fix the exponentiation gate and implement random access gate

README.md

@@ -45,7 +45,7 @@ Supported gates:
 - [x] PublicInputGate
 - [x] PoseidonGate
 - [ ] PoseidonMdsGate
-- [ ] RandomAccessGate
+- [X] RandomAccessGate
 - [ ] ReducingGate
 - [ ] ReducingExtensionGate
 

src/Gate/Computation.hs

@@ -90,22 +90,29 @@ instance Pretty v => Pretty (LocalDef v) where
 
 -- | A straightline program encoding the computation of constraints
 data StraightLine = MkStraightLine 
-  { localdefs :: [LocalDef Var_]         -- ^ local definitions, in reverse order
-  , commits   :: [Expr_]                 -- ^ committed constraints, in reverse order
+  { localdefs :: [LocalDef Var_]         -- ^ local definitions (during compilation, in reverse order)
+  , commits   :: [Expr_]                 -- ^ committed constraints (during compilation, in reverse order)
   , counter   :: Int                     -- ^ fresh variable counter
   }
   deriving Show
 
+reverseStraightLine :: StraightLine -> StraightLine
+reverseStraightLine (MkStraightLine{..}) = MkStraightLine 
+  { localdefs = reverse localdefs
+  , commits   = reverse commits
+  , counter   = counter
+  }
+
 emptyStraightLine :: StraightLine
 emptyStraightLine = MkStraightLine [] [] 0
 
 printStraightLine :: StraightLine -> IO ()
 printStraightLine (MkStraightLine{..}) = do
-  forM_ (reverse localdefs) $ \def  -> putStrLn (pretty def)
-  forM_ (reverse commits  ) $ \expr -> putStrLn $ "constraint 0 == " ++ (pretty expr)
+  forM_ (localdefs) $ \def  -> putStrLn (pretty def)
+  forM_ (commits  ) $ \expr -> putStrLn $ "constraint 0 == " ++ (pretty expr)
 
 compileToStraightLine :: Compute () -> StraightLine
-compileToStraightLine = fst . go emptyStraightLine where
+compileToStraightLine = reverseStraightLine . fst . go emptyStraightLine where
   go :: StraightLine -> Compute a -> (StraightLine,a) 
   go state instr = case instr of
      Return x       -> (state,x)
@@ -149,10 +156,13 @@ runStraightLine = runStraightLine' emptyScope
 
 runStraightLine' :: Scope FExt -> EvaluationVars FExt -> StraightLine -> [FExt]
 runStraightLine' iniScope vars (MkStraightLine{..}) = result where
-  finalScope = foldl' worker iniScope (reverse localdefs)
-  result = evalConstraints finalScope vars (reverse commits) 
+  finalScope = foldl' worker iniScope (localdefs)
+  result = evalConstraints finalScope vars (commits) 
   worker !scope (MkLocalDef i _ rhs) = IntMap.insert i (evalConstraint scope vars rhs) scope
 
+runComputation :: EvaluationVars FExt -> Compute () -> [FExt]
+runComputation evalvars action = runStraightLine evalvars (compileToStraightLine action)
+
 --------------------------------------------------------------------------------
 -- * Evaluation
 
@@ -169,6 +179,19 @@ data EvaluationVars a = MkEvaluationVars
   }
   deriving (Show,Functor)
 
+-- | used for testing the gate constraint
+testEvaluationVarsBase :: EvaluationVars F
+testEvaluationVarsBase = MkEvaluationVars
+  { local_selectors    = listArray (0,  0) []
+  , local_constants    = listArray (0,  1) [666,77]
+  , local_wires        = listArray (0,134) [ 1001 + 71 * fromInteger i | i<-[0..134] ]
+  , public_inputs_hash = [101,102,103,104]
+  }
+
+testEvaluationVarsExt :: EvaluationVars FExt
+testEvaluationVarsExt = fmap f testEvaluationVarsBase where
+  f x = MkExt x 13
+
 evalConstraint :: Scope FExt -> EvaluationVars FExt -> Constraint -> FExt
 evalConstraint scope (MkEvaluationVars{..}) expr = evalExprWith f expr where
   f var = case var of

src/Gate/Constraints.hs

@@ -22,6 +22,7 @@ import Gate.Base
 import Gate.Vars
 import Gate.Computation
 import Gate.Poseidon
+import Gate.RandomAccess
 
 import Misc.Aux
 
@@ -62,15 +63,7 @@ gateComputation gate =
 
     -- computes `out = base ^ (sum 2^i e_i)`
     -- order of witness variables: [ base, e[0],...,e[n-1], output, t[0]...t[n-1] ]
-    ExponentiationGate num_power_bits 
-      -> let base      = wire 0
-             exp_bit i = wire (i+1)
-             out       = wire (num_power_bits+1)
-             tmp_val 0 = 1
-             tmp_val i = wire (num_power_bits+1+i)
-             cur_bit i = exp_bit (num_power_bits - 1 - i)
-             eq i      = tmp_val (i-1) * (cur_bit i * base + 1 - cur_bit i) - tmp_val i
-         in  commitList $ [ eq i | i <- range num_power_bits ] ++ [ out - tmp_val (num_power_bits-1) ]
+    ExponentiationGate num_power_bits -> exponentiationGateConstraints num_power_bits
 
