generalize `ShufflingRef` acceleration logic (#5197) (#5204)

Split up the `ShufflingRef` acceleration logic into generically usable
parts and attester shuffling specific parts. The generic parts could be
used to accelerate other purposes, e.g., REST `/states/xxx/randao` API.

beacon_chain/rpc/rest_beacon_api.nim

@@ -749,6 +749,17 @@ proc installBeaconApiHandlers*(router: var RestRouter, node: BeaconNode) =
         # the state will be obtained.
         bslot.slot.epoch()
 
+    # Try to obtain RANDAO in an accelerated way
+    let bsi = node.dag.atSlot(bslot.bid, (qepoch + 1).start_slot - 1)
+    if bsi.isSome:
+      let mix = node.dag.computeRandaoMix(bsi.get.bid)
+      if mix.isSome:
+        return RestApiResponse.jsonResponseWOpt(
+          RestEpochRandao(randao: mix.get),
+          node.getBidOptimistic(bsi.get.bid)
+        )
+
+    # Fall back to full state computation
     node.withStateForBlockSlotId(bslot):
       withState(state):
         return RestApiResponse.jsonResponseWOpt(

beacon_chain/rpc/rest_utils.nim

@@ -279,6 +279,12 @@ func keysToIndices*(cacheTable: var Table[ValidatorPubKey, ValidatorIndex],
         indices[listIndex[]] = some(ValidatorIndex(validatorIndex))
   indices
 
+proc getBidOptimistic*(node: BeaconNode, bid: BlockId): Option[bool] =
+  if node.currentSlot().epoch() >= node.dag.cfg.BELLATRIX_FORK_EPOCH:
+    some[bool](node.dag.is_optimistic(bid))
+  else:
+    none[bool]()
+
 proc getShufflingOptimistic*(node: BeaconNode,
                              dependentSlot: Slot,
                              dependentRoot: Eth2Digest): Option[bool] =