diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..68df01b
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,6 @@
+.DS_Store
+*.hi
+*.o
+tmp/
+build/
+.ghc.environment.*
\ No newline at end of file
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..efd1bf7
--- /dev/null
+++ b/README.md
@@ -0,0 +1,55 @@
+
+Circom witness generators
+-------------------------
+
+This piece of software takes the computation graph files generated by 
+[`circom-witnesscalc`](https://github.com/iden3/circom-witnesscalc),
+and either interprets or compiles them to various algebra backends.
+
+### Implementation status
+
+Compiler (in Haskell):
+
+- [x] parsing the graph files
+- [x] naive interpreter
+- [ ] constantine backend
+- [ ] zikkurat backend
+- [ ] arkworks backend
+
+Nim witness generator:
+
+- [x] parsing the graph files
+- [ ] generating the witness
+- [ ] proper error handling
+
+### Testing & correctness
+
+I haven't yet done any proper testing, apart from "works for our purposes".
+
+### Circuit optimizations
+
+NOTE: you _have to_ run `circom` with the `--O2` options, otherwise the 
+witness format will be most probably incompatible with the the one generated
+by `circom-witnesscalc`.
+
+### Graph file format
+
+`circom-witnesscalc` produces binary files encoding a computation graph.
+
+This has the following format:
+
+- magic header: "wtns.graph.001" (14 bytes)
+- number of nodes (8 bytes little endian)
+- list of nodes, in protobuf serialized format. Each prefixed by a `varint` length
+- `GraphMetaData`, in protobuf 
+- 8 bytes offset (yes, _at the very end_...), pointing to the start of `GraphMetaData`
+
+Node format:
+
+- varint length prefix
+- protobuf tag-byte (lower 3 bits are `0x02`, upper bits are node type 1..5)
+- varint length 
+- several record fields
+    - protobuf tag-byte (lower 3 bits are 0x00, upper bits are field index 1,2,3,4)
+    - value is varint word32, except for ConstantNode when it's a little-endian bytes
+      (wrapped a few times, because protobuf is being protobuf...)
diff --git a/src/BN254.hs b/src/BN254.hs
new file mode 100644
index 0000000..e47ebfd
--- /dev/null
+++ b/src/BN254.hs
@@ -0,0 +1,191 @@
+
+-- | The BN254 scalar field
+
+{-# LANGUAGE Strict, BangPatterns #-} 
+module BN254 where
+
+--------------------------------------------------------------------------------
+
+import Prelude hiding (div)
+import qualified Prelude
+
+import Data.Bits
+import Data.Word
+import Data.Ratio
+
+import System.Random
+import Text.Printf
+
+import Misc
+
+--------------------------------------------------------------------------------
+
+fieldPrime :: Integer
+fieldPrime = 21888242871839275222246405745257275088548364400416034343698204186575808495617
+
+modP :: Integer -> Integer
+modP x = mod x fieldPrime
+
+halfPrimePlus1 :: Integer
+halfPrimePlus1 = 1 + Prelude.div fieldPrime 2
+
+--------------------------------------------------------------------------------
+
+newtype F 
+  = MkF Integer 
+  deriving (Eq,Show)
+
+fromF :: F -> Integer
+fromF (MkF x) = x
+
+-- from the circom docs: @val(z) = z-p  if p/2 +1 <= z < p@
+signedFromF :: F -> Integer
+signedFromF (MkF x) = if x >= halfPrimePlus1 then x - fieldPrime else x
+
+toF :: Integer -> F
+toF = MkF . modP
+
+isZero :: F -> Bool
+isZero (MkF x) = (x == 0)
+
+fromBool :: Bool -> F
+fromBool b = if b then 1 else 0
+
+toBool :: F -> Bool
+toBool = not . isZero
+
+--------------------------------------------------------------------------------
+
+neg :: F -> F
+neg (MkF x) = toF (negate x)
+
+add :: F -> F -> F
+add (MkF x) (MkF y) = toF (x+y)
+
+sub :: F -> F -> F
+sub (MkF x) (MkF y) = toF (x-y)
+
+mul :: F -> F -> F
+mul (MkF x) (MkF y) = toF (x*y)
+
+instance Num F where
+  fromInteger = toF 
+  negate = neg
+  (+) = add
+  (-) = sub
+  (*) = mul
