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Phase 0 -- The Beacon Chain

Table of contents

Introduction

This document represents the specification for Phase 0 -- The Beacon Chain.

At the core of Ethereum proof-of-stake is a system chain called the "beacon chain". The beacon chain stores and manages the registry of validators. In the initial deployment phases of proof-of-stake, the only mechanism to become a validator is to make a one-way ETH transaction to a deposit contract on the Ethereum proof-of-work chain. Activation as a validator happens when deposit receipts are processed by the beacon chain, the activation balance is reached, and a queuing process is completed. Exit is either voluntary or done forcibly as a penalty for misbehavior. The primary source of load on the beacon chain is "attestations". Attestations are simultaneously availability votes for a shard block (in a later upgrade) and proof-of-stake votes for a beacon block (Phase 0).

Notation

Code snippets appearing in this style are to be interpreted as Python 3 code.

Custom types

We define the following Python custom types for type hinting and readability:

Name SSZ equivalent Description
Slot uint64 a slot number
Epoch uint64 an epoch number
CommitteeIndex uint64 a committee index at a slot
ValidatorIndex uint64 a validator registry index
Gwei uint64 an amount in Gwei
Root Bytes32 a Merkle root
Hash32 Bytes32 a 256-bit hash
Version Bytes4 a fork version number
DomainType Bytes4 a domain type
ForkDigest Bytes4 a digest of the current fork data
Domain Bytes32 a signature domain
BLSPubkey Bytes48 a BLS12-381 public key
BLSSignature Bytes96 a BLS12-381 signature

Constants

The following values are (non-configurable) constants used throughout the specification.

Misc

Name Value
UINT64_MAX uint64(2**64 - 1)
UINT64_MAX_SQRT uint64(4294967295)
GENESIS_SLOT Slot(0)
GENESIS_EPOCH Epoch(0)
FAR_FUTURE_EPOCH Epoch(2**64 - 1)
BASE_REWARDS_PER_EPOCH uint64(4)
DEPOSIT_CONTRACT_TREE_DEPTH uint64(2**5) (= 32)
JUSTIFICATION_BITS_LENGTH uint64(4)
ENDIANNESS 'little'

Withdrawal prefixes

Name Value
BLS_WITHDRAWAL_PREFIX Bytes1('0x00')
ETH1_ADDRESS_WITHDRAWAL_PREFIX Bytes1('0x01')

Domain types

Name Value
DOMAIN_BEACON_PROPOSER DomainType('0x00000000')
DOMAIN_BEACON_ATTESTER DomainType('0x01000000')
DOMAIN_RANDAO DomainType('0x02000000')
DOMAIN_DEPOSIT DomainType('0x03000000')
DOMAIN_VOLUNTARY_EXIT DomainType('0x04000000')
DOMAIN_SELECTION_PROOF DomainType('0x05000000')
DOMAIN_AGGREGATE_AND_PROOF DomainType('0x06000000')
DOMAIN_APPLICATION_MASK DomainType('0x00000001')

Note: DOMAIN_APPLICATION_MASK reserves the rest of the bitspace in DomainType for application usage. This means for some DomainType DOMAIN_SOME_APPLICATION, DOMAIN_SOME_APPLICATION & DOMAIN_APPLICATION_MASK MUST be non-zero. This expression for any other DomainType in the consensus specs MUST be zero.

Preset

Note: The below configuration is bundled as a preset: a bundle of configuration variables which are expected to differ between different modes of operation, e.g. testing, but not generally between different networks. Additional preset configurations can be found in the configs directory.

Misc

Name Value
MAX_COMMITTEES_PER_SLOT uint64(2**6) (= 64)
TARGET_COMMITTEE_SIZE uint64(2**7) (= 128)
MAX_VALIDATORS_PER_COMMITTEE uint64(2**11) (= 2,048)
SHUFFLE_ROUND_COUNT uint64(90)
HYSTERESIS_QUOTIENT uint64(4)
HYSTERESIS_DOWNWARD_MULTIPLIER uint64(1)
HYSTERESIS_UPWARD_MULTIPLIER uint64(5)
  • For the safety of committees, TARGET_COMMITTEE_SIZE exceeds the recommended minimum committee size of 111; with sufficient active validators (at least SLOTS_PER_EPOCH * TARGET_COMMITTEE_SIZE), the shuffling algorithm ensures committee sizes of at least TARGET_COMMITTEE_SIZE. (Unbiasable randomness with a Verifiable Delay Function (VDF) will improve committee robustness and lower the safe minimum committee size.)

