github.com/prysmaticlabs/prysm@v1.4.4/tools/specs-checker/data/specs/phase0/validator.md

```python
def check_if_validator_active(state: BeaconState, validator_index: ValidatorIndex) -> bool:
    validator = state.validators[validator_index]
    return is_active_validator(validator, get_current_epoch(state))
```
```python
def get_committee_assignment(state: BeaconState,
                             epoch: Epoch,
                             validator_index: ValidatorIndex
                             ) -> Optional[Tuple[Sequence[ValidatorIndex], CommitteeIndex, Slot]]:
    """
    Return the committee assignment in the ``epoch`` for ``validator_index``.
    ``assignment`` returned is a tuple of the following form:
        * ``assignment[0]`` is the list of validators in the committee
        * ``assignment[1]`` is the index to which the committee is assigned
        * ``assignment[2]`` is the slot at which the committee is assigned
    Return None if no assignment.
    """
    next_epoch = Epoch(get_current_epoch(state) + 1)
    assert epoch <= next_epoch

    start_slot = compute_start_slot_at_epoch(epoch)
    committee_count_per_slot = get_committee_count_per_slot(state, epoch)
    for slot in range(start_slot, start_slot + SLOTS_PER_EPOCH):
        for index in range(committee_count_per_slot):
            committee = get_beacon_committee(state, Slot(slot), CommitteeIndex(index))
            if validator_index in committee:
                return committee, CommitteeIndex(index), Slot(slot)
    return None
```
```python
def is_proposer(state: BeaconState, validator_index: ValidatorIndex) -> bool:
    return get_beacon_proposer_index(state) == validator_index
```
```python
def get_epoch_signature(state: BeaconState, block: BeaconBlock, privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_RANDAO, compute_epoch_at_slot(block.slot))
    signing_root = compute_signing_root(compute_epoch_at_slot(block.slot), domain)
    return bls.Sign(privkey, signing_root)
```
```python
def compute_time_at_slot(state: BeaconState, slot: Slot) -> uint64:
    return uint64(state.genesis_time + slot * SECONDS_PER_SLOT)
```
```python
def voting_period_start_time(state: BeaconState) -> uint64:
    eth1_voting_period_start_slot = Slot(state.slot - state.slot % (EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH))
    return compute_time_at_slot(state, eth1_voting_period_start_slot)
```
```python
def is_candidate_block(block: Eth1Block, period_start: uint64) -> bool:
    return (
        block.timestamp + SECONDS_PER_ETH1_BLOCK * ETH1_FOLLOW_DISTANCE <= period_start
        and block.timestamp + SECONDS_PER_ETH1_BLOCK * ETH1_FOLLOW_DISTANCE * 2 >= period_start
    )
```
```python
def get_eth1_vote(state: BeaconState, eth1_chain: Sequence[Eth1Block]) -> Eth1Data:
    period_start = voting_period_start_time(state)
    # `eth1_chain` abstractly represents all blocks in the eth1 chain sorted by ascending block height
    votes_to_consider = [
        get_eth1_data(block) for block in eth1_chain
        if (
            is_candidate_block(block, period_start)
            # Ensure cannot move back to earlier deposit contract states
            and get_eth1_data(block).deposit_count >= state.eth1_data.deposit_count
        )
    ]

    # Valid votes already cast during this period
    valid_votes = [vote for vote in state.eth1_data_votes if vote in votes_to_consider]

    # Default vote on latest eth1 block data in the period range unless eth1 chain is not live
    # Non-substantive casting for linter
    state_eth1_data: Eth1Data = state.eth1_data
    default_vote = votes_to_consider[len(votes_to_consider) - 1] if any(votes_to_consider) else state_eth1_data

    return max(
        valid_votes,
        key=lambda v: (valid_votes.count(v), -valid_votes.index(v)),  # Tiebreak by smallest distance
        default=default_vote
    )
```
```python
def compute_new_state_root(state: BeaconState, block: BeaconBlock) -> Root:
    temp_state: BeaconState = state.copy()
    signed_block = SignedBeaconBlock(message=block)
    state_transition(temp_state, signed_block, validate_result=False)
    return hash_tree_root(temp_state)
```
```python
def get_block_signature(state: BeaconState, block: BeaconBlock, privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_BEACON_PROPOSER, compute_epoch_at_slot(block.slot))
    signing_root = compute_signing_root(block, domain)
    return bls.Sign(privkey, signing_root)
```
```python
def get_attestation_signature(state: BeaconState, attestation_data: AttestationData, privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_BEACON_ATTESTER, attestation_data.target.epoch)
    signing_root = compute_signing_root(attestation_data, domain)
    return bls.Sign(privkey, signing_root)
```
```python
def compute_subnet_for_attestation(committees_per_slot: uint64, slot: Slot, committee_index: CommitteeIndex) -> uint64:
    """
    Compute the correct subnet for an attestation for Phase 0.
    Note, this mimics expected future behavior where attestations will be mapped to their shard subnet.
    """
    slots_since_epoch_start = uint64(slot % SLOTS_PER_EPOCH)
    committees_since_epoch_start = committees_per_slot * slots_since_epoch_start

    return uint64((committees_since_epoch_start + committee_index) % ATTESTATION_SUBNET_COUNT)
```
```python
def get_slot_signature(state: BeaconState, slot: Slot, privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_SELECTION_PROOF, compute_epoch_at_slot(slot))
    signing_root = compute_signing_root(slot, domain)
    return bls.Sign(privkey, signing_root)
```
```python
def is_aggregator(state: BeaconState, slot: Slot, index: CommitteeIndex, slot_signature: BLSSignature) -> bool:
    committee = get_beacon_committee(state, slot, index)
    modulo = max(1, len(committee) // TARGET_AGGREGATORS_PER_COMMITTEE)
    return bytes_to_uint64(hash(slot_signature)[0:8]) % modulo == 0
```
```python
def get_aggregate_signature(attestations: Sequence[Attestation]) -> BLSSignature:
    signatures = [attestation.signature for attestation in attestations]
    return bls.Aggregate(signatures)
```
```python
def get_aggregate_and_proof(state: BeaconState,
                            aggregator_index: ValidatorIndex,
                            aggregate: Attestation,
                            privkey: int) -> AggregateAndProof:
    return AggregateAndProof(
        aggregator_index=aggregator_index,
        aggregate=aggregate,
        selection_proof=get_slot_signature(state, aggregate.data.slot, privkey),
    )
```
```python
def get_aggregate_and_proof_signature(state: BeaconState,
                                      aggregate_and_proof: AggregateAndProof,
                                      privkey: int) -> BLSSignature:
    aggregate = aggregate_and_proof.aggregate
    domain = get_domain(state, DOMAIN_AGGREGATE_AND_PROOF, compute_epoch_at_slot(aggregate.data.slot))
    signing_root = compute_signing_root(aggregate_and_proof, domain)
    return bls.Sign(privkey, signing_root)
```