diff --git a/lib/common.ex b/lib/common.ex
index b30c0ea..1fab35c 100644
--- a/lib/common.ex
+++ b/lib/common.ex
@@ -47,4 +47,14 @@ defmodule Aoc2023.Common do
   end
 
   def transpose(l), do: l |> Enum.zip |> Enum.map(&Tuple.to_list/1)
+
+  def collapse(enum, v) do
+    enum
+    |> Stream.zip(enum |> Stream.drop(1) |> Stream.concat([nil]))
+    |> Stream.filter(fn {a, b} -> a != v || b != v end)
+    |> Stream.map(pipe(elem(0)))
+  end
+
+  def repeat(lst, 0), do: []
+  def repeat(lst, n), do: Enum.concat(lst, repeat(lst, n - 1))
 end
diff --git a/lib/day12.ex b/lib/day12.ex
new file mode 100644
index 0000000..0d040a4
--- /dev/null
+++ b/lib/day12.ex
@@ -0,0 +1,93 @@
+defmodule Aoc2023.Day12 do
+  use Aoc2023
+
+  defmodule Cache do
+    use GenServer
+
+    def start do
+      {:ok, pid} = GenServer.start_link(Cache, nil)
+      pid
+    end
+
+    def memoize(cache, key, fun) do
+      got = GenServer.call(cache, {:get, key})
+      if got == nil do
+        val = fun.(key)
+        GenServer.call(cache, {:set, key, val})
+        val
+      else
+        got
+      end
+    end
+
+    @impl true
+    def init(_) do
+      {:ok, %{}}
+    end
+
+    @impl true
+    def handle_call({:get, key}, _, state) do
+      case state do
+        %{^key => value} ->
+          {:reply, value, state}
+
+        _ ->
+          {:reply, nil, state}
+      end
+    end
+
+    @impl true
+    def handle_call({:set, key, value}, _, state) do
+      {:reply, nil, Map.put_new(state, key, value)}
+    end
+  end
+
+  def parse(input) do
+    input
+    |> lines
+    |> Enum.map(
+      pipe(
+        String.split()
+        |> then(fn [rec, blocks] ->
+          {rec |> String.to_charlist(),
+           blocks |> String.split(",") |> Enum.map(&String.to_integer/1)}
+        end)
+      )
+    )
+  end
+
+  defp possibilities({rec, blocks}) do
+    loop = fn recur, arg ->
+      case arg do
+        {rec, []} -> if Enum.all?(rec, &(&1 in [??, ?.])) do 1 else 0 end
+        {rec, [block | blocks]} ->
+          # Expect rec to start with any number of ./?s up to the max allowing all remaining groups
+          # i.e. their lengths + an undamaged seperator
+          0..(length(rec) - Enum.sum(blocks) - length(blocks) - block)
+          |> Enum.map(fn i -> repeat('.', i) ++ repeat('#', block) ++ '.' end)
+          |> Enum.filter(pipe(Enum.zip_with(rec, &(&1 == &2 || &2 == ??)) |> Enum.all?))
+          |> Enum.map(&(recur.(recur, {Enum.drop(rec, length(&1)), blocks})))
+          |> Enum.sum
+      end
+    end
+    cache = Cache.start()
+    memoized = fn recur, arg -> Cache.memoize(cache, arg, fn arg -> loop.(recur, arg) end) end
+    memoized.(memoized, {rec, blocks})
+  end
+
+  def part1(input) do
+    input
+    |> Enum.map(pipe(possibilities()))
+    |> Enum.sum()
+  end
+
+  def part2(input) do
+    input
+    |> Enum.map(fn {rec, blocks} -> {
+        [rec] |> repeat(5) |> Enum.intersperse('?') |> Enum.flat_map(&id/1),
+        repeat(blocks, 5)
+      } end)
+    |> Enum.map(&possibilities/1)
+    |> Enum.sum()
+  end
+end
diff --git a/lib/mix_tasks.ex b/lib/mix_tasks.ex
index acdc81b..133e15e 100644
--- a/lib/mix_tasks.ex
+++ b/lib/mix_tasks.ex
@@ -13,6 +13,7 @@ defmodule Mix.Tasks.Aoc do
   defp module(9), do: Aoc2023.Day9
   defp module(10), do: Aoc2023.Day10
   defp module(11), do: Aoc2023.Day11
+  defp module(12), do: Aoc2023.Day12
   # [MODULE INSERTION POINT]  
 
   defp base_dir(), do: System.get_env("AOC_BASE")
diff --git a/tests/day12/1 b/tests/day12/1
new file mode 100644
index 0000000..e925935
--- /dev/null
+++ b/tests/day12/1
@@ -0,0 +1,6 @@
+???.### 1,1,3
+.??..??...?##. 1,1,3
+?#?#?#?#?#?#?#? 1,3,1,6
+????.#...#... 4,1,1
+????.######..#####. 1,6,5
+?###???????? 3,2,1
diff --git a/tests/day12/1.1 b/tests/day12/1.1
new file mode 100644
index 0000000..aabe6ec
--- /dev/null
+++ b/tests/day12/1.1
@@ -0,0 +1 @@
+21
diff --git a/tests/day12/1.2 b/tests/day12/1.2
new file mode 100644
index 0000000..aa2eea0
--- /dev/null
+++ b/tests/day12/1.2
@@ -0,0 +1 @@
+525152