     -- lookups are handled specially, no constraints here
     LookupGate      num_slots lut_hash              -> return ()
@@ -95,7 +88,7 @@ gateComputation gate =
       k  -> error ( "gateConstraints/PoseidonMdsGate: unsupported width " ++ show k)
 
     RandomAccessGate num_bits num_copies num_extra_constants 
-      -> todo
+      -> randomAccessGateConstraints (MkRACfg num_bits num_copies num_extra_constants)
 
     ReducingGate num_coeffs 
       -> todo
@@ -110,3 +103,28 @@ gateComputation gate =
     todo = error $ "gateConstraints: gate `" ++ takeWhile isAlpha (show gate) ++ "` not yet implemented"
 
 --------------------------------------------------------------------------------
+
+-- computes `out = base ^ (sum 2^i e_i)`
+-- order of witness variables: [ base, e[0],...,e[n-1], output, t[0]...t[n-1] ]
+exponentiationGateConstraints :: Int -> Compute ()
+exponentiationGateConstraints num_power_bits = 
+  do
+    let prev i = if i==0 then 1 else sqr (tmp_val (i-1))
+    let comp i = prev i * (cur_bit i * base + (1 - cur_bit i))
+    let eq   i = comp i - tmp_val i
+    commitList [ eq i | i <- range num_power_bits ] 
+    commit     ( out - tmp_val (num_power_bits-1) )
+  where
+    base       = wire 0
+    exp_bit i  = wire (i+1)
+    out        = wire (num_power_bits+1)
+    tmp_val i  = wire (num_power_bits+2+i)
+    cur_bit i  = exp_bit (num_power_bits - 1 - i)
+    sqr x      = x*x
+
+--------------------------------------------------------------------------------
+
+testExpoGate = runComputation testEvaluationVarsExt (gateComputation (ExponentiationGate 13))
+
+--------------------------------------------------------------------------------
+

src/Gate/RandomAccess.hs

@@ -0,0 +1,91 @@
+
+-- | The @RandomAccess@ gate
+
+{-# LANGUAGE StrictData, RecordWildCards #-}
+module Gate.RandomAccess where
+
+--------------------------------------------------------------------------------
+
+import Data.Foldable
+import Control.Monad
+
+import Algebra.Goldilocks
+import Algebra.Expr
+
+import Gate.Vars
+import Gate.Computation
+
+import Misc.Aux
+
+--------------------------------------------------------------------------------
+
+num_routed_columns = 80 :: Int
+
+--------------------------------------------------------------------------------
+
+data RandomAccessGateConfig = MkRACfg 
+  { ra_num_bits            :: Int        -- ^ number of bits in the index (so the vector has width @2^n@)
+  , ra_num_copies          :: Int        -- ^ how many copies of this operation is included in a row
+  , ra_num_extra_constants :: Int        -- ^ number of extra cells used as in ConstantGate
+  }
+  deriving Show
+
+randomAccessGateConfig :: Int -> RandomAccessGateConfig
+randomAccessGateConfig num_bits = ra_cfg where
+  veclen = 2 ^ num_bits
+  width  = 2 + veclen
+  copies = Prelude.div num_routed_columns width 
+  extra  = min 2 (num_routed_columns - copies*width)
+  ra_cfg = MkRACfg 
+    { ra_num_bits            = num_bits
+    , ra_num_copies          = copies
+    , ra_num_extra_constants = extra
+    }
+
+--------------------------------------------------------------------------------
+
+randomAccessGateConstraints :: RandomAccessGateConfig -> Compute ()
+randomAccessGateConstraints (MkRACfg{..}) = do
+
+  forM_ [0..ra_num_copies-1] $ \k -> do
+ 
+    -- index bits are actual bits
+    commitList [ bits k j * (bits k j - 1) | j<-[0..ra_num_bits-1] ]
+
+    -- bit decomposition is correct
+    let reconstr = foldr (\b acc -> 2*acc + b) 0 [ bits k j | j<-[0..ra_num_bits-1] ] 
+    commit $ reconstr - index k
+
+    let lkp_val = lookup_eq 
+          [ bits   k j | j<-[0..ra_num_bits-1] ]
+          [ inputs k i | i<-[0..veclen-1]       ]
+    commit $ lkp_val - output k
+
+  forM_ [0..ra_num_extra_constants-1] $ \j -> commit (cnst j - extra j)
+
+  where
+  
+    veclen   = 2 ^ ra_num_bits
+    width    = 2 + veclen
+
+    bits_start_at = width * ra_num_copies + ra_num_extra_constants
+
+    -- witness variables
+    index  k   = wire $ k*width + 0
+    output k   = wire $ k*width + 1
+    inputs k j = wire $ k*width + 2 + j
+    extra  j   = wire $ ra_num_copies * width + j
+    bits   k j = wire $ bits_start_at + k*ra_num_bits + j 
+
+    into_pairs []         = []
+    into_pairs (x:y:rest) = (x,y) : into_pairs rest
+    into_pairs [x]        = error "into_pairs: odd input"
+    
+    lookup_eq []       [z] = z
+    lookup_eq (b:bits) values = lookup_eq bits $ map (\(x,y) -> x + b*(y-x)) (into_pairs values)
+    lookup_eq bits     values = error $ "RandomAccessGate/lookup_eq: shouldn't happen: " ++ show (length bits, length values)
+
+--------------------------------------------------------------------------------
+
+testRAGate = runComputation testEvaluationVarsExt (randomAccessGateConstraints $ randomAccessGateConfig 4)
+