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+{-# LANGUAGE TemplateHaskell #-}
+--------------------------------------------------------------------------------
+-- |
+-- Module      : Xanthous.Entities.Common
+-- Description : Common data type definitions and utilities for entities
+--
+--------------------------------------------------------------------------------
+module Xanthous.Entities.Common
+  ( -- * Inventory
+    Inventory(..)
+  , backpack
+  , wielded
+  , items
+  , InventoryPosition(..)
+  , describeInventoryPosition
+  , inventoryPosition
+  , itemsWithPosition
+  , removeItemFromPosition
+
+    -- ** Wielded items
+  , Wielded(..)
+  , hands
+  , leftHand
+  , rightHand
+  , inLeftHand
+  , inRightHand
+  , doubleHanded
+  , wieldedItems
+  , WieldedItem(..)
+  , wieldedItem
+  , wieldableItem
+  , asWieldedItem
+  ) where
+--------------------------------------------------------------------------------
+import           Xanthous.Prelude
+--------------------------------------------------------------------------------
+import           Data.Aeson (ToJSON, FromJSON)
+import           Data.Aeson.Generic.DerivingVia
+import           Test.QuickCheck
+import           Test.QuickCheck.Checkers (EqProp)
+--------------------------------------------------------------------------------
+import           Xanthous.Data (Positioned(..), positioned)
+import           Xanthous.Util.QuickCheck
+import           Xanthous.Game.State
+import           Xanthous.Entities.Item
+import           Xanthous.Entities.RawTypes (WieldableItem, wieldable)
+import           Xanthous.Util (removeFirst, EqEqProp(..))
+--------------------------------------------------------------------------------
+
+data WieldedItem = WieldedItem
+  { _wieldedItem :: Item
+  , _wieldableItem :: WieldableItem
+    -- ^ Invariant: item ^. itemType . wieldable ≡ Just wieldableItem
+  }
+  deriving stock (Eq, Show, Ord, Generic)
+  deriving anyclass (NFData, CoArbitrary, Function)
+  deriving (ToJSON, FromJSON)
+       via WithOptions '[ FieldLabelModifier '[Drop 1] ]
+           WieldedItem
+makeFieldsNoPrefix ''WieldedItem
+
+asWieldedItem :: Prism' Item WieldedItem
+asWieldedItem = prism' hither yon
+ where
+   yon item = WieldedItem item <$> item ^. itemType . wieldable
+   hither (WieldedItem item _) = item
+
+instance Brain WieldedItem where
+  step ticks (Positioned p wi) =
+    over positioned (\i -> WieldedItem i $ wi ^. wieldableItem)
+    <$> step ticks (Positioned p $ wi ^. wieldedItem)
+
+instance Draw WieldedItem where
+  draw = draw . view wieldedItem
+
+instance Entity WieldedItem where
+  entityAttributes = entityAttributes . view wieldedItem
+  description = description . view wieldedItem
+  entityChar = entityChar . view wieldedItem
+
+instance Arbitrary WieldedItem where
+  arbitrary = genericArbitrary <&> \wi ->
+    wi & wieldedItem . itemType . wieldable ?~ wi ^. wieldableItem
+
+data Wielded
+  = DoubleHanded WieldedItem
+  | Hands { _leftHand :: !(Maybe WieldedItem)
+          , _rightHand :: !(Maybe WieldedItem)
+          }
+  deriving stock (Eq, Show, Ord, Generic)
+  deriving anyclass (NFData, CoArbitrary, Function)
+  deriving Arbitrary via GenericArbitrary Wielded
+  deriving (ToJSON, FromJSON)
+       via WithOptions '[ 'SumEnc 'ObjWithSingleField ]
+           Wielded
+
+nothingWielded :: Wielded
+nothingWielded = Hands Nothing Nothing
+
+hands :: Prism' Wielded (Maybe WieldedItem, Maybe WieldedItem)
+hands = prism' (uncurry Hands) $ \case
+  Hands l r -> Just (l, r)
+  _ -> Nothing
+
+leftHand :: Traversal' Wielded (Maybe WieldedItem)
+leftHand = hands . _1
+
+inLeftHand :: WieldedItem -> Wielded
+inLeftHand wi = Hands (Just wi) Nothing
+
+rightHand :: Traversal' Wielded (Maybe WieldedItem)
+rightHand = hands . _2
+
+inRightHand :: WieldedItem -> Wielded
+inRightHand wi = Hands Nothing (Just wi)
+
+doubleHanded :: Prism' Wielded WieldedItem
+doubleHanded = prism' DoubleHanded $ \case
+  DoubleHanded i -> Just i
+  _ -> Nothing
+
+wieldedItems :: Traversal' Wielded WieldedItem
+wieldedItems k (DoubleHanded wielded) = DoubleHanded <$> k wielded
+wieldedItems k (Hands l r) = Hands <$> _Just k l <*> _Just k r
+
+data Inventory = Inventory
+  { _backpack :: Vector Item
+  , _wielded :: Wielded
+  }
+  deriving stock (Eq, Show, Ord, Generic)
+  deriving anyclass (NFData, CoArbitrary, Function)
+  deriving Arbitrary via GenericArbitrary Inventory
+  deriving EqProp via EqEqProp Inventory
+  deriving (ToJSON, FromJSON)
+       via WithOptions '[ FieldLabelModifier '[Drop 1] ]
+           Inventory
+makeFieldsNoPrefix ''Inventory
+
+items :: Traversal' Inventory Item
+items k (Inventory bp w) = Inventory
