restore compute_committee

specs/core/0_beacon-chain.md

@@ -73,7 +73,7 @@
         - [`get_beacon_proposer_index`](#get_beacon_proposer_index)
         - [`verify_merkle_branch`](#verify_merkle_branch)
         - [`get_shuffled_index`](#get_shuffled_index)
-        - [`get_committee`](#get_committee)
+        - [`compute_committee`](#compute_committee)
         - [`get_crosslink_committee`](#get_crosslink_committee)
         - [`get_attesting_indices`](#get_attesting_indices)
         - [`int_to_bytes1`, `int_to_bytes2`, ...](#int_to_bytes1-int_to_bytes2-)
@@ -908,23 +908,25 @@ def get_shuffled_index(index: ValidatorIndex, index_count: int, seed: Bytes32) -
     return index
 ```
 
-### `get_committee`
+### `compute_committee`
 
 ```python
-def get_committee(state: BeaconState, epoch: Epoch, index: int, count: int) -> List[ValidatorIndex]:
-    active_indices = get_active_validator_indices(state, epoch)
-    return [
-        active_indices[get_shuffled_index(i, len(active_indices), generate_seed(state, epoch))]
-        for i in range((len(active_indices) * index) // count, (len(active_indices) * (index + 1)) // count)
-    ]
+def compute_committee(indices: List[ValidatorIndex], seed: Bytes32, index: int, count: int) -> List[ValidatorIndex]:
+    start = (len(indices) * index) // count
+    end = (len(indices) * (index + 1)) // count
+    return [indices[get_shuffled_index(i, len(indices), seed)] for i in range(start, end)]
 ```
 
 ### `get_crosslink_committee`
 
 ```python
 def get_crosslink_committee(state: BeaconState, epoch: Epoch, shard: Shard) -> List[ValidatorIndex]:
-    committee_index = (shard + SHARD_COUNT - get_epoch_start_shard(state, epoch)) % SHARD_COUNT
-    return get_committee(state, epoch, committee_index, get_epoch_committee_count(state, epoch))
+    return compute_committee(
+        indices=get_active_validator_indices(state, epoch),
+        seed=generate_seed(state, epoch),
+        index=(shard + SHARD_COUNT - get_epoch_start_shard(state, epoch)) % SHARD_COUNT,
+        count=get_epoch_committee_count(state, epoch),
+    )
 ```
 
 ### `get_attesting_indices`

specs/core/1_shard-data-chains.md

@@ -124,7 +124,12 @@ def get_period_committee(state: BeaconState, epoch: Epoch, shard: Shard, index:
     """
     Return committee for a period. Used to construct persistent committees.
     """
-    return get_committee(state, epoch, shard * count + index, SHARD_COUNT * count)
+    return compute_committee(
+        indices=get_active_validator_indices(state, epoch),
+        seed=generate_seed(state, epoch),
+        index=shard * count + index,
+        count=SHARD_COUNT * count,
+    )
 ```
 
 ### `get_switchover_epoch`