+  abs = id
+  signum _ = toF 1
+
+square :: F -> F
+square x = x*x
+
+rndF :: IO F
+rndF = MkF <$> randomRIO (0,fieldPrime-1)
+
+--------------------------------------------------------------------------------
+
+pow :: F -> Integer -> F
+pow x0 exponent
+  | exponent < 0  = error "power: expecting positive exponent"
+  | otherwise     = go 1 x0 exponent
+  where
+    go !acc _ 0 = acc
+    go !acc s e = go acc' s' (shiftR e 1) where
+      s'   = s*s
+      acc' = if e .&. 1 == 0 then acc else acc*s
+
+invNaive :: F -> F
+invNaive x = pow x (fieldPrime - 2)
+
+divNaive :: F -> F -> F
+divNaive x y = x * invNaive y
+
+--------------------------------------------------------------------------------
+
+instance Fractional F where
+  fromRational q = fromInteger (numerator q) / fromInteger (denominator q)
+  recip = inv
+  (/)   = div
+
+--------------------------------------------------------------------------------
+
+fromBytesLE :: [Word8] -> F
+fromBytesLE = toF . integerFromBytesLE
+
+integerFromBytesLE :: [Word8] -> Integer
+integerFromBytesLE = go where
+  go []     = 0
+  go (b:bs) = fromIntegral b + (shiftL (go bs) 8)
+
+--------------------------------------------------------------------------------
+
+instance ShowHex Integer where showHex = printf "0x%x"
+instance ShowHex F       where showHex (MkF x) = showHex x
+
+--------------------------------------------------------------------------------
+
+-- | Inversion (using Euclid's algorithm)
+inv :: F -> F
+inv (MkF a) 
+  | a == 0    = 0 -- error "field inverse of zero (generic prime)"
+  | otherwise = MkF (euclid 1 0 a fieldPrime) 
+
+-- | Division via Euclid's algorithm
+div :: F -> F -> F
+div (MkF a) (MkF b)
+  | b == 0    = 0 -- error "field division by zero (generic prime)"
+  | otherwise = MkF (euclid a 0 b fieldPrime) 
+
+--------------------------------------------------------------------------------
+-- * Euclidean algorithm
+
+-- | Extended binary Euclidean algorithm
+euclid :: Integer -> Integer -> Integer -> Integer -> Integer 
+euclid !x1 !x2 !u !v = go x1 x2 u v where
+
+  p = fieldPrime
+
+  halfp1 = shiftR (p+1) 1
+
+  modp :: Integer -> Integer
+  modp n = mod n p
+
+  -- Inverse using the binary Euclidean algorithm 
+  euclid :: Integer -> Integer
+  euclid a 
+    | a == 0     = 0
+    | otherwise  = go 1 0 a p
+  
+  go :: Integer -> Integer -> Integer -> Integer -> Integer
+  go !x1 !x2 !u !v 
+    | u==1       = x1
+    | v==1       = x2
+    | otherwise  = stepU x1 x2 u v
+
+  stepU :: Integer -> Integer -> Integer -> Integer -> Integer
+  stepU !x1 !x2 !u !v = if even u 
+    then let u'  = shiftR u 1
+             x1' = if even x1 then shiftR x1 1 else shiftR x1 1 + halfp1
+         in  stepU x1' x2 u' v
+    else     stepV x1  x2 u  v
+
+  stepV :: Integer -> Integer -> Integer -> Integer -> Integer
+  stepV !x1 !x2 !u !v = if even v
+    then let v'  = shiftR v 1
+             x2' = if even x2 then shiftR x2 1 else shiftR x2 1 + halfp1
+         in  stepV x1 x2' u v' 
+    else     final x1 x2  u v
+
+  final :: Integer -> Integer -> Integer -> Integer -> Integer
+  final !x1 !x2 !u !v = if u>=v
+
+    then let u'  = u-v
+             x1' = if x1 >= x2 then modp (x1-x2) else modp (x1+p-x2)
+         in  go x1' x2  u' v 
+
+    else let v'  = v-u
+             x2' = if x2 >= x1 then modp (x2-x1) else modp (x2+p-x1)
+         in  go x1  x2' u  v'
+
+--------------------------------------------------------------------------------
diff --git a/src/Graph.hs b/src/Graph.hs
new file mode 100644
index 0000000..f71dac0
--- /dev/null
+++ b/src/Graph.hs
@@ -0,0 +1,104 @@
+
+{-# LANGUAGE StrictData #-}
+module Graph where
+
+--------------------------------------------------------------------------------
+
+import Text.Printf
+