+  <$> traversed k bp
+  <*> (wieldedItems . wieldedItem) k w
+
+type instance Element Inventory = Item
+
+instance MonoFunctor Inventory where
+  omap = over items
+
+instance MonoFoldable Inventory where
+  ofoldMap = foldMapOf items
+  ofoldr = foldrOf items
+  ofoldl' = foldlOf' items
+  otoList = toListOf items
+  oall = allOf items
+  oany = anyOf items
+  onull = nullOf items
+  ofoldr1Ex = foldr1Of items
+  ofoldl1Ex' = foldl1Of' items
+  headEx = headEx . toListOf items
+  lastEx = lastEx . toListOf items
+
+instance MonoTraversable Inventory where
+  otraverse = traverseOf items
+
+instance Semigroup Inventory where
+  inv₁ <> inv₂ =
+    let backpack' = inv₁ ^. backpack <> inv₂ ^. backpack
+        (wielded', backpack'') = case (inv₁ ^. wielded, inv₂ ^. wielded) of
+          (wielded₁, wielded₂@(DoubleHanded _)) ->
+            (wielded₂, backpack' <> fromList (wielded₁ ^.. wieldedItems . wieldedItem))
+          (wielded₁, wielded₂@(Hands (Just _) (Just _))) ->
+            (wielded₂, backpack' <> fromList (wielded₁ ^.. wieldedItems . wieldedItem))
+          (wielded₁, Hands Nothing Nothing) -> (wielded₁, backpack')
+          (Hands Nothing Nothing, wielded₂) -> (wielded₂, backpack')
+          (Hands (Just l₁) Nothing, Hands Nothing (Just r₂)) ->
+            (Hands (Just l₁) (Just r₂), backpack')
+          (wielded₁@(DoubleHanded _), wielded₂) ->
+            (wielded₁, backpack' <> fromList (wielded₂ ^.. wieldedItems . wieldedItem))
+          (Hands Nothing (Just r₁), Hands Nothing (Just r₂)) ->
+            (Hands Nothing (Just r₂), r₁ ^. wieldedItem <| backpack')
+          (Hands Nothing r₁, Hands (Just l₂) Nothing) ->
+            (Hands (Just l₂) r₁, backpack')
+          (Hands (Just l₁) Nothing, Hands (Just l₂) Nothing) ->
+            (Hands (Just l₂) Nothing, l₁ ^. wieldedItem <| backpack')
+          (Hands (Just l₁) (Just r₁), Hands Nothing (Just r₂)) ->
+            (Hands (Just l₁) (Just r₂), r₁ ^. wieldedItem <| backpack')
+          (Hands (Just l₁) (Just r₁), Hands (Just l₂) Nothing) ->
+            (Hands (Just l₂) (Just r₁), l₁ ^. wieldedItem <| backpack')
+    in Inventory backpack'' wielded'
+
+instance Monoid Inventory where
+  mempty = Inventory mempty $ Hands Nothing Nothing
+
+-- | Representation for where in the inventory an item might be
+data InventoryPosition
+  = Backpack
+  | LeftHand
+  | RightHand
+  | BothHands
+  deriving stock (Eq, Show, Ord, Generic)
+  deriving anyclass (NFData, CoArbitrary, Function)
+  deriving Arbitrary via GenericArbitrary InventoryPosition
+
+-- | Return a human-readable description of the given 'InventoryPosition'
+describeInventoryPosition :: InventoryPosition -> Text
+describeInventoryPosition Backpack  = "In backpack"
+describeInventoryPosition LeftHand  = "Wielded, in left hand"
+describeInventoryPosition RightHand = "Wielded, in right hand"
+describeInventoryPosition BothHands = "Wielded, in both hands"
+
+-- | Given a position in the inventory, return a traversal on the inventory over
+-- all the items in that position
+inventoryPosition :: InventoryPosition -> Traversal' Inventory Item
+inventoryPosition Backpack = backpack . traversed
+inventoryPosition LeftHand = wielded . leftHand . _Just . wieldedItem
+inventoryPosition RightHand = wielded . leftHand . _Just . wieldedItem
+inventoryPosition BothHands = wielded . doubleHanded . wieldedItem
+
+-- | A fold over all the items in the inventory accompanied by their position in
+-- the inventory
+--
+-- Invariant: This will return items in the same order as 'items'
+itemsWithPosition :: Fold Inventory (InventoryPosition, Item)
+itemsWithPosition = folding $ (<>) <$> backpackItems <*> handItems
+  where
+    backpackItems = toListOf $ backpack . folded . to (Backpack ,)
+    handItems inventory = case inventory ^. wielded of
+       DoubleHanded i -> pure (BothHands, i ^. wieldedItem)
+       Hands l r -> (l ^.. folded . wieldedItem . to (LeftHand ,))
+                 <> (r ^.. folded . wieldedItem . to (RightHand ,))
+
+-- | Remove the first item equal to 'Item' from the given position in the
+-- inventory
+removeItemFromPosition :: InventoryPosition -> Item -> Inventory -> Inventory
+removeItemFromPosition Backpack item inv
+  = inv & backpack %~ removeFirst (== item)
+removeItemFromPosition LeftHand item inv
+  = inv & wielded . leftHand %~ filter ((/= item) . view wieldedItem)
+removeItemFromPosition RightHand item inv
+  = inv & wielded . rightHand %~ filter ((/= item) . view wieldedItem)
+removeItemFromPosition BothHands item inv
+  | has (wielded . doubleHanded . wieldedItem . filtered (== item)) inv
+  = inv & wielded .~ nothingWielded
+  | otherwise
+  = inv