81bef96148c8f6b8e72ad4541caa4a24373e900a36da07de03a3"
+"checksum num-traits 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)" = "c62be47e61d1842b9170f0fdeec8eba98e60e90e5446449a0545e5152acd7096"
+"checksum regex 1.3.3 (registry+https://github.com/rust-lang/crates.io-index)" = "b5508c1941e4e7cb19965abef075d35a9a8b5cdf0846f30b4050e9b55dc55e87"
+"checksum regex-syntax 0.6.13 (registry+https://github.com/rust-lang/crates.io-index)" = "e734e891f5b408a29efbf8309e656876276f49ab6a6ac208600b4419bd893d90"
+"checksum thread_local 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "88ddf1ad580c7e3d1efff877d972bcc93f995556b9087a5a259630985c88ceab"
diff --git a/day12/Cargo.toml b/day12/Cargo.toml
new file mode 100644
index 0000000..6772fd6
--- /dev/null
+++ b/day12/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "day12"
+version = "0.1.0"
+authors = ["Tyler Hallada <tyler@hallada.net>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+lazy_static = "1.4.0"
+num = "0.2.1"
+regex = "1.3.3"
diff --git a/day12/input/input.txt b/day12/input/input.txt
new file mode 100644
index 0000000..c6134ab
--- /dev/null
+++ b/day12/input/input.txt
@@ -0,0 +1,4 @@
+<x=3, y=2, z=-6>
+<x=-13, y=18, z=10>
+<x=-8, y=-1, z=13>
+<x=5, y=10, z=4>
diff --git a/day12/input/test1.txt b/day12/input/test1.txt
new file mode 100644
index 0000000..89cc805
--- /dev/null
+++ b/day12/input/test1.txt
@@ -0,0 +1,4 @@
+<x=-1, y=0, z=2>
+<x=2, y=-10, z=-7>
+<x=4, y=-8, z=8>
+<x=3, y=5, z=-1>
diff --git a/day12/input/test2.txt b/day12/input/test2.txt
new file mode 100644
index 0000000..1078293
--- /dev/null
+++ b/day12/input/test2.txt
@@ -0,0 +1,4 @@
+<x=-8, y=-10, z=0>
+<x=5, y=5, z=10>
+<x=2, y=-7, z=3>
+<x=9, y=-8, z=-3>
diff --git a/day12/src/main.rs b/day12/src/main.rs
new file mode 100644
index 0000000..68eec99
--- /dev/null
+++ b/day12/src/main.rs
@@ -0,0 +1,418 @@
+#[macro_use]
+extern crate lazy_static;
+
+use std::collections::HashSet;
+use std::error::Error;
+use std::fs::File;
+use std::io::{prelude::*, BufReader};
+use std::ops::AddAssign;
+use std::ops::Index;
+use std::result;
+use std::str::FromStr;
+
+use num::integer::lcm;
+use regex::Regex;
+
+type Result<T> = result::Result<T, Box<dyn Error>>;
+
+const INPUT: &str = "input/input.txt";
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy, Hash)]
+struct Vector {
+    x: i64,
+    y: i64,
+    z: i64,
+}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+struct Body {
+    position: Vector,
+    velocity: Vector,
+}
+
+#[derive(Debug, PartialEq, Eq, Hash, Clone)]
+struct NBody {
+    bodies: Vec<Body>,
+}
+
+impl FromStr for Body {
+    type Err = Box<dyn Error>;
+
+    fn from_str(s: &str) -> Result<Body> {
+        lazy_static! {
+            static ref RE: Regex =
+                Regex::new(r"<x=(?P<x>-?\d+), y=(?P<y>-?\d+), z=(?P<z>-?\d+)>").unwrap();
+        }
+
+        let captures = match RE.captures(s) {
+            None => {
+                return Err(From::from("Malformed scan, no positions could be found"));
+            }
+            Some(captures) => captures,
+        };
+
+        Ok(Body {
+            position: Vector {
+                x: captures["x"].parse()?,
+                y: captures["y"].parse()?,
+                z: captures["z"].parse()?,
+            },
+            velocity: Vector::new(),
+        })
+    }
+}
+
+impl Vector {
+    fn new() -> Vector {
+        Vector { x: 0, y: 0, z: 0 }
+    }
+}
+
+impl AddAssign for Vector {
+    fn add_assign(&mut self, other: Self) {
+        *self = Self {
+            x: self.x + other.x,
+            y: self.y + other.y,
+            z: self.z + other.z,
+        }
+    }
+}
+
+impl Index<&str> for Vector {
+    type Output = i64;
+
+    fn index(&self, index: &str) -> &i64 {
+        match index {
+            "x" => &self.x,
+            "y" => &self.y,
+            "z" => &self.z,
+            _ => panic!("unknown field: {}", index),
+        }
+    }
+}
+
+impl Body {
+    fn add_gravity(&self, gravity: &mut Vector, other: &Self) {
+        if self.position.x > other.position.x {
+            gravity.x -= 1;
+        } else if self.position.x < other.position.x {
+            gravity.x += 1;
+        }
+
+        if self.position.y > other.position.y {
+            gravity.y -= 1;
+        } else if self.position.y < other.position.y {
+            gravity.y += 1;
+        }
+
+        if self.position.z > other.position.z {
+            gravity.z -= 1;
+        } else if self.position.z < other.position.z {
+            gravity.z += 1;
+        }
+    }
+}
