more initialize_validator_exit optimization (#6146)

beacon_chain/spec/beaconstate.nim

@@ -11,13 +11,11 @@ import
   stew/assign2,
   json_serialization/std/sets,
   chronicles,
-  ../extras,
   ./datatypes/[phase0, altair, bellatrix],
   "."/[eth2_merkleization, forks, signatures, validator]
 
 from std/algorithm import fill
 from std/sequtils import anyIt, mapIt, toSeq
-
 from ./datatypes/capella import BeaconState, ExecutionPayloadHeader, Withdrawal
 
 export extras, forks, validator, chronicles
@@ -104,25 +102,31 @@ func initiate_validator_exit*(
   # Return if validator already initiated exit
   let validator = addr state.validators.mitem(index)
 
-  var exit_queue_epoch = compute_activation_exit_epoch(get_current_epoch(state))
+  var
+    exit_queue_epoch = compute_activation_exit_epoch(get_current_epoch(state))
+    exit_queue_churn: uint64
   # Compute max exit epoch
   for idx in 0..<state.validators.len:
     let exit_epoch = state.validators.item(idx).exit_epoch
     if exit_epoch != FAR_FUTURE_EPOCH and exit_epoch > exit_queue_epoch:
       exit_queue_epoch = exit_epoch
 
-  var exit_queue_churn: uint64
-  let validator_churn_limit = get_validator_churn_limit(cfg, state, cache)
-  for idx in 0..<state.validators.len:
-    if state.validators.item(idx).exit_epoch == exit_queue_epoch:
+      # Reset exit queue churn counter as the expected exit_queue_epoch updates
+      # via this essentially max()-but-not-FAR_FUTURE_EPOCH loop to restart the
+      # counting the second for loop in spec version does. Only the last count,
+      # the one corresponding to the ultimately correct exit_queue_epoch, won't
+      # be reset.
+      exit_queue_churn = 0
+
+    # Second spec loop body, which there is responsible for taking the already
+    # known exit_queue_epoch, scanning for all validators with that exit epoch
+    # and checking if they'll reach validator_churn_limit(state). Do that here
+    # incrementally to fuse the two loops and save an all-validator iteration.
+    if exit_epoch == exit_queue_epoch:
       inc exit_queue_churn
 
-      # In spec version, this check occurs only after looping through all
-      # validators, but it is ultimately a binary decision. Once matching
-      # get_validator_churn_limit(state), it can't validly become less.
-      if exit_queue_churn >= validator_churn_limit:
-        inc exit_queue_epoch
-        break
+  if exit_queue_churn >= get_validator_churn_limit(cfg, state, cache):
+    inc exit_queue_epoch
 
   # Set validator exit epoch and withdrawable epoch
   validator.exit_epoch = exit_queue_epoch