    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Data.SBV.Examples.CodeGeneration.Uninterpreted
    -- Copyright   :  (c) Levent Erkok
    -- License     :  BSD3
    -- Maintainer  :  erkokl@gmail.com
    -- Stability   :  experimental
    --
    -- Demonstrates the use of uninterpreted functions for the purposes of
    -- code generation. This facility is important when we want to take
    -- advantage of native libraries in the target platform, or when we'd
    -- like to hand-generate code for certain functions for various
    -- purposes, such as efficiency, or reliability.
    -----------------------------------------------------------------------------

    module Data.SBV.Examples.CodeGeneration.Uninterpreted where

    import Data.Maybe (fromMaybe)
    import Data.SBV

    -- | A definition of shiftLeft that can deal with variable length shifts.
    -- (Note that the ``shiftL`` method from the 'Bits' class requires an 'Int' shift
    -- amount.) Unfortunately, this'll generate rather clumsy C code due to the
    -- use of tables etc., so we uninterpret it for code generation purposes
    -- using the 'cgUninterpret' function.
    shiftLeft :: SWord32 -> SWord32 -> SWord32
    shiftLeft = cgUninterpret "SBV_SHIFTLEFT" cCode hCode
      where -- the C code we'd like SBV to spit out when generating code. Note that this is
            -- arbitrary C code. In this case we just used a macro, but it could be a function,
            -- text that includes files etc. It should essentially bring the name SBV_SHIFTLEFT
            -- used above into scope when compiled. If no code is needed, one can also just
            -- provide the empty list for the same effect. Also see 'cgAddDecl', 'cgAddLDFlags',
            -- and 'cgAddPrototype' functions for further variations.
            cCode = ["#define SBV_SHIFTLEFT(x, y) ((x) << (y))"]
            -- the Haskell code we'd like SBV to use when running inside Haskell or when
            -- translated to SMTLib for verification purposes. This is good old Haskell
            -- code, as one would typically write.
            hCode x = select [x * literal (bit b) | b <- [0.. bs x - 1]] (literal 0)
            bs x = fromMaybe (error "SBV.Example.CodeGeneration.Uninterpreted.shiftLeft: Unexpected non-finite usage!") (bitSizeMaybe x)

    -- | Test function that uses shiftLeft defined above. When used as a normal Haskell function
    -- or in verification the definition is fully used, i.e., no uninterpretation happens. To wit,
    -- we have:
    --
    --  >>> tstShiftLeft 3 4 5
    --  224 :: SWord32
    --
    --  >>> prove $ \x y -> tstShiftLeft x y 0 .== x + y
    --  Q.E.D.
    tstShiftLeft ::  SWord32 -> SWord32 -> SWord32 -> SWord32
    tstShiftLeft x y z = x `shiftLeft` z + y `shiftLeft` z

    -- | Generate C code for "tstShiftLeft". In this case, SBV will *use* the user given definition
    -- verbatim, instead of generating code for it. (Also see the functions 'cgAddDecl', 'cgAddLDFlags',
    -- and 'cgAddPrototype'.)
    genCCode :: IO ()
    genCCode = compileToC Nothing "tst" $ do
                    [x, y, z] <- cgInputArr 3 "vs"
                    cgReturn $ tstShiftLeft x y z