Add get_attestation_participants

Author: hwwhww

eth/beacon/helpers.py

@@ -18,6 +18,10 @@
     Hash32,
 )
 
+from eth.utils.bitfield import (
+    get_bitfield_length,
+    has_voted,
+)
 from eth.utils.numeric import (
     clamp,
 )
@@ -399,3 +403,53 @@ def get_beacon_proposer_index(state: 'BeaconState',
         )
 
     return first_shard_committee.committee[slot % len(first_shard_committee.committee)]
+
+
+#
+# Bitfields
+#
+@to_tuple
+def _get_shard_committees(shard_committees: Sequence[ShardCommittee],
+                          shard: int) -> Iterable[ShardCommittee]:
+    for item in shard_committees:
+        if item.shard == shard:
+            yield item
+
+
+@to_tuple
+def get_attestation_participants(state: 'BeaconState',
+                                 slot: int,
+                                 shard: int,
+                                 participation_bitfield: bytes,
+                                 epoch_length: int) -> Iterable[int]:
+    """
+    Return the participant indices at for the ``slot`` of shard ``shard``
+    from ``participation_bitfield``.
+    """
+    # Find the relevant committee
+    shard_committees = _get_shard_committees(
+        shard_committees=get_shard_committees_at_slot(
+            state,
+            slot,
+            epoch_length,
+        ),
+        shard=shard,
+    )
+    try:
+        shard_committee = shard_committees[0]
+    except IndexError:
+        raise ValueError("shard_committees should not be empty.")
+
+    if len(participation_bitfield) != get_bitfield_length(len(shard_committee.committee)):
+        raise ValueError(
+            'Invalid bitfield length,'
+            "\texpected: %s, found: %s" % (
+                get_bitfield_length(len(shard_committee.committee)),
+                len(participation_bitfield),
+            )
+        )
+
+    # Find the participating attesters in the committee
+    for bitfield_index, validator_index in enumerate(shard_committee.committee):
+        if has_voted(participation_bitfield, bitfield_index):
+            yield validator_index

tests/beacon/test_helpers.py

@@ -11,6 +11,7 @@
 from eth.beacon.helpers import (
     _get_element_from_recent_list,
     get_active_validator_indices,
+    get_attestation_participants,
     get_attestation_indices,
     get_beacon_proposer_index,
     get_block_hash,
@@ -579,3 +580,90 @@ def test_get_active_validator_indices(sample_validator_record_params):
     )
     active_validator_indices = get_active_validator_indices(validators)
     assert len(active_validator_indices) == 2
+
+
+@pytest.mark.parametrize(
+    (
+        'num_validators,'
+        'epoch_length,'
+        'committee,'
+        'participation_bitfield,'
+        'expected'
+    ),
+    [
+        (
+            100,
+            64,
+            (10, 11, 12),
+            b'\00',
+            (),
+        ),
+        (
+            100,
+            64,
+            (10, 11, 12),
+            b'\x80',
+            (10,),
+        ),
+        (
+            100,
+            64,
+            (10, 11, 12),
+            b'\xc0',
+            (10, 11),
+        ),
+        (
+            100,
+            64,
+            (10, 11, 12),
+            b'\x00\x00',
+            ValueError(),
+        ),
+    ]
+)
+def test_get_attestation_participants(
+        monkeypatch,
+        num_validators,
+        epoch_length,
+        committee,
+        participation_bitfield,
+        expected,
+        sample_state):
+    from eth.beacon import helpers
+
+    def mock_get_shard_committees_at_slot(state,
+                                          slot,
+                                          epoch_length):
+        return (
+            ShardCommittee(
+                shard=0,
+                committee=committee,
+                total_validator_count=num_validators,
+            ),
+        )
+
+    monkeypatch.setattr(
+        helpers,
+        'get_shard_committees_at_slot',
+        mock_get_shard_committees_at_slot
+    )
+
+    if isinstance(expected, Exception):
+        with pytest.raises(ValueError):
+            get_attestation_participants(
+                state=sample_state,
+                slot=0,
+                shard=0,
+                participation_bitfield=participation_bitfield,
+                epoch_length=epoch_length,
+            )
+    else:
+        result = get_attestation_participants(
+            state=sample_state,
+            slot=0,
+            shard=0,
+            participation_bitfield=participation_bitfield,
+            epoch_length=epoch_length,
+        )
+
+        assert result == expected