diff --git a/day21/day21.rb b/day21/day21.rb
new file mode 100644
index 0000000..64a6fd2
--- /dev/null
+++ b/day21/day21.rb
@@ -0,0 +1,88 @@
+#!/usr/bin/env ruby
+
+grid = [ ['.', '#', '.'],
+         ['.', '.', '#'],
+         ['#', '#', '#'] ]
+
+def string_to_grid(string)
+  string.chomp.split("/").map do |row|
+    row.split("")
+  end
+end
+
+def get_permutations(grid, depth = 0)
+  return [grid] if depth == 4
+  flipped = grid.map(&:reverse)
+  rotated = grid.transpose.map(&:reverse)
+  flipped_and_rotated = rotated.map(&:reverse)
+  [ grid, flipped, flipped_and_rotated ].concat(get_permutations(rotated, depth + 1)).uniq
+end
+
+def get_input_rules(filename)
+  sum = {}
+  File.readlines(filename).each do |line|
+    source, dest = line.split(" => ")
+    sgrid = string_to_grid(source)
+    dgrid = string_to_grid(dest)
+    get_permutations(sgrid).each do |perm|
+      sum[perm] = dgrid
+    end
+  end
+  sum
+end
+
+def break_grid_into(grid, size)
+  results = []
+  split = grid[0].size / size
+  split.times do |i|
+    grid[i * size, size].each_with_index do |row, index|
+      split.times do |j|
+        results[i] ||= []
+        results[i][j] ||= []
+        results[i][j] << row[j * size, size]
+      end
+    end
+  end
+  results
+end
+
+def recombine_grid(grid)
+  results = []
+  size = grid[0][0].size
+  grid.each_with_index do |row, k|
+    size.times do |i|
+      row_index = size * k + i
+      row.size.times do |j|
+        results[row_index] ||= []
+        results[row_index].concat(row[j][i])
+      end
+    end
+  end
+  results
+end
+
+def process_grid(grid, rules, depth = 5)
+  if depth == 0
+    grid.each { |row| puts row.join("") }
+    puts grid.flatten.select { |x| x == "#" }.size
+    return
+  end
+  new_size = grid[0].length.even? ? 2 : 3
+  new_grid = break_grid_into(grid, new_size)
+  mutated = new_grid.map do |row|
+    row.map do |entry|
+      if rules[entry].nil?
+        rules.each { |k, v| puts "#{k} => #{v}" }
+        raise "#{entry.inspect} has no match"
+      else
+        rules[entry]
+      end
+    end
+  end
+  mutated_grid = recombine_grid(mutated)
+  process_grid(mutated_grid, rules, depth - 1)
+end
+
+process_grid(grid, get_input_rules('sample'), 2)
+process_grid(grid, get_input_rules('problem'), 5)
+process_grid(grid, get_input_rules('problem'), 18)
diff --git a/day21/problem b/day21/problem
new file mode 100644
index 0000000..65f3e97
--- /dev/null
+++ b/day21/problem
@@ -0,0 +1,108 @@
+../.. => ..#/.#./...
+#./.. => .../#../.##
+##/.. => .##/###/##.
+.#/#. => #.#/..#/#.#
+##/#. => .../.##/...
+##/## => ##./..#/..#
+.../.../... => ##../..../##../.###
+#../.../... => ...#/.#.#/.#../.#.#
+.#./.../... => #.#./...#/#.#./.##.
+##./.../... => ..#./#.##/#.../.###
+#.#/.../... => ##../##.#/..#./#.##
+###/.../... => ..../.#.#/.###/#..#
+.#./#../... => #..#/#.../.##./....
+##./#../... => #.##/..##/####/.###
+..#/#../... => ..#./#.##/####/####
+#.#/#../... => .##./#.##/#.#./##.#
+.##/#../... => #.##/####/.###/...#
+###/#../... => ..../#.#./##.#/..##
+.../.#./... => .###/.##./##../.##.
+#../.#./... => ..../#.##/...#/#.#.
+.#./.#./... => ...#/####/.##./#...
+##./.#./... => .###/#.##/###./....
+#.#/.#./... => #.##/###./..../..#.
+###/.#./... => .#../#.#./#.##/#.##
+.#./##./... => .###/##../..##/#..#
+##./##./... => ..#./#.#./.#.#/##.#
+..#/##./... => .#../####/...#/..##
+#.#/##./... => ..../##.#/.##./....
+.##/##./... => .#.#/.#.#/.##./####
+###/##./... => ##.#/..../..../....
+.../#.#/... => ..##/##../##.#/###.
+#../#.#/... => ####/#.##/#.../###.
+.#./#.#/... => ..../#..#/..##/.#..
+##./#.#/... => #.../..##/##../..#.
