Add eth.beacon.helpers.is_surround_vote and implement tests

Author: onyb

eth/beacon/helpers.py

@@ -430,3 +430,21 @@ def is_double_vote(attestation_data_1: 'AttestationData',
     due to a 'double vote'.
     """
     return attestation_data_1.slot == attestation_data_2.slot
+
+
+def is_surround_vote(attestation_data_1: 'AttestationData',
+                     attestation_data_2: 'AttestationData') -> bool:
+    """
+    Assumes ``attestation_data_1`` is distinct from ``attestation_data_2``.
+
+    Returns True if the provided ``AttestationData`` are slashable
+    due to a 'surround vote'.
+
+    Note: parameter order matters as this function only checks
+    that ``attestation_data_1`` surrounds ``attestation_data_2``.
+    """
+    return (
+        (attestation_data_1.justified_slot < attestation_data_2.justified_slot) and
+        (attestation_data_2.justified_slot + 1 == attestation_data_2.slot) and
+        (attestation_data_2.slot < attestation_data_1.slot)
+    )

tests/beacon/test_helpers.py

@@ -36,6 +36,7 @@
     _get_shard_committees_at_slot,
     get_block_committees_info,
     is_double_vote,
+    is_surround_vote,
 )
 
 
@@ -757,3 +758,22 @@ def test_is_double_vote(sample_attestation_data_params):
     attestation_data_3 = AttestationData(**attestation_data_3_params)
 
     assert not is_double_vote(attestation_data_1, attestation_data_3)
+
+
+def test_is_surround_vote(sample_attestation_data_params):
+    attestation_data_1_params = {
+        **sample_attestation_data_params,
+        'slot': 4,
+        'justified_slot': 0,
+
+    }
+    attestation_data_1 = AttestationData(**attestation_data_1_params)
+
+    attestation_data_2_params = {
+        **sample_attestation_data_params,
+        'slot': 3,
+        'justified_slot': 2,
+    }
+    attestation_data_2 = AttestationData(**attestation_data_2_params)
+
+    assert is_surround_vote(attestation_data_1, attestation_data_2)