+--------------------------------------------------------------------------------
+
+import Data.Word
+
+data Graph = Graph
+  { graphNodes :: [Node]
+  , graphMeta  :: GraphMetaData
+  }
+  deriving Show
+
+--------------------------------------------------------------------------------
+
+-- | Unary operations
+data UnoOp
+  = Neg       -- ^ @= 0@
+  | Id        -- ^ @= 1@
+  deriving (Eq,Enum,Bounded,Show)
+
+data DuoOp 
+  = Mul       -- ^ @=  0@
+  | Div       -- ^ @=  1@
+  | Add       -- ^ @=  2@
+  | Sub       -- ^ @=  3@
+  | Pow       -- ^ @=  4@
+  | Idiv      -- ^ @=  5@
+  | Mod       -- ^ @=  6@
+  | Eq_       -- ^ @=  7@
+  | Neq       -- ^ @=  8@
+  | Lt        -- ^ @=  9@
+  | Gt        -- ^ @= 10@
+  | Leq       -- ^ @= 11@
+  | Geq       -- ^ @= 12@
+  | Land      -- ^ @= 13@
+  | Lor       -- ^ @= 14@
+  | Shl       -- ^ @= 15@
+  | Shr       -- ^ @= 16@
+  | Bor       -- ^ @= 17@
+  | Band      -- ^ @= 18@
+  | Bxor      -- ^ @= 19@
+  deriving (Eq,Enum,Bounded,Show)
+
+data TresOp
+  = TernCond  -- ^ @= 0@
+  deriving (Eq,Enum,Bounded,Show)
+
+--------------------------------------------------------------------------------
+
+newtype BigUInt 
+  = BigUInt [Word8]      -- ^ little endian
+
+showBigUInt :: BigUInt -> String
+showBigUInt (BigUInt bytes) = "0x" ++ concatMap f (reverse bytes) where
+  f :: Word8 -> String
+  f = printf "%02x"
+
+instance Show BigUInt where show = showBigUInt
+
+newtype InputNode 
+  = InputNode Word32
+  deriving (Show)
+ 
+newtype ConstantNode 
+  = ConstantNode BigUInt
+  deriving (Show)
+
+data UnoOpNode  = UnoOpNode  !UnoOp  !Word32                 deriving (Show)
+data DuoOpNode  = DuoOpNode  !DuoOp  !Word32 !Word32         deriving (Show)
+data TresOpNode = TresOpNode !TresOp !Word32 !Word32 !Word32 deriving (Show)
+
+data Node 
+  = AnInputNode    InputNode         -- @= 1@
+  | AConstantNode  ConstantNode      -- @= 2@
+  | AnUnoOpNode    UnoOpNode         -- @= 3@
+  | ADuoOpNode     DuoOpNode         -- @= 4@
+  | ATresOpNode    TresOpNode        -- @= 5@
+  deriving (Show)
+
+data SignalDescription = SignalDescription
+  { signalOffset :: !Word32
+  , signalLength :: !Word32
+  }
+  deriving (Show)
+
+newtype WitnessMapping 
+  = WitnessMapping { fromWitnessMapping :: [Word32] }
+  deriving (Show)
+
+type CircuitInputs = [(String, SignalDescription)]
+
+data GraphMetaData = GraphMetaData 
+  { witnessMapping :: WitnessMapping
+  , inputSignals   :: CircuitInputs  
+  }
+  deriving (Show)
+
+--------------------------------------------------------------------------------
diff --git a/src/Main.hs b/src/Main.hs
new file mode 100644
index 0000000..e4db71d
--- /dev/null
+++ b/src/Main.hs
@@ -0,0 +1,34 @@
+
+module Main where
+
+--------------------------------------------------------------------------------
+
+import Data.Map (Map)
+import qualified Data.Map as Map
+
+import Witness
+import Parser
+
+--------------------------------------------------------------------------------
+
+(~>) :: String -> a -> (String, a)
+(~>) = (,)
+
+infix 2 ~>
+
+testInputs :: Map String [Integer]
+testInputs = Map.fromList 
+  [ "a" ~> [0xff01] 
+  , "b" ~> [0xff02]
+  ]
+
+--------------------------------------------------------------------------------
+
+main :: IO ()
+main = do
+  Right graph <- parseGraphFile "../tmp/graph2.bin"
+  putStrLn ""
+  print graph
+  let wtns = witnessCalc testInputs graph
+  putStrLn ""
+  print wtns
\ No newline at end of file
diff --git a/src/Misc.hs b/src/Misc.hs
new file mode 100644
index 0000000..19bf8ae
--- /dev/null
+++ b/src/Misc.hs
@@ -0,0 +1,33 @@
+
+{-# LANGUAGE Strict #-}
+module Misc where
+
+--------------------------------------------------------------------------------
+
+import Data.Bits
+
+--------------------------------------------------------------------------------
+
+class ShowHex a where
