diff --git a/2019/day12/part1.rb b/2019/day12/part1.rb
new file mode 100644
index 0000000..64be5fa
--- /dev/null
+++ b/2019/day12/part1.rb
@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+Position = Struct.new(:x, :y, :z) do
+  def to_s
+    "(#{x}, #{y}, #{z})"
+  end
+
+  def plus(v)
+    Position.new(x + v.x, y + v.y, z + v.z)
+  end
+end
+
+Velocity = Struct.new(:x, :y, :z) do
+  def to_s
+    "(#{x}, #{y}, #{z})"
+  end
+
+  def plus(dx, dy, dz)
+    Velocity.new(x + dx, y + dy, z + dz)
+  end
+end
+
+Moon = Struct.new(:pos, :vel) do
+  def to_s
+    "pos=#{pos} vel=#{vel}"
+  end
+
+  def energy
+    potential_energy * kinetic_energy
+  end
+
+  def potential_energy
+    pos.x.abs + pos.y.abs + pos.z.abs
+  end
+
+  def kinetic_energy
+    vel.x.abs + vel.y.abs + vel.z.abs
+  end
+end
+
+def parse(input)
+  positions = []
+  input.lines.each do |line|
+    coords = {}
+    line.strip[1...-1].split(',').each do |chunk|
+      key, val = chunk.split('=')
+      coords[key.strip] = val.to_i
+    end
+    positions << Position.new(coords['x'], coords['y'], coords['z'])
+  end
+  positions
+end
+
+def pairs_with_indices(arr)
+  (0...(arr.length)).each do |i|
+    ((i + 1)...(arr.length)).each do |j|
+      yield(arr[i], i, arr[j], j)
+    end
+  end
+end
+
+def gravity(moons)
+  dvs = 4.times.map { Velocity.new(0, 0, 0) }
+  pairs_with_indices(moons) do |a, i, b, j|
+    # From a's perspective
+    dx = b.pos.x <=> a.pos.x
+    dy = b.pos.y <=> a.pos.y
+    dz = b.pos.z <=> a.pos.z
+    dvs[i] = dvs[i].plus(dx, dy, dz)
+    dvs[j] = dvs[j].plus(-dx, -dy, -dz)
+  end
+  moons.each_with_index.map do |moon, i|
+    dv = dvs[i]
+    Moon.new(moon.pos, moon.vel.plus(dv.x, dv.y, dv.z))
+  end
+end
+
+def move(moons)
+  moons.map do |moon|
+    Moon.new(moon.pos.plus(moon.vel), moon.vel)
+  end
+end
+
+input = File.read(File.join(__dir__, 'input'))
+positions = parse(input)
+moons = positions.map { |pos| Moon.new(pos, Velocity.new(0, 0, 0)) }
+
+1000.times do
+  moons = move(gravity(moons))
+end
+
+puts moons.sum(&:energy)