Merge pull request #3769 from ethDreamer/fix_compute_proposer_index

Electra: Properly Calculate Proposer Probabilities

specs/electra/beacon-chain.md

@@ -42,6 +42,7 @@
     - [`BeaconState`](#beaconstate)
 - [Helper functions](#helper-functions)
   - [Predicates](#predicates)
+    - [Updated `compute_proposer_index`](#updated-compute_proposer_index)
     - [Updated `is_eligible_for_activation_queue`](#updated-is_eligible_for_activation_queue)
     - [New `is_compounding_withdrawal_credential`](#new-is_compounding_withdrawal_credential)
     - [New `has_compounding_withdrawal_credential`](#new-has_compounding_withdrawal_credential)
@@ -438,6 +439,27 @@ class BeaconState(Container):
 
 ### Predicates
 
+#### Updated `compute_proposer_index`
+
+```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
+    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]
+        effective_balance = state.validators[candidate_index].effective_balance
+        # [Modified in Electra:EIP7251]
+        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE_ELECTRA * random_byte:
+            return candidate_index
+        i += 1
+```
+
 #### Updated `is_eligible_for_activation_queue`
 
 ```python