reduce the frequency of client and server diverging before they reach startTick

the generator for startTick is now heavily weighted to pick a lower value

Author: fraser-iohk

ouroboros-consensus/test/consensus-test/Test/Consensus/MiniProtocol/ChainSync/Client.hs

@@ -766,7 +766,7 @@ instance Arbitrary ChainSyncClientSetup where
           , lastTick (getClientUpdates clientUpdates) - 1
           , lastTick (getServerUpdates serverUpdates) - 1
           ]
-    startTick <- choose (1, maxStartTick)
+    startTick <- chooseExponential 3 (1, maxStartTick)
     let trapBlocks =
           [ blockHash b
           | AddBlock b <- joinSchedule (getServerUpdates serverUpdates)
@@ -829,6 +829,13 @@ instance Arbitrary ChainSyncClientSetup where
     , y <- shrink x
     ]
 
+chooseExponential :: Enum a => Int -> (a, a) -> Gen a
+chooseExponential decayFactor (low, up) =
+  frequency
+    [ (decayFactor, pure low)
+    , (1, chooseExponential decayFactor (succ low, up))
+    ]
+
 prettyChainSyncClientSetup :: ChainSyncClientSetup -> String
 prettyChainSyncClientSetup testSetup =
     unlines