Gwei values

Name Value
MIN_DEPOSIT_AMOUNT Gwei(2**0 * 10**9) (= 1,000,000,000)
MAX_EFFECTIVE_BALANCE Gwei(2**5 * 10**9) (= 32,000,000,000)
EFFECTIVE_BALANCE_INCREMENT Gwei(2**0 * 10**9) (= 1,000,000,000)

Time parameters

Name Value Unit Duration
MIN_ATTESTATION_INCLUSION_DELAY uint64(2**0) (= 1) slots 12 seconds
SLOTS_PER_EPOCH uint64(2**5) (= 32) slots 6.4 minutes
MIN_SEED_LOOKAHEAD uint64(2**0) (= 1) epochs 6.4 minutes
MAX_SEED_LOOKAHEAD uint64(2**2) (= 4) epochs 25.6 minutes
MIN_EPOCHS_TO_INACTIVITY_PENALTY uint64(2**2) (= 4) epochs 25.6 minutes
EPOCHS_PER_ETH1_VOTING_PERIOD uint64(2**6) (= 64) epochs ~6.8 hours
SLOTS_PER_HISTORICAL_ROOT uint64(2**13) (= 8,192) slots ~27 hours

State list lengths

Name Value Unit Duration
EPOCHS_PER_HISTORICAL_VECTOR uint64(2**16) (= 65,536) epochs ~0.8 years
EPOCHS_PER_SLASHINGS_VECTOR uint64(2**13) (= 8,192) epochs ~36 days
HISTORICAL_ROOTS_LIMIT uint64(2**24) (= 16,777,216) historical roots ~52,262 years
VALIDATOR_REGISTRY_LIMIT uint64(2**40) (= 1,099,511,627,776) validators

Rewards and penalties

Name Value
BASE_REWARD_FACTOR uint64(2**6) (= 64)
WHISTLEBLOWER_REWARD_QUOTIENT uint64(2**9) (= 512)
PROPOSER_REWARD_QUOTIENT uint64(2**3) (= 8)
INACTIVITY_PENALTY_QUOTIENT uint64(2**26) (= 67,108,864)
MIN_SLASHING_PENALTY_QUOTIENT uint64(2**7) (= 128)
PROPORTIONAL_SLASHING_MULTIPLIER uint64(1)
  • The INACTIVITY_PENALTY_QUOTIENT equals INVERSE_SQRT_E_DROP_TIME**2 where INVERSE_SQRT_E_DROP_TIME := 2**13 epochs (about 36 days) is the time it takes the inactivity penalty to reduce the balance of non-participating validators to about 1/sqrt(e) ~= 60.6%. Indeed, the balance retained by offline validators after n epochs is about (1 - 1/INACTIVITY_PENALTY_QUOTIENT)**(n**2/2); so after INVERSE_SQRT_E_DROP_TIME epochs, it is roughly (1 - 1/INACTIVITY_PENALTY_QUOTIENT)**(INACTIVITY_PENALTY_QUOTIENT/2) ~= 1/sqrt(e). Note this value will be upgraded to 2**24 after Phase 0 mainnet stabilizes to provide a faster recovery in the event of an inactivity leak.
  • The PROPORTIONAL_SLASHING_MULTIPLIER is set to 1 at initial mainnet launch, resulting in one-third of the minimum accountable safety margin in the event of a finality attack. After Phase 0 mainnet stabilizes, this value will be upgraded to 3 to provide the maximal minimum accountable safety margin.

Max operations per block

Name Value
MAX_PROPOSER_SLASHINGS 2**4 (= 16)
MAX_ATTESTER_SLASHINGS 2**1 (= 2)
MAX_ATTESTATIONS 2**7 (= 128)
MAX_DEPOSITS 2**4 (= 16)
MAX_VOLUNTARY_EXITS 2**4 (= 16)

Configuration

Note: The default mainnet configuration values are included here for illustrative purposes. Defaults for this more dynamic type of configuration are available with the presets in the configs directory. Testnets and other types of chain instances may use a different configuration.

