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+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE LambdaCase #-}
+module SC.Exp where
+
+import qualified Data.Array.Accelerate.AST as A
+import Data.Array.Accelerate.AST.LeftHandSide
+import Data.Array.Accelerate.AST.Var
+import Data.Array.Accelerate.Representation.Array
+import Data.Array.Accelerate.Representation.Shape
+import Data.Array.Accelerate.Representation.Type
+import Data.Array.Accelerate.Type
+
+import qualified Language.C as C
+import SC.Defs
+import SC.Monad
+
+
+data CompiledFun aenv t1 t2 =
+    CompiledFun
+        C.FunDef  -- ^ expression function implementation
+        (Exprs t1 -> Names t2 -> [C.Expr])
+            -- ^ arguments builder. Given:
+            --   - expressions that compute the direct arguments;
+            --   - names that the output values should be stored in;
+            --   returns the list of arguments to be passed to the compiled
+            --   function. The outputs will be stored by storing to pointers to
+            --   the given names.
+            --   The arguments will refer to array variable names found in the
+            --   original array environment.
+
+-- | The function must be single-argument. Uncurry if necessary (e.g. for zipWith).
+compileFun :: AVarEnv aenv -> A.Fun aenv (t1 -> t2) -> SC (CompiledFun aenv t1 t2)
+compileFun aenv (A.Lam lhs (A.Body body)) = do
+    funname <- genName "expfun_"
+    (argnames, env) <- genVarsEnv lhs VENil
+    outnames <- itupmap (\(TypedName t n) -> TypedName (C.TPtr t) n)
+                    <$> genVars (A.expType body)
+    (usedA, res) <- compileExp' aenv env body
+    (sts1, retexprs) <- toStExprs (A.expType body) res
+    let sts2 = genoutstores outnames retexprs
+        arguments =
+            map (\(TypedAName t n) -> (t, n)) usedA
+            ++ map (\(TypedName t n) -> (t, n)) (itupList argnames)
+            ++ map (\(TypedName t n) -> (t, n)) (itupList outnames)
+    return $ CompiledFun
+        (C.ProcDef funname arguments (sts1 ++ sts2))
+        (\argexprs destnames ->
+            map (\(TypedAName _ n) -> C.EVar n) usedA
+            ++ itupList argexprs
+            ++ map (\(TypedName _ n) -> C.EPtrTo (C.EVar n)) (itupList destnames))
+  where
+    genoutstores :: Names t -> Exprs t -> [C.Stmt]
+    genoutstores ITupIgnore _ = []
+    genoutstores (ITupSingle (TypedName _ n)) (ITupSingle e) = [C.SStore n (C.ELit "0") e]
+    genoutstores (ITupPair n1 n2) (ITupPair e1 e2) = genoutstores n1 e1 ++ genoutstores n2 e2
+    genoutstores _ _ = error "wat"
+compileFun _ _ = error "compileFun: Not single-argument function"
+
+compileExp :: AVarEnv aenv -> A.Exp aenv t -> SC (CompiledFun aenv () t)
+compileExp aenv expr = compileFun aenv (A.Lam (LeftHandSideWildcard TupRunit) (A.Body expr))
+
+compileExp' :: AVarEnv aenv -> VarEnv env -> A.OpenExp env aenv t
+            -> SC ([TypedAName], Either (Names t -> [C.Stmt]) ([C.Stmt], Exprs t))
+compileExp' aenv env = \case
+    A.Let lhs rhs body -> do
+        (names, env') <- genVarsEnv lhs env
+        let sts1 = [C.SDecl t n Nothing | TypedName t n <- itupList names]
+        (usedA2, sts2) <- fmap (`toStoring` names) <$> compileExp' aenv env rhs
+        (usedA3, res3) <- compileExp' aenv env' body
+        return (usedA2 ++ usedA3
+               ,fmap (\(sts, exs) -> (sts1 ++ sts2 ++ sts, exs)) res3)
+
+    A.Evar (Var _ idx) ->
+        return ([], Right ([], ITupSingle (C.EVar (veprj env idx))))
+
+    A.Pair a b -> do
+        (usedA1, res1) <- compileExp' aenv env a
+        (usedA2, res2) <- compileExp' aenv env b
+        return (usedA1 ++ usedA2, Left (\case
+            ITupPair n1 n2 -> toStoring res1 n1 ++ toStoring res2 n2
+            ITupIgnore -> []
+            ITupSingle _ -> error "wat"))
+
+    A.PrimApp (A.PrimAdd _) e -> binary aenv env "+" e
+    A.PrimApp (A.PrimSub _) e -> binary aenv env "-" e