+  showHex  :: a -> String
+
+printHex :: ShowHex a => a -> IO ()
+printHex x = putStrLn (showHex x)
+
+--------------------------------------------------------------------------------
+-- Integer logarithm
+
+-- | Largest integer @k@ such that @2^k@ is smaller or equal to @n@
+integerLog2 :: Integer -> Int
+integerLog2 n = go n where
+  go 0 = -1
+  go k = 1 + go (shiftR k 1)
+
+-- | Smallest integer @k@ such that @2^k@ is larger or equal to @n@
+ceilingLog2 :: Integer -> Int
+ceilingLog2 0 = 0
+ceilingLog2 n = 1 + go (n-1) where
+  go 0 = -1
+  go k = 1 + go (shiftR k 1)
+  
+--------------------------------------------------------------------------------
diff --git a/src/Parser.hs b/src/Parser.hs
new file mode 100644
index 0000000..140b83f
--- /dev/null
+++ b/src/Parser.hs
@@ -0,0 +1,345 @@
+
+-- | Parsing the graph binary format
+
+{-# LANGUAGE Strict, PackageImports, BangPatterns #-}
+module Parser where
+
+--------------------------------------------------------------------------------
+
+import Data.Bits
+import Data.Word
+import Data.List
+import Data.Ord
+
+import Control.Monad
+import Control.Applicative
+import System.IO
+
+import Data.ByteString.Lazy (ByteString) 
+import qualified Data.ByteString.Lazy       as L
+import qualified Data.ByteString.Lazy.Char8 as LC
+
+import "binary" Data.Binary.Get
+import "binary" Data.Binary.Builder as Builder
+
+import Graph
+
+--------------------------------------------------------------------------------
+
+{-
+test :: IO ()
+test = do
+  Right graph <- parseGraphFile "../tmp/graph.bin"
+  print graph
+-}
+
+{-
+nodeEx1  = 0x06 : nodeEx1'            :: [Word8]
+nodeEx2  = 0x08 : nodeEx2'            :: [Word8]
+
+nodeEx1' = 0x22 : 0x04 : duoNodeEx1   :: [Word8]
+nodeEx2' = 0x22 : 0x06 : duoNodeEx2   :: [Word8]
+
+duoNodeEx1 = [ 0x10 , 0x05 , 0x18 , 0x05 ]                 :: [Word8]
+duoNodeEx2 = [ 0x08 , 0x02 , 0x10 , 0x03 , 0x18 , 0x04 ]   :: [Word8]
+-}
+
+--------------------------------------------------------------------------------
+
+type Msg = String
+
+parseGraphFile :: FilePath -> IO (Either Msg Graph)
+parseGraphFile fname = do
+  h    <- openBinaryFile fname ReadMode 
+  ei   <- readGraphFile h
+  hClose h
+  return ei
+
+hGetBytes :: Handle -> Int -> IO ByteString
+hGetBytes h n = L.hGet h (fromIntegral n)
+
+hSeekInt :: Handle -> Int -> IO ()
+hSeekInt h ofs = hSeek h AbsoluteSeek (fromIntegral ofs)
+
+readGraphFile :: Handle -> IO (Either Msg Graph)
+readGraphFile h = do
+  flen  <- (fromIntegral :: Integer -> Int) <$> hFileSize h 
+  magic <- hGetBytes h (length magicHeader)
+  if magic /= LC.pack magicHeader
+    then return $ Left "magic header not found or invalid"
+    else do
+      hSeekInt h (flen - 8)
+      offset <- (fromIntegral . runGet getWord64le) <$> hGetBytes h 8
+      putStrLn $ "metadata offset = " ++ show offset
+      if (offset >= flen) || (offset <= 18)
+        then return $ Left "invalid final `graphMetaData` offset bytes"
+        else do
+          hSeekInt h (length magicHeader)
+          part1 <- hGetBytes h (offset - length magicHeader)
+          part2 <- hGetBytes h (flen - offset - 8)
+          return (Right $ Graph (parseNodes part1) (parseMeta part2))
+
+magicHeader :: String
+magicHeader = "wtns.graph.001"
+
+parseNodes :: ByteString -> [Node]
+parseNodes = runGet getNodes
+
+parseMeta :: ByteString -> GraphMetaData
+parseMeta = runGet getMetaData
+
+--------------------------------------------------------------------------------
+
+varInt' :: Get Word64
+varInt' = go 0 where
+  go !cnt = if cnt >= 8 then return 0 else do
+    w <- getWord8
+    if (w < 128) 
+      then return (fromIntegral w)
+      else do
+        let x = fromIntegral (w .&. 127) 
+        y <- go (cnt+1)
+        return (x + 128*y)
+
+varInt :: Get Int