Genesis settings

Name Value
MIN_GENESIS_ACTIVE_VALIDATOR_COUNT uint64(2**14) (= 16,384)
MIN_GENESIS_TIME uint64(1606824000) (Dec 1, 2020, 12pm UTC)
GENESIS_FORK_VERSION Version('0x00000000')
GENESIS_DELAY uint64(604800) (7 days)

Time parameters

Name Value Unit Duration
SECONDS_PER_SLOT uint64(12) seconds 12 seconds
SECONDS_PER_ETH1_BLOCK uint64(14) seconds 14 seconds
MIN_VALIDATOR_WITHDRAWABILITY_DELAY uint64(2**8) (= 256) epochs ~27 hours
SHARD_COMMITTEE_PERIOD uint64(2**8) (= 256) epochs ~27 hours
ETH1_FOLLOW_DISTANCE uint64(2**11) (= 2,048) Eth1 blocks ~8 hours

Validator cycle

Name Value
EJECTION_BALANCE Gwei(2**4 * 10**9) (= 16,000,000,000)
MIN_PER_EPOCH_CHURN_LIMIT uint64(2**2) (= 4)
CHURN_LIMIT_QUOTIENT uint64(2**16) (= 65,536)

Containers

The following types are SimpleSerialize (SSZ) containers.

Note: The definitions are ordered topologically to facilitate execution of the spec.

Note: Fields missing in container instantiations default to their zero value.

Misc dependencies

Fork

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class Fork(Container):
    previous_version: Version
    current_version: Version
    epoch: Epoch  # Epoch of latest fork

ForkData

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class ForkData(Container):
    current_version: Version
    genesis_validators_root: Root

Checkpoint

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class Checkpoint(Container):
    epoch: Epoch
    root: Root

Validator

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class Validator(Container):
    pubkey: BLSPubkey
    withdrawal_credentials: Bytes32  # Commitment to pubkey for withdrawals
    effective_balance: Gwei  # Balance at stake
    slashed: boolean
    # Status epochs
    activation_eligibility_epoch: Epoch  # When criteria for activation were met
    activation_epoch: Epoch
    exit_epoch: Epoch
    withdrawable_epoch: Epoch  # When validator can withdraw funds

AttestationData

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class AttestationData(Container):
    slot: Slot
    index: CommitteeIndex
    # LMD GHOST vote
    beacon_block_root: Root
    # FFG vote
    source: Checkpoint
    target: Checkpoint

IndexedAttestation

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class IndexedAttestation(Container):
    attesting_indices: List[ValidatorIndex, MAX_VALIDATORS_PER_COMMITTEE]
    data: AttestationData
    signature: BLSSignature

PendingAttestation

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class PendingAttestation(Container):
    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
    data: AttestationData
    inclusion_delay: Slot
    proposer_index: ValidatorIndex

Eth1Data

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class Eth1Data(Container):
    deposit_root: Root
    deposit_count: uint64
    block_hash: Hash32

HistoricalBatch

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class HistoricalBatch(Container):
    block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]

DepositMessage

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class DepositMessage(Container):
    pubkey: BLSPubkey
    withdrawal_credentials: Bytes32
    amount: Gwei

DepositData

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class DepositData(Container):
    pubkey: BLSPubkey
    withdrawal_credentials: Bytes32
    amount: Gwei
    signature: BLSSignature  # Signing over DepositMessage

BeaconBlockHeader

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class BeaconBlockHeader(Container):
    slot: Slot
    proposer_index: ValidatorIndex
    parent_root: Root
    state_root: Root
    body_root: Root

SigningData

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class SigningData(Container):
    object_root: Root
    domain: Domain

Beacon operations

ProposerSlashing

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class ProposerSlashing(Container):
    signed_header_1: SignedBeaconBlockHeader
    signed_header_2: SignedBeaconBlockHeader

AttesterSlashing

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class AttesterSlashing(Container):
    attestation_1: IndexedAttestation
    attestation_2: IndexedAttestation

Attestation

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class Attestation(Container):
    aggregation_bits: Bitlist[MAX_VALIDATORS_PER_COMMITTEE]
    data: AttestationData
    signature: BLSSignature

Deposit

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class Deposit(Container):
    proof: Vector[Bytes32, DEPOSIT_CONTRACT_TREE_DEPTH + 1]  # Merkle path to deposit root
    data: DepositData

VoluntaryExit

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class VoluntaryExit(Container):
    epoch: Epoch  # Earliest epoch when voluntary exit can be processed
    validator_index: ValidatorIndex