+    A.PrimApp (A.PrimMul _) e -> binary aenv env "*" e
+    A.PrimApp (A.PrimQuot _) e -> binary aenv env "/" e
+    A.PrimApp (A.PrimRem _) e -> binary aenv env "%" e
+
+    A.Shape (Var _ idx) ->
+        let (shnames, _) = aveprj aenv idx
+            buildExprs :: ShNames sh -> Exprs sh
+            buildExprs ShZ = ITupIgnore
+            buildExprs (ShS n names) = ITupPair (buildExprs names) (ITupSingle (C.EVar n))
+        in return ([], Right ([], buildExprs shnames))
+
+    A.ToIndex shr she idxe -> do
+        let build :: ShapeR sh -> Exprs sh -> Exprs sh -> C.Expr
+            build ShapeRz _ _ = C.ELit "0"
+            build (ShapeRsnoc ShapeRz) _ (ITupPair _ (ITupSingle idxe')) = idxe'
+            build (ShapeRsnoc shr') (ITupPair shes' (ITupSingle she'))
+                                    (ITupPair idxes' (ITupSingle idxe')) =
+                C.EOp (C.EOp (build shr' shes' idxes') "*" she') "+" idxe'
+            build _ _ _ = error "wat"
+        (usedA1, res1) <- compileExp' aenv env she
+        (sts1, shes) <- toStExprs (shapeType shr) res1
+        (usedA2, res2) <- compileExp' aenv env idxe
+        (sts2, idxes) <- toStExprs (shapeType shr) res2
+        return (usedA1 ++ usedA2, Right (sts1 ++ sts2, ITupSingle (build shr shes idxes)))
+
+    A.Index avar@(Var (ArrayR shr _) _) she ->
+        compileExp' aenv env $
+            A.LinearIndex avar (A.ToIndex shr (A.Shape avar) she)
+
+    A.LinearIndex (Var _ idx) e -> do
+        temp <- genName "i"
+        let sts0 = [C.SDecl (C.TInt C.B64) temp Nothing]
+        (usedA1, sts1) <- fmap (`toStoring` ITupSingle (TypedName (C.TInt C.B64) temp))
+                              <$> compileExp' aenv env e
+        let (_, anames) = aveprj aenv idx
+            usedA = itupList anames ++ usedA1
+        return (usedA, Right (sts0 ++ sts1
+                             ,itupmap (\(TypedAName _ name) -> C.EIndex name (C.EVar temp)) anames))
+
+    _ -> throw "Unsupported Exp constructor"
+  where
+    binary :: AVarEnv aenv -> VarEnv env -> String -> A.OpenExp env aenv (a, b)
+           -> SC ([TypedAName], Either (Names t -> [C.Stmt]) ([C.Stmt], Exprs t))
+    binary aenv' env' op e' = do
+        (usedA, res) <- compileExp' aenv' env' e'
+        (sts, ITupPair (ITupSingle e1) (ITupSingle e2)) <-
+            toStExprs (A.expType e') res
+        return (usedA, Right (sts, ITupSingle (C.EOp e1 op e2)))
+
+toStExprs :: TypeR t -> Either (Names t -> [C.Stmt]) ([C.Stmt], Exprs t) -> SC ([C.Stmt], Exprs t)
+toStExprs ty (Left fun) = do
+    names <- genVars ty
+    let sts1 = fun names
+    return (sts1, itupmap (\(TypedName _ n) -> C.EVar n) names)
+toStExprs _ (Right pair) = return pair
+
+toStoring :: Either (Names t -> [C.Stmt]) ([C.Stmt], Exprs t) -> Names t -> [C.Stmt]
+toStoring (Left f) = f
+toStoring (Right (sts, exs)) = (sts ++) . flip go exs
+  where
+    go :: Names t -> Exprs t -> [C.Stmt]
+    go (ITupSingle (TypedName _ name)) (ITupSingle ex) = [C.SAsg name ex]
+    go (ITupSingle _) _ = error "wat"
+    go ITupIgnore _ = []
+    go (ITupPair ns1 ns2) (ITupPair es1 es2) = go ns1 es1 ++ go ns2 es2
+    go (ITupPair _ _) _ = error "wat"
+
+genVarsEnv :: A.ELeftHandSide t env env' -> VarEnv env -> SC (Names t, VarEnv env')
+genVarsEnv (LeftHandSideWildcard _) env = return (ITupIgnore, env)
+genVarsEnv (LeftHandSideSingle ty) env = do
+    name <- genName "x"
+    ty' <- cvtType ty
+    return (ITupSingle (TypedName ty' name), VEPush name env)
+genVarsEnv (LeftHandSidePair lhs1 lhs2) env = do
+    (n1, env1) <- genVarsEnv lhs1 env
+    (n2, env2) <- genVarsEnv lhs2 env1
+    return (ITupPair n1 n2, env2)
+
+genVars :: TypeR t -> SC (Names t)
+genVars TupRunit = return ITupIgnore
+genVars (TupRsingle ty) = genVar ty
+genVars (TupRpair t1 t2) = ITupPair <$> genVars t1 <*> genVars t2
+
+genVar :: ScalarType t -> SC (Names t)
+genVar ty = ITupSingle <$> (TypedName <$> cvtType ty <*> genName "x")