diff --git a/2019/day10/part1.rb b/2019/day10/part1.rb
new file mode 100644
index 0000000..4a1f3ca
--- /dev/null
+++ b/2019/day10/part1.rb
@@ -0,0 +1,116 @@
+# frozen_string_literal: true
+
+input = File.read(File.join(__dir__, 'input'))
+
+class AsteroidBelt
+  def initialize(text)
+    @grid = parse(text)
+  end
+
+  def asteroids
+    return @asteroids unless @asteroids.nil?
+
+    @asteroids = []
+    @grid.each_with_index do |line, y|
+      line.chars.each_with_index do |cell, x|
+        @asteroids << Asteroid.new(x, y) if cell == '#'
+      end
+    end
+    @asteroids
+  end
+
+  private
+
+  def parse(text)
+    text.lines
+  end
+end
+
+class Asteroid
+  attr_reader :x, :y
+
+  def initialize(x, y)
+    @x = x
+    @y = y
+  end
+
+  def neighbors(asteroids)
+    by_direction = {}
+    asteroids.each do |asteroid|
+      next if asteroid == self
+
+      theta = direction_to(asteroid).normalize
+      # Since we only care about count, we don't care *which* one we can see at
+      # in a given direction.  If we do care about it, sort by distance and
+      # take the closer one.
+      by_direction[theta] = asteroid
+    end
+    by_direction
+  end
+
+  def direction_to(other)
+    dx = other.x - x
+    dy = other.y - y
+    Vector.new(dx, dy)
+  end
+
+  def inspect
+    "<Asteroid: #{x},#{y}>"
+  end
+
+  def to_s
+    "(#{x},#{y})"
+  end
+end
+
+class Vector
+  attr_reader :x, :y
+
+  def initialize(x, y, normal = false)
+    @x = x
+    @y = y
+    @normal = normal
+  end
+
+  def normalize
+    return self if @normal
+
+    div = gcd(@x.abs, @y.abs)
+    Vector.new(Rational(@x / div), Rational(@y / div), true)
+  end
+
+  def ==(other)
+    s = normalize
+    o = other.normalize
+
+    s.x == o.x && s.y == o.y
+  end
+
+  def eql?(other)
+    self == other
+  end
+
+  def hash
+    n = normalize
+    [n.x, n.y].hash
+  end
+
+  def inspect
+    "<Vector: #{x},#{y}>"
+  end
+end
+
+def gcd(n, m)
+  # Invariant: n >= m
+  n, m = m, n if n < m
+  n, m = m, n % m until m.zero?
+  n
+end
+
+belt = AsteroidBelt.new input
+
+best = belt.asteroids.max_by do |asteroid|
+  asteroid.neighbors(belt.asteroids).count
+end
+pp best
+pp best.neighbors(belt.asteroids).count