Beacon blocks

BeaconBlockBody

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class BeaconBlockBody(Container):
    randao_reveal: BLSSignature
    eth1_data: Eth1Data  # Eth1 data vote
    graffiti: Bytes32  # Arbitrary data
    # Operations
    proposer_slashings: List[ProposerSlashing, MAX_PROPOSER_SLASHINGS]
    attester_slashings: List[AttesterSlashing, MAX_ATTESTER_SLASHINGS]
    attestations: List[Attestation, MAX_ATTESTATIONS]
    deposits: List[Deposit, MAX_DEPOSITS]
    voluntary_exits: List[SignedVoluntaryExit, MAX_VOLUNTARY_EXITS]

BeaconBlock

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class BeaconBlock(Container):
    slot: Slot
    proposer_index: ValidatorIndex
    parent_root: Root
    state_root: Root
    body: BeaconBlockBody

Beacon state

BeaconState

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class BeaconState(Container):
    # Versioning
    genesis_time: uint64
    genesis_validators_root: Root
    slot: Slot
    fork: Fork
    # History
    latest_block_header: BeaconBlockHeader
    block_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    state_roots: Vector[Root, SLOTS_PER_HISTORICAL_ROOT]
    historical_roots: List[Root, HISTORICAL_ROOTS_LIMIT]
    # Eth1
    eth1_data: Eth1Data
    eth1_data_votes: List[Eth1Data, EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH]
    eth1_deposit_index: uint64
    # Registry
    validators: List[Validator, VALIDATOR_REGISTRY_LIMIT]
    balances: List[Gwei, VALIDATOR_REGISTRY_LIMIT]
    # Randomness
    randao_mixes: Vector[Bytes32, EPOCHS_PER_HISTORICAL_VECTOR]
    # Slashings
    slashings: Vector[Gwei, EPOCHS_PER_SLASHINGS_VECTOR]  # Per-epoch sums of slashed effective balances
    # Attestations
    previous_epoch_attestations: List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH]
    current_epoch_attestations: List[PendingAttestation, MAX_ATTESTATIONS * SLOTS_PER_EPOCH]
    # Finality
    justification_bits: Bitvector[JUSTIFICATION_BITS_LENGTH]  # Bit set for every recent justified epoch
    previous_justified_checkpoint: Checkpoint  # Previous epoch snapshot
    current_justified_checkpoint: Checkpoint
    finalized_checkpoint: Checkpoint

Signed envelopes

SignedVoluntaryExit

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class SignedVoluntaryExit(Container):
    message: VoluntaryExit
    signature: BLSSignature

SignedBeaconBlock

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class SignedBeaconBlock(Container):
    message: BeaconBlock
    signature: BLSSignature

SignedBeaconBlockHeader

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class SignedBeaconBlockHeader(Container):
    message: BeaconBlockHeader
    signature: BLSSignature

Helper functions

Note: The definitions below are for specification purposes and are not necessarily optimal implementations.

Math

integer_squareroot

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def integer_squareroot(n: uint64) -> uint64:
    """
    Return the largest integer ``x`` such that ``x**2 <= n``.
    """
    if n == UINT64_MAX:
        return UINT64_MAX_SQRT
    x = n
    y = (x + 1) // 2
    while y < x:
        x = y
        y = (x + n // x) // 2
    return x

xor

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def xor(bytes_1: Bytes32, bytes_2: Bytes32) -> Bytes32:
    """
    Return the exclusive-or of two 32-byte strings.
    """
    return Bytes32(a ^ b for a, b in zip(bytes_1, bytes_2))

uint_to_bytes

def uint_to_bytes(n: uint) -> bytes is a function for serializing the uint type object to bytes in ENDIANNESS-endian. The expected length of the output is the byte-length of the uint type.

bytes_to_uint64

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def bytes_to_uint64(data: bytes) -> uint64:
    """
    Return the integer deserialization of ``data`` interpreted as ``ENDIANNESS``-endian.
    """
    return uint64(int.from_bytes(data, ENDIANNESS))

saturating_sub

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def saturating_sub(a: int, b: int) -> int:
    """
    Computes a - b, saturating at numeric bounds.
    """
    return a - b if a > b else 0

Crypto

hash

def hash(data: bytes) -> Bytes32 is SHA256.

hash_tree_root

def hash_tree_root(object: SSZSerializable) -> Root is a function for hashing objects into a single root by utilizing a hash tree structure, as defined in the SSZ spec.