+varInt = fromIntegral <$> varInt'
+
+varUInt :: Get Word32
+varUInt = fromIntegral <$> varInt'
+
+--------------------------------------------------------------------------------
+
+getNodes :: Get [Node]
+getNodes = do
+  n <- getWord64le 
+  replicateM (fromIntegral n) getNode
+
+-- | with varint length prefix
+getNode :: Get Node
+getNode = do
+  len <- varInt
+  bs  <- getLazyByteString (fromIntegral len)
+  return (runGet getNode' bs)
+
+-- | without varint length prefix
+getNode' :: Get Node
+getNode' = do
+  nodetype <- getFieldId LEN
+  case nodetype of
+    1 -> AnInputNode   <$> getInputNode   
+    2 -> AConstantNode <$> getConstantNode
+    3 -> AnUnoOpNode   <$> getUnoOpNode   
+    4 -> ADuoOpNode    <$> getDuoOpNode   
+    5 -> ATresOpNode   <$> getTresOpNode  
+    _ -> error "unexpected node type"
+
+getInputNode :: Get InputNode
+getInputNode = do
+  SomeNode idx _ _ _ <- getSomeNode 
+  return (InputNode idx)
+
+getConstantNode :: Get ConstantNode
+getConstantNode = do
+  len  <- varInt
+  bs   <- getLazyByteString (fromIntegral len)
+  return $ ConstantNode (runGet getBigUInt bs)
+
+getBigUInt :: Get BigUInt
+getBigUInt = do
+  fld <- getFieldId LEN
+  if fld /= 1 
+    then error "getBigUInt"
+    else do
+      len <- varInt
+      bs  <- getLazyByteString (fromIntegral len)
+      return $ BigUInt (runGet getByteList bs)
+
+getByteList :: Get [Word8]
+getByteList = do
+  fld <- getFieldId LEN
+  if fld /= 1 
+    then error "getByteList"
+    else do
+      len <- varInt
+      bs  <- getLazyByteString (fromIntegral len)
+      return (L.unpack bs)
+
+getUnoOpNode :: Get UnoOpNode
+getUnoOpNode = do
+  SomeNode op arg1 _ _ <- getSomeNode 
+  return (UnoOpNode (wordToEnum op) arg1)
+
+getDuoOpNode :: Get DuoOpNode
+getDuoOpNode = do
+  SomeNode op arg1 arg2 _ <- getSomeNode 
+  return (DuoOpNode (wordToEnum op) arg1 arg2)
+
+getTresOpNode :: Get TresOpNode
+getTresOpNode = do
+  SomeNode op arg1 arg2 arg3 <- getSomeNode 
+  return (TresOpNode (wordToEnum op) arg1 arg2 arg3)
+
+wordToEnum :: Enum a => Word32 -> a
+wordToEnum w = toEnum (fromIntegral w)
+
+--------------------------------------------------------------------------------
+
+data SomeNode = SomeNode
+  { field1 :: Word32
+  , field2 :: Word32
+  , field3 :: Word32
+  , field4 :: Word32
+  }
+  deriving Show
+
+defaultSomeNode :: SomeNode
+defaultSomeNode = SomeNode 0 0 0 0
+
+insert1 :: (Int,Word32) -> SomeNode -> SomeNode
+insert1 (idx,val) old = case idx of
+  1 -> old { field1 = val }
+  2 -> old { field2 = val } 
+  3 -> old { field3 = val } 
+  4 -> old { field4 = val }
+
+insertMany :: [(Int,Word32)] -> SomeNode -> SomeNode
+insertMany list old = foldl' (flip insert1) old list
+
+getSomeNode :: Get SomeNode
+getSomeNode = do
+  len  <- varInt
+  bs   <- getLazyByteString (fromIntegral len)
+  let list = runGet getRecord bs
+  return $ insertMany list defaultSomeNode 
+
+--------------------------------------------------------------------------------
+-- TODO: refactor this mess
+
+getMetaData :: Get GraphMetaData
+getMetaData = do
+  len <- varInt
+  mapping <- getWitnessMapping
+  inputs  <- getCircuitInputs
+  return $ GraphMetaData mapping inputs
+
+getWitnessMapping :: Get WitnessMapping
+getWitnessMapping = do
+  fld <- getFieldId LEN 
+  if fld /= 1 
+    then error "getWitnessMapping: expecting field 1"
+    else do
+      len <- varInt
+      bs  <- getLazyByteString (fromIntegral len)
+      return $ WitnessMapping (runGet worker bs)
+  where
+    worker :: Get [Word32]
+    worker = isEmpty >>= \b -> if b 
+      then return [] 
+      else (:) <$> varUInt <*> worker
+
+getCircuitInputs :: Get CircuitInputs
+getCircuitInputs = worker where
+
+  worker :: Get [(String, SignalDescription)]
+  worker = isEmpty >>= \b -> if b 
+    then return [] 
