diff --git a/2019/day12/input b/2019/day12/input
new file mode 100644
index 0000000..1754c6a
--- /dev/null
+++ b/2019/day12/input
@@ -0,0 +1,4 @@
+<x=15, y=-2, z=-6>
+<x=-5, y=-4, z=-11>
+<x=0, y=-6, z=0>
+<x=5, y=9, z=6>
diff --git a/2019/day12/main.go b/2019/day12/main.go
new file mode 100644
index 0000000..be327ef
--- /dev/null
+++ b/2019/day12/main.go
@@ -0,0 +1,178 @@
+package main
+
+import (
+	"fmt"
+
+	helpers "git.bullercodeworks.com/brian/adventofcode/helpers"
+)
+
+func main() {
+	inp := helpers.StdinToStringSlice()
+
+	part1(inp)
+	part2(inp)
+}
+
+func part1(inp []string) {
+	s := NewSystem(inp)
+	for i := 1; i <= 1000; i++ {
+		s.Step()
+	}
+	fmt.Println("System Energy:", s.GetTotalEnergy())
+}
+
+func part2(inp []string) {
+	s := NewSystem(inp)
+	sInit := NewSystem(inp)
+	// Find how long it takes for all planets to repeat
+	done := make([]bool, 3)
+	count := make([]int, 3)
+	for !AllTrue(done...) {
+		s.Step()
+		for i := 0; i < len(done); i++ {
+			if !done[i] {
+				equal := true
+				for pi := range s.moons {
+					switch i {
+					case 0:
+						if s.moons[pi].posX != sInit.moons[pi].posX || s.moons[pi].velX != sInit.moons[pi].velX {
+							equal = false
+						}
+					case 1:
+						if s.moons[pi].posY != sInit.moons[pi].posY || s.moons[pi].velY != sInit.moons[pi].velY {
+							equal = false
+						}
+					case 2:
+						if s.moons[pi].posZ != sInit.moons[pi].posZ || s.moons[pi].velZ != sInit.moons[pi].velZ {
+							equal = false
+						}
+					}
+				}
+				if equal {
+					done[i] = true
+				} else {
+					count[i]++
+				}
+			}
+		}
+	}
+	fmt.Println(count[0], count[1], count[2])
+	fmt.Println(helpers.Lcm(count[0]+1, count[1]+1, count[2]+1))
+}
+
+func AllTrue(done ...bool) bool {
+	for _, v := range done {
+		if !v {
+			return false
+		}
+	}
+	return true
+}
+
+type System struct {
+	moons []*Moon
+}
+
+func NewSystem(inp []string) *System {
+	s := &System{}
+	for k := range inp {
+		var x, y, z int
+		_, err := fmt.Sscanf(inp[k], "<x=%d, y=%d, z=%d>", &x, &y, &z)
+		if err != nil {
+			panic(err)
+		}
+		s.AddMoon(NewMoon(x, y, z))
+	}
+	return s
+}
+
+func (s *System) AddMoon(m *Moon) {
+	s.moons = append(s.moons, m)
+}
+
+func (s *System) Step() {
+	for i := range s.moons {
+		for j := i + 1; j < len(s.moons); j++ {
+			s.moons[i].Interact(s.moons[j])
+		}
+	}
+	for i := range s.moons {
+		s.moons[i].Step()
+	}
+}
+
+func (s *System) GetTotalEnergy() int {
+	var ret int
+	for v := range s.moons {
+		ret = ret + (s.moons[v].GetPotentialEnergy() * s.moons[v].GetKineticEnergy())
+	}
+	return ret
+}
+
+func (s System) String() string {
+	var ret string
+	for k := range s.moons {
+		ret = ret + s.moons[k].String() + "\n"
+	}
+	return ret
+}
+
+type Moon struct {
+	posX, posY, posZ int
+	velX, velY, velZ int
+}
+
+func NewMoon(x, y, z int) *Moon {
+	return &Moon{
+		posX: x, posY: y, posZ: z,
+	}
+}
+
+func (m *Moon) GetPotentialEnergy() int {
+	return helpers.AbsInt(m.posX) + helpers.AbsInt(m.posY) + helpers.AbsInt(m.posZ)
+}
+
+func (m *Moon) GetKineticEnergy() int {
+	return helpers.AbsInt(m.velX) + helpers.AbsInt(m.velY) + helpers.AbsInt(m.velZ)
+}
+
+func (m *Moon) Interact(n *Moon) {
+	if n.posX > m.posX {
+		n.velX--
+		m.velX++
+	} else if n.posX < m.posX {
+		n.velX++
+		m.velX--
+	}
+	if n.posY > m.posY {
+		n.velY--
+		m.velY++
+	} else if n.posY < m.posY {
+		n.velY++
+		m.velY--
+	}
+	if n.posZ > m.posZ {
+		n.velZ--
+		m.velZ++
+	} else if n.posZ < m.posZ {
+		n.velZ++
+		m.velZ--
+	}
+}
+
+func (m *Moon) Step() {
+	m.posX += m.velX
+	m.posY += m.velY
+	m.posZ += m.velZ
+}
+
+func (m *Moon) PosEquals(n *Moon) (bool, bool, bool) {
+	return (m.posX == n.posX), (m.posY == n.posY), (m.posZ == n.posZ)
+}
+
+func (m Moon) String() string {
+	return fmt.Sprintf("pos=<x=%3d, y=%3d, z=%3d>, vel=<x=%3d, y=%3d, z=%3d>",
+		m.posX, m.posY, m.posZ,
+		m.velX, m.velY, m.velZ,
+	)
+}
diff --git a/2019/day12/testinput1 b/2019/day12/testinput1
new file mode 100644
index 0000000..89cc805
--- /dev/null
+++ b/2019/day12/testinput1
@@ -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/2019/day12/testinput2 b/2019/day12/testinput2
new file mode 100644
index 0000000..aaff01e
--- /dev/null
+++ b/2019/day12/testinput2
@@ -0,0 +1,4 @@
+<x=-8, y=-10, z=0>
+<x=5, y=5, z=10>
+<x=1, y=-7, z=3>
+<x=9, y=-8, z=-3>
diff --git a/helpers/helpers.go b/helpers/helpers.go
index 6369216..9a7432a 100644
--- a/helpers/helpers.go
+++ b/helpers/helpers.go
@@ -27,6 +27,23 @@ const (
 	MIN_INT = -MAX_INT - 1
 )
 
+// Find the greatest common denominator
+func Gcd(x, y int) int {
+	for y != 0 {
+		x, y = y, x%y
+	}
+	return x
+}
+
+// Find the least common multiple, using gcd
+func Lcm(a, b int, integers ...int) int {
+	result := a * b / Gcd(a, b)
+	for i := 0; i < len(integers); i++ {
+		result = Lcm(result, integers[i])
+	}
+	return result
+}
+
 func AbsInt(i int) int {
 	if i < 0 {
 		return i * -1