+
+impl NBody {
+    fn run_step(&mut self) {
+        let mut gravities = Vec::new();
+        for body in self.bodies.iter() {
+            let mut gravity = Vector::new();
+            for other_body in self.bodies.iter() {
+                body.add_gravity(&mut gravity, other_body);
+            }
+            gravities.push(gravity);
+        }
+
+        for (index, gravity) in gravities.into_iter().enumerate() {
+            self.bodies[index].velocity += gravity;
+            let velocity = self.bodies[index].velocity;
+            self.bodies[index].position += velocity;
+        }
+    }
+
+    fn total_energy(&self) -> i64 {
+        let mut total_energy = 0;
+        for body in self.bodies.iter() {
+            let potential_energy =
+                body.position.x.abs() + body.position.y.abs() + body.position.z.abs();
+            let kinetic_energy =
+                body.velocity.x.abs() + body.velocity.y.abs() + body.velocity.z.abs();
+            total_energy += potential_energy * kinetic_energy;
+        }
+        total_energy
+    }
+
+    fn state(&self, component: &str) -> [(i64, i64); 4] {
+        [
+            (
+                self.bodies[0].position[component],
+                self.bodies[0].velocity[component],
+            ),
+            (
+                self.bodies[1].position[component],
+                self.bodies[1].velocity[component],
+            ),
+            (
+                self.bodies[2].position[component],
+                self.bodies[2].velocity[component],
+            ),
+            (
+                self.bodies[3].position[component],
+                self.bodies[3].velocity[component],
+            ),
+        ]
+    }
+}
+
+fn read_moon_scan(filename: &str) -> Result<NBody> {
+    let file = File::open(filename)?;
+    let reader = BufReader::new(file);
+    let mut moons = vec![];
+
+    for line in reader.lines() {
+        moons.push(line?.parse()?);
+    }
+
+    Ok(NBody { bodies: moons })
+}
+
+fn solve_part1(filename: &str) -> Result<i64> {
+    let mut nbody = read_moon_scan(filename)?;
+    for _ in 0..1000 {
+        nbody.run_step();
+    }
+    Ok(nbody.total_energy())
+}
+
+fn solve_part2(filename: &str) -> Result<u64> {
+    let mut step_count = 0;
+    let mut x_states: HashSet<[(i64, i64); 4]> = HashSet::new();
+    let mut y_states: HashSet<[(i64, i64); 4]> = HashSet::new();
+    let mut z_states: HashSet<[(i64, i64); 4]> = HashSet::new();
+    let mut x_repeated_step_count = None;
+    let mut y_repeated_step_count = None;
+    let mut z_repeated_step_count = None;
+    let mut nbody = read_moon_scan(filename)?;
+    while x_repeated_step_count == None
+        || y_repeated_step_count == None
+        || z_repeated_step_count == None
+    {
+        if x_repeated_step_count == None {
+            let x_state = nbody.state("x");
+            if x_states.contains(&x_state) {
+                x_repeated_step_count = Some(step_count);
+            } else {
+                x_states.insert(x_state);
+            }
+        }
+
+        if y_repeated_step_count == None {
+            let y_state = nbody.state("y");
+            if y_states.contains(&y_state) {
+                y_repeated_step_count = Some(step_count);
+            } else {
+                y_states.insert(y_state);
+            }
+        }
+
+        if z_repeated_step_count == None {
+            let z_state = nbody.state("z");
+            if z_states.contains(&z_state) {
+                z_repeated_step_count = Some(step_count);
+            } else {
+                z_states.insert(z_state);
+            }
+        }
+
+        nbody.run_step();
+        step_count += 1;
+    }
+
+    Ok(lcm(
+        x_repeated_step_count.unwrap(),
+        lcm(
+            y_repeated_step_count.unwrap(),
+            z_repeated_step_count.unwrap(),
+        ),
+    ))
+}
+
+fn main() -> Result<()> {
+    println!("Part 1: {}", solve_part1(INPUT)?);
+    println!("Part 2: {}", solve_part2(INPUT)?);
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST_INPUT1: &str = "input/test1.txt";
+    const TEST_INPUT2: &str = "input/test2.txt";
+    fn nbody_1() -> NBody {
+        NBody {
+            bodies: vec![
+                Body {
+                    position: Vector { x: -1, y: 0, z: 2 },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+                Body {
+                    position: Vector {
+                        x: 2,
+                        y: -10,
+                        z: -7,