+    else (:) <$> getSingleInput <*> worker
+
+  getSingleInput :: Get (String, SignalDescription)
+  getSingleInput = do
+    fld <- getFieldId LEN 
+    if fld /= 2
+      then error "getCircuitInputs: expecting field 2"
+      else do
+        len <- varInt
+        bs  <- getLazyByteString (fromIntegral len)
+        return $ runGet inputHelper bs
+
+  inputHelper = do
+    name   <- getName
+    signal <- getSignal
+    return (name,signal)
+
+  getName :: Get String
+  getName = do
+    fld <- getFieldId LEN 
+    if fld /= 1
+      then error "getCircuitInputs/getName: expecting field 1"
+      else do
+        len <- varInt
+        bs  <- getLazyByteString (fromIntegral len)
+        return (LC.unpack bs)
+
+  getSignal :: Get SignalDescription
+  getSignal = do
+    fld <- getFieldId LEN 
+    if fld /= 2
+      then error "getCircuitInputs/getSignal: expecting field 2"
+      else do
+        len <- varInt
+        bs  <- getLazyByteString (fromIntegral len)
+        return $ runGet signalHelper bs
+
+  signalHelper = do
+    ofs <- getSignalOffset
+    len <- getSignalLength
+    return $ SignalDescription { signalOffset = ofs , signalLength = len }
+
+  getSignalOffset = do
+    fld <- getFieldId VARINT
+    if fld /= 1
+      then error "getCircuitInputs/getSignalOffset: expecting field 1"
+      else varUInt
+
+  getSignalLength = do
+    fld <- getFieldId VARINT
+    if fld /= 2
+      then error "getCircuitInputs/getSignalLength: expecting field 2"
+      else varUInt
+
+--------------------------------------------------------------------------------
+-- * protobuf stuff
+
+-- | There are six wire types: VARINT, I64, LEN, SGROUP, EGROUP, and I32
+data WireType
+  = VARINT    -- ^ used for: int32, int64, uint32, uint64, sint32, sint64, bool, enum
+  | I64       -- ^ used for: fixed64, sfixed64, double
+  | LEN       -- ^ used for: string, bytes, embedded messages, packed repeated fields
+  | SGROUP    -- ^ used for: group start (deprecated)
+  | EGROUP    -- ^ used for: group end (deprecated)
+  | I32       -- ^ fixed32, sfixed32, float
+  deriving (Eq,Show,Enum,Bounded)
+
+type FieldId = Int
+
+getFieldId :: WireType -> Get FieldId
+getFieldId wty = do
+  tag <- getWord8
+  let (fld,wty') = decodeTag tag
+  if wty == wty' 
+    then return fld
+    else error "getFieldId: unexpected protobuf wire type"
+
+decodeTag_ :: Word8 -> FieldId
+decodeTag_ = fst . decodeTag
+
+decodeTag :: Word8 -> (FieldId, WireType)
+decodeTag w = (fld , wty) where
+  fld = fromIntegral (shiftR w 3) 
+  wty = toEnum (fromIntegral (w .&. 7))
+
+-- (index, value) pair
+getEntry :: Get (Int,Word32)
+getEntry = do
+  idx <- getFieldId VARINT
+  val <- varUInt
+  return (idx,val)
+
+-- list of (index, value) pairs
+getRecord :: Get [(Int,Word32)]
+getRecord = sort <$> go where
+  go = isEmpty >>= \b -> if b
+    then return []
+    else (:) <$> getEntry <*> go
+
+--------------------------------------------------------------------------------
diff --git a/src/Semantics.hs b/src/Semantics.hs
new file mode 100644
index 0000000..ddc1f36
--- /dev/null
+++ b/src/Semantics.hs
@@ -0,0 +1,101 @@
+
+-- | See <https://docs.circom.io/circom-language/basic-operators> for the official
+-- semantics of the operations
+
+{-# LANGUAGE StrictData, DeriveFunctor #-}
+module Semantics where
+
+--------------------------------------------------------------------------------
+
+import Data.Bits
+
+import BN254
+import Misc
+
+--------------------------------------------------------------------------------
+
+data PrimOp a
+  -- unary
+  = Neg  a
+  | Id   a
+  -- binary
+  | Mul  a a
+  | Div  a a
+  | Add  a a
+  | Sub  a a
+  | Pow  a a
+  | Idiv a a
+  | Mod  a a
+  | Eq_  a a
+  | Neq  a a
+  | Lt   a a
+  | Gt   a a
+  | Leq  a a
+  | Geq  a a
+  | Land a a
