diff --git a/2018/ruby/day_15.rb b/2018/ruby/day_15.rb
new file mode 100644
index 0000000..154d324
--- /dev/null
+++ b/2018/ruby/day_15.rb
@@ -0,0 +1,94 @@
+require "minitest"
+
+class Combat
+end
+
+class Grid
+  def self.parse(s)
+    occupied = {}
+    s.chomp.lines.each.with_index do |row, y|
+      row.chomp.chars.each.with_index do |pos, x|
+        case pos
+        when ?#
+          occupied[[y, x]] = :wall
+        when ?E
+          occupied[[y, x]] = Elf.new
+        when ?G
+          occupied[[y, x]] = Goblin.new
+        when ?.
+        else
+          fail "Invalid character: #{pos.inspect}"
+        end
+      end
+    end
+    self.new(occupied)
+  end
+
+  def initialize(occupied)
+    @occupied = occupied
+  end
+
+  def [](pos)
+    y, x = pos
+    @occupied[[y, x]]
+  end
+
+  def to_s
+    max_y = @occupied.keys.map(&:first).max
+    max_x = @occupied.keys.map(&:last).max
+    (0..max_y).map {|y|
+      (0..max_x).map {|x|
+        case @occupied[[y, x]]
+        when :wall
+          ?#
+        when Elf
+          ?E
+        when Goblin
+          ?G
+        when nil
+          ?.
+        else
+          fail "Unexpected object: #{@occupied[[y, x]]}"
+        end
+      }.join
+    }.join(?\n)
+  end
+end
+
+class TestGrid < Minitest::Test
+  def test_parse
+    grid = Grid.parse(<<~GRID)
+      #######
+      #E..G.#
+      #...#.#
+      #.G.#G#
+      #######
+    GRID
+
+    assert_equal :wall, grid[[0,0]]
+    assert_equal :wall, grid[[2,4]]
+    assert_equal :wall, grid[[4,6]]
+
+    assert_instance_of Elf, grid[[1,1]]
+    assert_instance_of Goblin, grid[[1,4]]
+    assert_instance_of Goblin, grid[[3,5]]
+
+    assert_nil grid[[1,2]]
+    assert_nil grid[[2,5]]
+  end
+end
+
+class Unit
+end
+
+class Elf < Unit
+end
+
+class Goblin < Unit
+end
+
+if __FILE__ == $0
+  require "minitest/autorun" and exit if ENV["TEST"]
+
+
+end