+                    },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+                Body {
+                    position: Vector { x: 4, y: -8, z: 8 },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+                Body {
+                    position: Vector { x: 3, y: 5, z: -1 },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+            ],
+        }
+    }
+    fn nbody_1_after_10_steps() -> NBody {
+        NBody {
+            bodies: vec![
+                Body {
+                    position: Vector { x: 2, y: 1, z: -3 },
+                    velocity: Vector { x: -3, y: -2, z: 1 },
+                },
+                Body {
+                    position: Vector { x: 1, y: -8, z: 0 },
+                    velocity: Vector { x: -1, y: 1, z: 3 },
+                },
+                Body {
+                    position: Vector { x: 3, y: -6, z: 1 },
+                    velocity: Vector { x: 3, y: 2, z: -3 },
+                },
+                Body {
+                    position: Vector { x: 2, y: 0, z: 4 },
+                    velocity: Vector { x: 1, y: -1, z: -1 },
+                },
+            ],
+        }
+    }
+    fn nbody_2() -> NBody {
+        NBody {
+            bodies: vec![
+                Body {
+                    position: Vector {
+                        x: -8,
+                        y: -10,
+                        z: 0,
+                    },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+                Body {
+                    position: Vector { x: 5, y: 5, z: 10 },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+                Body {
+                    position: Vector { x: 2, y: -7, z: 3 },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+                Body {
+                    position: Vector { x: 9, y: -8, z: -3 },
+                    velocity: Vector { x: 0, y: 0, z: 0 },
+                },
+            ],
+        }
+    }
+    fn nbody_2_after_100_steps() -> NBody {
+        NBody {
+            bodies: vec![
+                Body {
+                    position: Vector {
+                        x: 8,
+                        y: -12,
+                        z: -9,
+                    },
+                    velocity: Vector { x: -7, y: 3, z: 0 },
+                },
+                Body {
+                    position: Vector {
+                        x: 13,
+                        y: 16,
+                        z: -3,
+                    },
+                    velocity: Vector {
+                        x: 3,
+                        y: -11,
+                        z: -5,
+                    },
+                },
+                Body {
+                    position: Vector {
+                        x: -29,
+                        y: -11,
+                        z: -1,
+                    },
+                    velocity: Vector { x: -3, y: 7, z: 4 },
+                },
+                Body {
+                    position: Vector {
+                        x: 16,
+                        y: -13,
+                        z: 23,
+                    },
+                    velocity: Vector { x: 7, y: 1, z: 1 },
+                },
+            ],
+        }
+    }
+
+    #[test]
+    fn reads_moon_scan_file() {
+        assert_eq!(read_moon_scan(TEST_INPUT1).unwrap(), nbody_1());
+        assert_eq!(read_moon_scan(TEST_INPUT2).unwrap(), nbody_2());
+    }
+
+    #[test]
+    fn runs_10_steps() {
+        let mut nbody = read_moon_scan(TEST_INPUT1).unwrap();
+        for _ in 0..10 {
+            nbody.run_step();
+        }
+        assert_eq!(nbody, nbody_1_after_10_steps());
+    }
+
+    #[test]
+    fn runs_100_steps() {
+        let mut nbody = read_moon_scan(TEST_INPUT2).unwrap();
+        for _ in 0..100 {
+            nbody.run_step();
+        }
+        assert_eq!(nbody, nbody_2_after_100_steps());
+    }
+
+    #[test]
+    fn calculates_total_energy_after_10_steps() {
+        let mut nbody = read_moon_scan(TEST_INPUT1).unwrap();
+        for _ in 0..10 {
+            nbody.run_step();
+        }
+        assert_eq!(nbody.total_energy(), 179);
+    }
+
+    #[test]
+    fn calculates_total_energy_after_100_steps() {
+        let mut nbody = read_moon_scan(TEST_INPUT2).unwrap();
+        for _ in 0..100 {
+            nbody.run_step();
+        }
+        assert_eq!(nbody.total_energy(), 1940);
+    }
+
+    #[test]
+    fn finds_repeated_states() {
+        assert_eq!(solve_part2(TEST_INPUT1).unwrap(), 2772);
+        assert_eq!(solve_part2(TEST_INPUT2).unwrap(), 4686774924);
+    }
+}