+  | Lor  a a
+  | Shl  a a
+  | Shr  a a
+  | Bor  a a
+  | Band a a
+  | Bxor a a
+  -- ternary
+  | Cond a a a
+  deriving (Show,Functor)
+
+--------------------------------------------------------------------------------
+
+evalPrimOp :: PrimOp F -> F
+evalPrimOp prim = case prim of
+  Neg  x     -> neg x
+  Id   x     -> x
+  Mul  x y   -> mul x y
+  Div  x y   -> BN254.div x y
+  Add  x y   -> add x y
+  Sub  x y   -> sub x y
+  Pow  x y   -> pow x (fromF y)
+  Idiv x y   -> if isZero y then 0 else toF $ Prelude.div (fromF x) (fromF y)
+  Mod  x y   -> if isZero y then 0 else toF $ Prelude.mod (fromF x) (fromF y)
+  Eq_  x y   -> fromBool (x == y)
+  Neq  x y   -> fromBool (x /= y)
+  Lt   x y   -> fromBool (signedFromF x <  signedFromF y)
+  Gt   x y   -> fromBool (signedFromF x >  signedFromF y)
+  Leq  x y   -> fromBool (signedFromF x <= signedFromF y)
+  Geq  x y   -> fromBool (signedFromF x >= signedFromF y)
+  Land x y   -> fromBool (toBool x && toBool y)
+  Lor  x y   -> fromBool (toBool x || toBool y)
+  Shl  x y   -> shiftLeft  x (fromF y)
+  Shr  x y   -> shiftRight x (fromF y)
+  Bor  x y   -> toF (fromF x  .|.  fromF y)
+  Band x y   -> toF (fromF x  .&.  fromF y)
+  Bxor x y   -> toF (fromF x `xor` fromF y) 
+  Cond b x y -> if toBool b then x else y
+
+--------------------------------------------------------------------------------
+
+fieldMask :: Integer
+fieldMask = shiftL 1 fieldBits - 1
+
+fieldBits :: Int
+fieldBits = ceilingLog2 fieldPrime
+
+shiftRight :: F -> Integer -> F
+shiftRight (MkF x) k = if k < halfPrimePlus1
+  then if k > fromIntegral fieldBits 
+    then 0 
+    else MkF (shiftR x (fromInteger k))
+  else shiftLeft (MkF x) (fieldPrime - k)
+
+shiftLeft :: F -> Integer -> F
+shiftLeft (MkF x) k = if k < halfPrimePlus1
+  then if k > fromIntegral fieldBits
+    then 0
+    else toF (shiftL x (fromIntegral k) .&. fieldMask)
+  else shiftRight (MkF x) (fieldPrime - k)
+
+--------------------------------------------------------------------------------
+
+{-
+notYet :: a
+notYet = error "not yet implemented"
+-}
diff --git a/src/Witness.hs b/src/Witness.hs
new file mode 100644
index 0000000..002f1ea
--- /dev/null
+++ b/src/Witness.hs
@@ -0,0 +1,104 @@
+
+{-# LANGUAGE StrictData #-}
+module Witness where
+
+--------------------------------------------------------------------------------
+
+import Data.Array
+import Data.Word
+
+import qualified Data.Map    as Map    ; import Data.Map    (Map   )
+import qualified Data.IntMap as IntMap ; import Data.IntMap (IntMap)
+
+import BN254
+
+import qualified Semantics as S ; import Semantics ( PrimOp , evalPrimOp )
+import qualified Graph     as G ; import Graph     ( Graph(..) , Node(..) , UnoOpNode(..) , DuoOpNode(..) , TresOpNode(..) , SignalDescription(..) ) 
+
+--------------------------------------------------------------------------------
+
+type Witness = Array Int F
+
+type Inputs = Map String [Integer]
+
+witnessCalc :: Inputs -> Graph -> Witness
+witnessCalc inputs (Graph nodes meta) = witness where
+  nodesArr   = listArray (0,length nodes-1) nodes
+  rawWitness = evaluateNodes rawInputs nodesArr
+  rawInputs  = convertInputs (G.inputSignals meta) inputs 
+  mapping_   = G.fromWitnessMapping (G.witnessMapping meta)
+  wtnslen    = length mapping_
+  mapping    = listArray (0,wtnslen-1) mapping_
+  witness    = listArray (0,wtnslen-1) $ [ rawWitness!(fromIntegral (mapping!i)) | i<-[0..wtnslen-1] ]
+
+convertInputs :: [(String,SignalDescription)] -> Map String [Integer] -> IntMap F
+convertInputs descTable inputTable = IntMap.fromList $ concatMap f descTable where
+  f :: (String,SignalDescription) -> [(Int,F)]
+  f (name,desc) = case Map.lookup name inputTable of
+    Nothing     -> error $ "input signal `" ++ name ++ "` not found in the given inputs!"
