Speed up shrinking

lib/Echidna/ABI.hs

@@ -1,7 +1,7 @@
 module Echidna.ABI where
 
 import Control.Monad (liftM2, liftM3, foldM, replicateM)
-import Control.Monad.Random.Strict (MonadRandom, join, getRandom, getRandoms, getRandomR)
+import Control.Monad.Random.Strict (MonadRandom, join, getRandom, getRandoms, getRandomR, uniform, fromList)
 import Control.Monad.Random.Strict qualified as Random
 import Data.Binary.Put (runPut, putWord32be)
 import Data.BinaryWord (unsignedWord)
@@ -274,7 +274,30 @@ shrinkAbiValue = \case
 
 -- | Given a 'SolCall', generate a random \"smaller\" (simpler) call.
 shrinkAbiCall :: MonadRandom m => SolCall -> m SolCall
-shrinkAbiCall = traverse $ traverse shrinkAbiValue
+shrinkAbiCall (name, vals) = do
+  let numShrinkable = length $ filter canShrinkAbiValue vals
+
+  halfwayVal <- getRandomR (0, numShrinkable)
+  -- This list was made arbitrarily. Feel free to change
+  let numToShrinkOptions = [1, 2, halfwayVal, numShrinkable]
+
+  numToShrink <- min numShrinkable <$> uniform numToShrinkOptions
+  shrunkVals <- shrinkVals (fromIntegral numShrinkable) (fromIntegral numToShrink) vals
+  pure (name, shrunkVals)
+  where
+    shrinkVals _ 0 l = pure l
+    shrinkVals _ _ [] = pure []
+    shrinkVals numShrinkable numToShrink (h:t)
+      | not (canShrinkAbiValue h) = (h:) <$> shrinkVals numShrinkable numToShrink t
+      | otherwise = do
+          -- We want to pick which ones to shrink uniformly from the vals list.
+          -- Odds of shrinking one element is numToShrink/numShrinkable.
+          shouldShrink <- fromList [(True, numToShrink), (False, numShrinkable-numToShrink)]
+          h' <- if shouldShrink then shrinkAbiValue h else pure h
+          let
+            numShrinkable' = numShrinkable-1
+            numToShrink' = if shouldShrink then numToShrink-1 else numToShrink
+          (h':) <$> shrinkVals numShrinkable' numToShrink' t
 
 -- | Given an 'AbiValue', generate a random \"similar\" value of the same 'AbiType'.
 mutateAbiValue :: MonadRandom m => AbiValue -> m AbiValue