diff --git a/2020/ruby/day_20.rb b/2020/ruby/day_20.rb
index 7c89960..bc0e80d 100644
--- a/2020/ruby/day_20.rb
+++ b/2020/ruby/day_20.rb
@@ -1,19 +1,41 @@
+# all rotations are clockwise
+# all flips are vertical
+
 class Tile
-  SIZE = 10
+  attr_reader :id, :raw
 
-  attr_reader :raw
+  def initialize(id, raw)
+    @id, @raw = id, raw
+  end
 
-  def initialize(raw)
-    @raw = raw
+  def [](y, x)
+    @raw.fetch(y).fetch(x)
   end
 
-  def edges
+  def edge_keys
     {
-      top: @raw.fetch(0),
-      bottom: @raw.fetch(SIZE-1),
-      left: @raw.transpose.fetch(0),
-      right: @raw.transpose.fetch(SIZE-1),
-    }
+      top: @raw.first,
+      down: @raw.last,
+      left: @raw.transpose.first,
+      right: @raw.transpose.last,
+    }.transform_values(&:join)
+  end
+
+  def permutations
+    (0..3).map {|i|
+      i.times.inject(self) {|x,_| x.rotated }
+    }.flat_map {|x| [x, x.flipped] }
+  end
+
+  def rotated
+    self.class.new(
+      @id,
+      @raw.map {|row| [row].transpose }.reverse.inject(&:zip).map(&:flatten),
+    )
+  end
+
+  def flipped
+    self.class.new(@id, @raw.reverse)
   end
 
   def to_s
@@ -21,35 +43,74 @@ class Tile
   end
 end
 
-input = ARGF.read
+Edge = Struct.new(*%i[ key from to rotations is_mirrored ]) do
+  def mirrored?
+    is_mirrored
+  end
+end
+
+tiles = ARGF.read
   .scan(/Tile (\d+):\n((?:[.#]+\n)+)/m)
   .to_h
-  .transform_keys(&:to_i)
   .transform_values {|v| v.split("\n").map {|row| row.chars } }
-  .transform_values {|v| Tile.new(v) }
+  .map {|k,v| [k, Tile.new(k, v)] }
+  .to_h
+
+keys_to_tiles = Hash.new {|h,k| h[k] = [] }
+tiles.values.each do |tile|
+  tile.edge_keys.values.each do |key|
+    keys_to_tiles[key] << tile.id
+    keys_to_tiles[key.reverse] << tile.id
+  end
+end
 
-edges = input.flat_map {|id,tile|
-  tile.edges.map {|edge,data| ["#{id}_#{edge}", data.join] }
-}.to_h
+start_corner = tiles.values.find {|tile|
+  tile.edge_keys.values_at(:left, :top).all? {|key| keys_to_tiles.fetch(key).size == 1 }
+}
 
-inverted_edges = edges.each.with_object(Hash.new {|h,k| h[k] = [] }) do |(id,data),hash|
-  hash[data] << id
+image = [[start_corner]]
+until image.map(&:size).sum == tiles.size
+  current = image.last.last
+  right_key = current.edge_keys.fetch(:right)
+  if right_id = keys_to_tiles.fetch(right_key, nil).find {|id| id != current.id }
+    right_tile = tiles.fetch(right_id).permutations.find {|p| p.edge_keys.fetch(:left) == right_key }
+    image.last << right_tile
+  else
+    current = image.last.first
+    down_key = current.edge_keys.fetch(:down)
+    break unless down_id = keys_to_tiles.fetch(down_key).find {|id| id != current.id }
+
+    down_tile = tiles.fetch(down_id).permutations.find {|p| p.edge_keys.fetch(:top) == down_key }
+    image << [down_tile]
+  end
 end
 
-matches = edges.map {|id,data|
-  [
-    id,
-    [
-      inverted_edges[data],
-      inverted_edges[data.reverse],
-    ].flatten.compact.reject {|other| other == id },
-  ]
-}.to_h
-
-corners = input.keys.select {|id|
-  %w[ top bottom left right ].count {|edge|
-    matches.fetch("#{id}_#{edge}").empty?
-  } == 2
+full_raw = image.map {|row| row.map {|tile|
+  tile.raw[1..-2].map {|row| row[1..-2] }
+}}.flat_map {|row| row.inject(&:zip).map(&:flatten) }
+full_tile = Tile.new(nil, full_raw)
+
+sea_monster = <<~SEA_MONSTER.split("\n").map(&:chars)
+                  # 
+#    ##    ##    ###
+ #  #  #  #  #  #   
+SEA_MONSTER
+needle = sea_monster.flat_map.with_index {|row,y|
+  row.filter_map.with_index {|c,x|
+    c == ?# ? [y,x] : nil
+  }
 }
 
-p corners.inject(&:*)
+num_monsters = full_tile.permutations.map {|tile|
+  needle_maxes = needle.transpose.map(&:max)
+  haystack = (0...tile.raw.size-needle_maxes.first).flat_map {|y|
+    (0...tile.raw.first.size-needle_maxes.last).map {|x|
+      [y, x]
+    }
+  }
+  haystack.count {|y,x|
+    needle.all? {|dy,dx| tile[y+dy, x+dx] == ?# }
+  }
+}.sum
+
+puts full_tile.raw.map {|row| row.count(?#) }.sum - (num_monsters * needle.size)