+#.#/#.#/... => ...#/#.#./#.#./#...
+###/#.#/... => ###./###./##.#/###.
+.../###/... => ..#./###./##.#/####
+#../###/... => ##.#/..#./##../..##
+.#./###/... => #.../#.##/##../....
+##./###/... => ..##/.#.#/#..#/#.##
+#.#/###/... => #.##/..#./.#../..##
+###/###/... => ..#./#..#/####/.##.
+..#/.../#.. => ##.#/#.##/...#/###.
+#.#/.../#.. => #..#/..#./##../###.
+.##/.../#.. => ..#./.#../###./#.#.
+###/.../#.. => ...#/...#/.#.#/.##.
+.##/#../#.. => ##../#.#./#..#/##..
+###/#../#.. => ##../.#.#/##../#..#
+..#/.#./#.. => ##.#/##.#/...#/.#..
+#.#/.#./#.. => .###/.#.#/###./....
+.##/.#./#.. => #..#/###./####/..#.
+###/.#./#.. => ..#./.###/.###/...#
+.##/##./#.. => #.##/..##/...#/.###
+###/##./#.. => ####/##.#/#.##/#..#
+#../..#/#.. => ..../.##./#.##/#...
+.#./..#/#.. => #..#/##../...#/#...
+##./..#/#.. => ..#./.###/..##/.#.#
+#.#/..#/#.. => .##./..##/..#./#..#
+.##/..#/#.. => ####/.#.#/#.../.#.#
+###/..#/#.. => ..../..##/#.##/###.
+#../#.#/#.. => #.##/.#.#/.#../.##.
+.#./#.#/#.. => ..##/###./.###/###.
+##./#.#/#.. => ##.#/##.#/#.#./##..
+..#/#.#/#.. => ###./###./.#.#/.#..
+#.#/#.#/#.. => ##../..#./##../....
+.##/#.#/#.. => .###/#.#./##.#/##..
+###/#.#/#.. => ##.#/#.#./.#.#/#...
+#../.##/#.. => .#.#/...#/.#.#/..#.
+.#./.##/#.. => ###./##../##.#/....
+##./.##/#.. => ..##/###./#.#./#.#.
+#.#/.##/#.. => ##.#/..##/#..#/####
+.##/.##/#.. => ..../####/..#./##..
+###/.##/#.. => .###/#..#/..../.#..
+#../###/#.. => #..#/.#../.#.#/#...
+.#./###/#.. => .#../..../.##./.###
+##./###/#.. => ##.#/.#../.#.#/#..#
+..#/###/#.. => #.##/##../..##/#...
+#.#/###/#.. => ####/..##/.#../##.#
+.##/###/#.. => .###/#..#/.###/#.##
+###/###/#.. => ..##/.##./##../#..#
+.#./#.#/.#. => ..##/.##./.##./.###
+##./#.#/.#. => ..##/...#/.##./####
+#.#/#.#/.#. => .###/.###/#.#./.#..
+###/#.#/.#. => ##.#/###./##.#/####
+.#./###/.#. => ...#/..#./.#.#/.#..
+##./###/.#. => ###./##.#/#.../#.#.
+#.#/###/.#. => .##./#.#./...#/..#.
+###/###/.#. => .#.#/.#../..##/####
+#.#/..#/##. => .##./...#/#..#/.###
+###/..#/##. => #.##/.#.#/...#/..##
+.##/#.#/##. => ###./.###/...#/....
+###/#.#/##. => .##./.##./#.#./#...
+#.#/.##/##. => #.#./.##./.#.#/.###
+###/.##/##. => ..../####/.#.#/#.##
+.##/###/##. => .##./.###/###./.#..
+###/###/##. => #.../###./.##./##.#
+#.#/.../#.# => #.#./..../#.##/###.
+###/.../#.# => .#../.#.#/#.../.###
+###/#../#.# => ###./#..#/####/##..
+#.#/.#./#.# => ###./##.#/..../.#..
+###/.#./#.# => ####/.#.#/.#../..##
+###/##./#.# => #.#./####/..##/#...
+#.#/#.#/#.# => #.#./#.#./#.../#.##
+###/#.#/#.# => #.##/.#../..#./.##.
+#.#/###/#.# => .###/..##/####/#..#
+###/###/#.# => #.../..#./..#./#.##
+###/#.#/### => .#.#/.###/#.##/..##
+###/###/### => #.#./...#/.#../.#.#
diff --git a/day21/sample b/day21/sample
new file mode 100644
index 0000000..f6d24df
--- /dev/null
+++ b/day21/sample
@@ -0,0 +1,2 @@
+../.# => ##./#../...
+.#./..#/### => #..#/..../..../#..#