diff --git a/pissoir/Pissoir.hs b/pissoir/Pissoir.hs
new file mode 100644
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--- /dev/null
+++ b/pissoir/Pissoir.hs
@@ -0,0 +1,60 @@
+module Pissoir where
+
+import Data.Tree
+import Data.List
+import Data.Function (on)
+import qualified Data.Set as Set
+
+type Urinal = Int
+
+-- |Occupied ∪ Free| = n (size of pissoir)
+-- Occupied ∩ Free = ∅
+data Pissoir = Pissoir {occupied :: Set.Set Urinal, free :: Set.Set Urinal}
+  deriving (Show)
+
+maximumsBy :: (Ord o, Eq a) => (a -> o) -> [a] -> [a]
+maximumsBy f xs = filter ((== theMaximum) . f) xs
+  where theMaximum = maximum $ map f xs
+
+mkPissoir :: Int -> Pissoir
+mkPissoir number = Pissoir {occupied = Set.empty, free = Set.fromList [1..number]}
+
+next :: Pissoir -> [Pissoir]
+next pissoir
+  | Set.null (occupied pissoir) = insertPossibilites (Set.toList (free pissoir))
+  | Set.null (free pissoir) = []
+  | otherwise =
+      let distances = (\freeOne -> map (abs . subtract freeOne) $ Set.toList $ occupied pissoir)
+          urinals = maximumsBy (minimum . distances) $ Set.toList $ free pissoir
+      in insertPossibilites urinals
+  where
+    insertPossibilites =
+      map
+        (\urinal ->
+          pissoir
+          { occupied = Set.insert urinal (occupied pissoir)
+          , free = Set.delete urinal (free pissoir)
+          }
+        )
+
+renderPissoir :: Pissoir -> String
+renderPissoir pissoir =
+  map
+    (\x ->
+      if Set.member x (occupied pissoir)
+        then '■'
+        else if Set.member x (free pissoir)
+          then '□'
+          else '?')
+    allUrinals
+  where allUrinals = sort (Set.toList (Set.union (occupied pissoir) (free pissoir)))
+
+pissoirTree :: Pissoir -> Tree Pissoir
+pissoirTree initialPissoir = Node initialPissoir (map pissoirTree $ next initialPissoir)
+
+printPissoirTree :: Tree Pissoir -> IO ()
+printPissoirTree = putStrLn . drawTree . fmap renderPissoir
+
+-- [1,2,4,8,20,48,216,576,1392,7200,43200,184320,
+pissoirProblemN :: Int -> Int
+pissoirProblemN = length . last . levels . pissoirTree . mkPissoir
diff --git a/pissoir/README.md b/pissoir/README.md
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index 0000000..6ecfe08
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+++ b/pissoir/README.md
@@ -0,0 +1 @@
+See [here](https://www.youtube.com/watch?v=a36zHPlbd2g) for context.
diff --git a/pissoir/shell.nix b/pissoir/shell.nix
new file mode 100644
index 0000000..7cf7f41
--- /dev/null
+++ b/pissoir/shell.nix
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+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  buildInputs = [ pkgs.ghc ];
+}