Merge pull request #4039 from jtraglia/16-bit-filter

Use 16-bit random value in validator filter

specs/electra/beacon-chain.md

@@ -427,23 +427,26 @@ class BeaconState(Container):
 
 #### Modified `compute_proposer_index`
 
-*Note*: The function `compute_proposer_index` is modified to use `MAX_EFFECTIVE_BALANCE_ELECTRA`.
+*Note*: The function `compute_proposer_index` is modified to use `MAX_EFFECTIVE_BALANCE_ELECTRA` and to use a 16-bit random value instead of an 8-bit random byte in the effective balance filter.
 
 ```python
 def compute_proposer_index(state: BeaconState, indices: Sequence[ValidatorIndex], seed: Bytes32) -> ValidatorIndex:
     """
     Return from ``indices`` a random index sampled by effective balance.
     """
     assert len(indices) > 0
-    MAX_RANDOM_BYTE = 2**8 - 1
+    MAX_RANDOM_VALUE = 2**16 - 1  # [Modified in Electra]
     i = uint64(0)
     total = uint64(len(indices))
     while True:
         candidate_index = indices[compute_shuffled_index(i % total, total, seed)]
-        random_byte = hash(seed + uint_to_bytes(uint64(i // 32)))[i % 32]
+        # [Modified in Electra]
+        random_bytes = hash(seed + uint_to_bytes(i // 16))
+        offset = i % 16 * 2
+        random_value = bytes_to_uint64(random_bytes[offset:offset + 2])
         effective_balance = state.validators[candidate_index].effective_balance
         # [Modified in Electra:EIP7251]
-        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE_ELECTRA * random_byte:
+        if effective_balance * MAX_RANDOM_VALUE >= MAX_EFFECTIVE_BALANCE_ELECTRA * random_value:
             return candidate_index
         i += 1
 ```
@@ -617,7 +620,7 @@ def get_attesting_indices(state: BeaconState, attestation: Attestation) -> Set[V
 
 #### Modified `get_next_sync_committee_indices`
 
-*Note*: The function `get_next_sync_committee_indices` is modified to use `MAX_EFFECTIVE_BALANCE_ELECTRA`.
+*Note*: The function `get_next_sync_committee_indices` is modified to use `MAX_EFFECTIVE_BALANCE_ELECTRA` and to use a 16-bit random value instead of an 8-bit random byte in the effective balance filter.
 
 ```python
 def get_next_sync_committee_indices(state: BeaconState) -> Sequence[ValidatorIndex]:
@@ -626,19 +629,22 @@ def get_next_sync_committee_indices(state: BeaconState) -> Sequence[ValidatorInd
     """
     epoch = Epoch(get_current_epoch(state) + 1)
 
-    MAX_RANDOM_BYTE = 2**8 - 1
+    MAX_RANDOM_VALUE = 2**16 - 1  # [Modified in Electra]
     active_validator_indices = get_active_validator_indices(state, epoch)
     active_validator_count = uint64(len(active_validator_indices))
     seed = get_seed(state, epoch, DOMAIN_SYNC_COMMITTEE)
-    i = 0
+    i = uint64(0)
     sync_committee_indices: List[ValidatorIndex] = []
     while len(sync_committee_indices) < SYNC_COMMITTEE_SIZE:
         shuffled_index = compute_shuffled_index(uint64(i % active_validator_count), active_validator_count, seed)
         candidate_index = active_validator_indices[shuffled_index]
-        random_byte = hash(seed + uint_to_bytes(uint64(i // 32)))[i % 32]
+        # [Modified in Electra]
+        random_bytes = hash(seed + uint_to_bytes(i // 16))
+        offset = i % 16 * 2
+        random_value = bytes_to_uint64(random_bytes[offset:offset + 2])
         effective_balance = state.validators[candidate_index].effective_balance
         # [Modified in Electra:EIP7251]
-        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE_ELECTRA * random_byte:
+        if effective_balance * MAX_RANDOM_VALUE >= MAX_EFFECTIVE_BALANCE_ELECTRA * random_value:
             sync_committee_indices.append(candidate_index)
         i += 1
     return sync_committee_indices