add compute_offset_slots

specs/phase1/beacon-chain.md

@@ -517,11 +517,18 @@ def get_latest_slot_for_shard(state: BeaconState, shard: Shard) -> Slot:
     return state.shard_states[shard].slot
 ```
 
+#### `compute_offset_slots`
+
+```python
+def compute_offset_slots(start_slot: Slot, end_slot: Slot) -> Sequence[Slot]:
+    return [Slot(start_slot + x) for x in SHARD_BLOCK_OFFSETS if start_slot + x < end_slot]
+```
+
 #### `get_offset_slots`
 
 ```python
-def get_offset_slots(state: BeaconState, latest_shard_slot: Slot) -> Sequence[Slot]:
-    return [Slot(latest_shard_slot + x) for x in SHARD_BLOCK_OFFSETS if latest_shard_slot + x < state.slot]
+def get_offset_slots(state: BeaconState, shard: Shard) -> Sequence[Slot]:
+    return compute_offset_slots(state.shard_states[shard].slot, state.slot)
 ```
 
 ### Predicates
@@ -630,14 +637,13 @@ def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
         assert attestation.data.source == state.previous_justified_checkpoint
 
     shard = get_shard(state, attestation)
-    latest_shard_slot = get_latest_slot_for_shard(state, shard)
 
     # Type 1: on-time attestations
     if attestation.custody_bits_blocks != []:
         # Ensure on-time attestation
         assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY == state.slot
         # Correct data root count
-        assert len(attestation.custody_bits_blocks) == len(get_offset_slots(state, latest_shard_slot))
+        assert len(attestation.custody_bits_blocks) == len(get_offset_slots(state, shard))
         # Correct parent block root
         assert data.beacon_block_root == get_block_root_at_slot(state, get_previous_slot(state.slot))
     # Type 2: no shard transition, no custody bits  # TODO: could only allow for older attestations.
@@ -655,11 +661,8 @@ def validate_attestation(state: BeaconState, attestation: Attestation) -> None:
 
 ```python
 def apply_shard_transition(state: BeaconState, shard: Shard, transition: ShardTransition) -> None:
-    # Slot the attestation starts counting from
-    latest_slot = get_latest_slot_for_shard(state, shard)
-
     # Correct data root count
-    offset_slots = get_offset_slots(state, latest_slot)
+    offset_slots = get_offset_slots(state, shard)
     assert (
         len(transition.shard_data_roots)
         == len(transition.shard_states)

tests/core/pyspec/eth2spec/test/helpers/attestations.py

@@ -1,7 +1,7 @@
 from typing import List
 
 from eth2spec.test.context import expect_assertion_error
-from eth2spec.test.helpers.state import next_slot, state_transition_and_sign_block
+from eth2spec.test.helpers.state import state_transition_and_sign_block
 from eth2spec.test.helpers.block import build_empty_block_for_next_slot
 from eth2spec.test.helpers.keys import privkeys
 from eth2spec.utils import bls
@@ -77,10 +77,7 @@ def build_attestation_data(spec, state, slot, index):
 
 def convert_to_valid_on_time_attestation(spec, state, attestation, signed=False):
     shard = spec.get_shard(state, attestation)
-
-    next_state = state.copy()
-    next_slot(spec, next_state)
-    offset_slots = spec.get_offset_slots(next_state, spec.get_latest_slot_for_shard(next_state, shard))
+    offset_slots = spec.compute_offset_slots(spec.get_latest_slot_for_shard(state, shard), state.slot + 1)
     for offset_slot in offset_slots:
         attestation.custody_bits_blocks.append(
             Bitlist[spec.MAX_VALIDATORS_PER_COMMITTEE]([0 for _ in attestation.aggregation_bits])

tests/core/pyspec/eth2spec/test/helpers/phase1/attestations.py

@@ -3,7 +3,6 @@ from eth2spec.utils import bls
 
 from eth2spec.test.helpers.keys import privkeys
 import eth2spec.test.helpers.attestations as phase0_attestations
-from eth2spec.test.helpers.state import next_slot
 
 
 def get_valid_on_time_attestation(spec, state, index=None, signed=False):
@@ -15,10 +14,8 @@ def get_valid_on_time_attestation(spec, state, index=None, signed=False):
 
     attestation = phase0_attestations.get_valid_attestation(spec, state, state.slot, index, False)
     shard = spec.get_shard(state, attestation)
+    offset_slots = spec.compute_offset_slots(spec.get_latest_slot_for_shard(state, shard), state.slot + 1)
 
-    next_state = state.copy()
-    next_slot(spec, next_state)
-    offset_slots = spec.get_offset_slots(next_state, spec.get_latest_slot_for_shard(next_state, shard))
     for offset_slot in offset_slots:
         attestation.custody_bits_blocks.append(
             Bitlist[spec.MAX_VALIDATORS_PER_COMMITTEE]([0 for _ in attestation.aggregation_bits])