BLS signatures

The IETF BLS signature draft standard v4 with ciphersuite BLS_SIG_BLS12381G2_XMD:SHA-256_SSWU_RO_POP_ defines the following functions:

  • def Sign(privkey: int, message: Bytes) -> BLSSignature
  • def Verify(pubkey: BLSPubkey, message: Bytes, signature: BLSSignature) -> bool
  • def Aggregate(signatures: Sequence[BLSSignature]) -> BLSSignature
  • def FastAggregateVerify(pubkeys: Sequence[BLSPubkey], message: Bytes, signature: BLSSignature) -> bool
  • def AggregateVerify(pubkeys: Sequence[BLSPubkey], messages: Sequence[Bytes], signature: BLSSignature) -> bool
  • def KeyValidate(pubkey: BLSPubkey) -> bool

The above functions are accessed through the bls module, e.g. bls.Verify.

Predicates

is_active_validator

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def is_active_validator(validator: Validator, epoch: Epoch) -> bool:
    """
    Check if ``validator`` is active.
    """
    return validator.activation_epoch <= epoch < validator.exit_epoch

is_eligible_for_activation_queue

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def is_eligible_for_activation_queue(validator: Validator) -> bool:
    """
    Check if ``validator`` is eligible to be placed into the activation queue.
    """
    return (
        validator.activation_eligibility_epoch == FAR_FUTURE_EPOCH
        and validator.effective_balance == MAX_EFFECTIVE_BALANCE
    )

is_eligible_for_activation

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def is_eligible_for_activation(state: BeaconState, validator: Validator) -> bool:
    """
    Check if ``validator`` is eligible for activation.
    """
    return (
        # Placement in queue is finalized
        validator.activation_eligibility_epoch <= state.finalized_checkpoint.epoch
        # Has not yet been activated
        and validator.activation_epoch == FAR_FUTURE_EPOCH
    )

is_slashable_validator

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def is_slashable_validator(validator: Validator, epoch: Epoch) -> bool:
    """
    Check if ``validator`` is slashable.
    """
    return (not validator.slashed) and (validator.activation_epoch <= epoch < validator.withdrawable_epoch)

is_slashable_attestation_data

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def is_slashable_attestation_data(data_1: AttestationData, data_2: AttestationData) -> bool:
    """
    Check if ``data_1`` and ``data_2`` are slashable according to Casper FFG rules.
    """
    return (
        # Double vote
        (data_1 != data_2 and data_1.target.epoch == data_2.target.epoch) or
        # Surround vote
        (data_1.source.epoch < data_2.source.epoch and data_2.target.epoch < data_1.target.epoch)
    )

is_valid_indexed_attestation

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def is_valid_indexed_attestation(state: BeaconState, indexed_attestation: IndexedAttestation) -> bool:
    """
    Check if ``indexed_attestation`` is not empty, has sorted and unique indices and has a valid aggregate signature.
    """
    # Verify indices are sorted and unique
    indices = indexed_attestation.attesting_indices
    if len(indices) == 0 or not indices == sorted(set(indices)):
        return False
    # Verify aggregate signature
    pubkeys = [state.validators[i].pubkey for i in indices]
    domain = get_domain(state, DOMAIN_BEACON_ATTESTER, indexed_attestation.data.target.epoch)
    signing_root = compute_signing_root(indexed_attestation.data, domain)
    return bls.FastAggregateVerify(pubkeys, signing_root, indexed_attestation.signature)

is_valid_merkle_branch

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def is_valid_merkle_branch(leaf: Bytes32, branch: Sequence[Bytes32], depth: uint64, index: uint64, root: Root) -> bool:
    """
    Check if ``leaf`` at ``index`` verifies against the Merkle ``root`` and ``branch``.
    """
    value = leaf
    for i in range(depth):
        if index // (2**i) % 2:
            value = hash(branch[i] + value)
        else:
            value = hash(value + branch[i])
    return value == root

Misc

compute_shuffled_index

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def compute_shuffled_index(index: uint64, index_count: uint64, seed: Bytes32) -> uint64:
    """
    Return the shuffled index corresponding to ``seed`` (and ``index_count``).
    """
    assert index < index_count