+    Just values -> if length values /= fromIntegral (signalLength desc)
+      then error $ "input signal `" ++ name ++ "` has incorrect size"
+      else let ofs = fromIntegral (signalOffset desc) :: Int
+           in  (0,1) : zip [ofs..] (map toF values)
+
+--------------------------------------------------------------------------------
+
+type RawInputs = IntMap F
+
+evaluateNodes :: RawInputs -> Array Int Node -> Array Int F
+evaluateNodes inputs nodes = witness where
+
+  (a,b) = bounds nodes
+  witness = array (0,b-a) $ [ (i-a, worker i) | i<-[a..b] ] 
+
+  lkp :: Word32 -> F
+  lkp i = witness!(fromIntegral i)
+
+  worker i = case nodes!i of
+    AnInputNode   (G.InputNode    idx)  -> getInputValue idx
+    AConstantNode (G.ConstantNode big)  -> fromBigUInt big
+    AnUnoOpNode   unoOpNode             -> evalPrimOp (fmap lkp $ unoToPrimOp  unoOpNode )
+    ADuoOpNode    duoOpNode             -> evalPrimOp (fmap lkp $ duoToPrimOp  duoOpNode )
+    ATresOpNode   tresOpNode            -> evalPrimOp (fmap lkp $ tresToPrimOp tresOpNode)
+
+  getInputValue j = case IntMap.lookup (fromIntegral j) inputs of
+    Just y  -> y
+    Nothing -> error ("input value not found at index " ++ show j)
+
+  fromBigUInt (G.BigUInt bs) = fromBytesLE bs
+
+--------------------------------------------------------------------------------
+
+unoToPrimOp :: UnoOpNode -> PrimOp Word32
+unoToPrimOp (UnoOpNode op arg1) = case op of
+  G.Neg -> S.Neg arg1
+  G.Id  -> S.Id  arg1
+
+duoToPrimOp :: DuoOpNode -> PrimOp Word32
+duoToPrimOp (DuoOpNode op arg1 arg2) = case op of
+  G.Mul  -> S.Mul  arg1 arg2
+  G.Div  -> S.Div  arg1 arg2
+  G.Add  -> S.Add  arg1 arg2
+  G.Sub  -> S.Sub  arg1 arg2
+  G.Pow  -> S.Pow  arg1 arg2
+  G.Idiv -> S.Idiv arg1 arg2
+  G.Mod  -> S.Mod  arg1 arg2
+  G.Eq_  -> S.Eq_  arg1 arg2
+  G.Neq  -> S.Neq  arg1 arg2
+  G.Lt   -> S.Lt   arg1 arg2
+  G.Gt   -> S.Gt   arg1 arg2
+  G.Leq  -> S.Leq  arg1 arg2
+  G.Geq  -> S.Geq  arg1 arg2
+  G.Land -> S.Land arg1 arg2
+  G.Lor  -> S.Lor  arg1 arg2
+  G.Shl  -> S.Shl  arg1 arg2
+  G.Shr  -> S.Shr  arg1 arg2
+  G.Bor  -> S.Bor  arg1 arg2
+  G.Band -> S.Band arg1 arg2
+  G.Bxor -> S.Bxor arg1 arg2
+
+tresToPrimOp :: TresOpNode -> PrimOp Word32
+tresToPrimOp (TresOpNode op arg1 arg2 arg3) = case op of
+  G.TernCond -> S.Cond arg1 arg2 arg3
+
+--------------------------------------------------------------------------------