1 files changed, 94 insertions, 0 deletions
diff --git a/src/Transpile.hs b/src/Transpile.hs
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+++ b/src/Transpile.hs
@@ -0,0 +1,94 @@
+-- This code partly uses algorithms (drip/abstract) created by John Tromp.
+-- Since they did not license their code (afaik), I assume it's okay to reuse it.
+
+module Transpile
+  ( transpileSKI
+  , transpileBirb
+  ) where
+
+import           Term
+import           Utils
+
+data SKI = S | K | I | AppSKI SKI SKI | IdxSKI Int
+  deriving (Eq, Ord)
+
+data Birb = Swan | Kool | Idiot | Quacky
+  deriving (Eq, Ord)
+
+instance Show Birb where
+  showsPrec _ Swan   = showString "\x1F9A2"
+  showsPrec _ Kool   = showString "\x1F425"
+  showsPrec _ Idiot  = showString "\x1F426"
+  showsPrec _ Quacky = showString "\x1F986"
+
+instance Show SKI where
+  showsPrec _ S = showString "s"
+  showsPrec _ K = showString "k"
+  showsPrec _ I = showString "i"
+  showsPrec _ (AppSKI x y) =
+    showString "(" . shows x . showString " " . shows y . showString ")"
+  showsPrec _ (IdxSKI i) = shows i
+
+drip :: SKI -> SKI
+drip i@(AppSKI (AppSKI S K) _) = i
+drip (  IdxSKI 0             ) = invalid
+drip (  IdxSKI i             ) = IdxSKI (i - 1)
+drip (  AppSKI x y           ) = AppSKI (drip x) (drip y)
+drip x                         = x
+
+abstract :: SKI -> SKI
+abstract (AppSKI sk@(AppSKI S K) _) = sk
+abstract e = if freeIn (== 0) e then occabstract e else AppSKI K (drip e) where
+  freeIn _  (AppSKI (AppSKI S K) _) = False
+  freeIn fv (AppSKI x            y) = freeIn fv x || freeIn fv y
+  freeIn fv (IdxSKI i             ) = fv i
+  freeIn _  _                       = False
+  isConst = not . freeIn (const True)
+  occabstract (IdxSKI 0) = I
+  occabstract (AppSKI m (IdxSKI 0)) | not (freeIn (== 0) m) = drip m
+  occabstract (AppSKI (AppSKI l m) l') | l == l' =
+    occabstract (AppSKI (AppSKI (AppSKI (AppSKI S S) K) l) m)
+  occabstract (AppSKI m (AppSKI n l)) | isConst m && isConst n =
+    occabstract (AppSKI (AppSKI (AppSKI S (abstract m)) n) l)
+  occabstract (AppSKI (AppSKI m n) l) | isConst m && isConst l =
+    occabstract (AppSKI (AppSKI (AppSKI S m) (abstract l)) n)
+  occabstract (AppSKI (AppSKI m l) (AppSKI n l'))
+    | l == l' && isConst m && isConst n = occabstract
+      (AppSKI (AppSKI (AppSKI S m) n) l)
+  occabstract (AppSKI e1 e2) = AppSKI (AppSKI S (abstract e1)) (abstract e2)
+  occabstract _              = invalid
+
+transpileSKI :: Term -> SKI
+transpileSKI (Idx i  ) = IdxSKI i
+transpileSKI (App m n) = AppSKI (transpileSKI m) (transpileSKI n)
+transpileSKI (Abs m  ) = abstract (transpileSKI m)
+
+fromSKI :: SKI -> Birb
+fromSKI S = Swan
+fromSKI K = Kool
+fromSKI I = Idiot
+fromSKI _ = invalid
+
+jotify :: SKI -> [Bool]
+jotify = reverse . go
+ where
+  go S            = [True, True, True, True, True, False, False, False]
+  go K            = [True, True, True, False, False]
+  go I            = go $ AppSKI (AppSKI S K) K
+  go (AppSKI a b) = True : (go a ++ go b)
+  go _            = invalid
+
+dejotify :: [Bool] -> SKI
+dejotify (False : js) = AppSKI (AppSKI (dejotify js) S) K
+dejotify (True  : js) = AppSKI S (AppSKI K (dejotify js))
+dejotify _            = I
+
+transpileBirb :: SKI -> [Birb]
+transpileBirb = go . dejotify . jotify
+ where
+  go (AppSKI a b) = case [a, b] of
+    [AppSKI x l, r] -> [Idiot, Idiot] ++ go x ++ [fromSKI l, fromSKI r]
+    [l, AppSKI r x] -> [Idiot, Quacky] ++ go x ++ [fromSKI r, fromSKI l]
+    [l, r] -> [fromSKI l, fromSKI r]
+  go (IdxSKI _) = invalid
+  go s          = [fromSKI s]