-----------------------------------------------------------------------------
-- |
-- Module    : Documentation.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.
-----------------------------------------------------------------------------

{-# OPTIONS_GHC -Wall -Werror #-}

module Documentation.SBV.Examples.CodeGeneration.Uninterpreted where

import Data.Maybe (fromMaybe)

import Data.SBV
import Data.SBV.Tools.CodeGen

-- | 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