    # Swap or not (https://link.springer.com/content/pdf/10.1007%2F978-3-642-32009-5_1.pdf)
    # See the 'generalized domain' algorithm on page 3
    for current_round in range(SHUFFLE_ROUND_COUNT):
        pivot = bytes_to_uint64(hash(seed + uint_to_bytes(uint8(current_round)))[0:8]) % index_count
        flip = (pivot + index_count - index) % index_count
        position = max(index, flip)
        source = hash(
            seed
            + uint_to_bytes(uint8(current_round))
            + uint_to_bytes(uint32(position // 256))
        )
        byte = uint8(source[(position % 256) // 8])
        bit = (byte >> (position % 8)) % 2
        index = flip if bit else index

    return index

compute_proposer_index

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def compute_proposer_index(state: BeaconState, indices: Sequence[ValidatorIndex], seed: Bytes32) -> ValidatorIndex:
    """
    Return from ``indices`` a random index sampled by effective balance.
    """
    assert len(indices) > 0
    MAX_RANDOM_BYTE = 2**8 - 1
    i = uint64(0)
    total = uint64(len(indices))
    while True:
        candidate_index = indices[compute_shuffled_index(i % total, total, seed)]
        random_byte = hash(seed + uint_to_bytes(uint64(i // 32)))[i % 32]
        effective_balance = state.validators[candidate_index].effective_balance
        if effective_balance * MAX_RANDOM_BYTE >= MAX_EFFECTIVE_BALANCE * random_byte:
            return candidate_index
        i += 1

compute_committee

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def compute_committee(indices: Sequence[ValidatorIndex],
                      seed: Bytes32,
                      index: uint64,
                      count: uint64) -> Sequence[ValidatorIndex]:
    """
    Return the committee corresponding to ``indices``, ``seed``, ``index``, and committee ``count``.
    """
    start = (len(indices) * index) // count
    end = (len(indices) * uint64(index + 1)) // count
    return [indices[compute_shuffled_index(uint64(i), uint64(len(indices)), seed)] for i in range(start, end)]

compute_epoch_at_slot

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def compute_epoch_at_slot(slot: Slot) -> Epoch:
    """
    Return the epoch number at ``slot``.
    """
    return Epoch(slot // SLOTS_PER_EPOCH)

compute_start_slot_at_epoch

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def compute_start_slot_at_epoch(epoch: Epoch) -> Slot:
    """
    Return the start slot of ``epoch``.
    """
    return Slot(epoch * SLOTS_PER_EPOCH)

compute_activation_exit_epoch

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def compute_activation_exit_epoch(epoch: Epoch) -> Epoch:
    """
    Return the epoch during which validator activations and exits initiated in ``epoch`` take effect.
    """
    return Epoch(epoch + 1 + MAX_SEED_LOOKAHEAD)

compute_fork_data_root

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def compute_fork_data_root(current_version: Version, genesis_validators_root: Root) -> Root:
    """
    Return the 32-byte fork data root for the ``current_version`` and ``genesis_validators_root``.
    This is used primarily in signature domains to avoid collisions across forks/chains.
    """
    return hash_tree_root(ForkData(
        current_version=current_version,
        genesis_validators_root=genesis_validators_root,
    ))

compute_fork_digest

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def compute_fork_digest(current_version: Version, genesis_validators_root: Root) -> ForkDigest:
    """
    Return the 4-byte fork digest for the ``current_version`` and ``genesis_validators_root``.
    This is a digest primarily used for domain separation on the p2p layer.
    4-bytes suffices for practical separation of forks/chains.
    """
    return ForkDigest(compute_fork_data_root(current_version, genesis_validators_root)[:4])

compute_domain

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def compute_domain(domain_type: DomainType, fork_version: Version=None, genesis_validators_root: Root=None) -> Domain:
    """
    Return the domain for the ``domain_type`` and ``fork_version``.
    """
    if fork_version is None:
        fork_version = GENESIS_FORK_VERSION
    if genesis_validators_root is None:
        genesis_validators_root = Root()  # all bytes zero by default
    fork_data_root = compute_fork_data_root(fork_version, genesis_validators_root)
    return Domain(domain_type + fork_data_root[:28])

compute_signing_root

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def compute_signing_root(ssz_object: SSZObject, domain: Domain) -> Root:
    """
    Return the signing root for the corresponding signing data.
    """
    return hash_tree_root(SigningData(
        object_root=hash_tree_root(ssz_object),
        domain=domain,
    ))

Beacon state accessors

get_current_epoch

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def get_current_epoch(state: BeaconState) -> Epoch:
    """
    Return the current epoch.
    """
    return compute_epoch_at_slot(state.slot)

get_previous_epoch

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def get_previous_epoch(state: BeaconState) -> Epoch:
    """`
    Return the previous epoch (unless the current epoch is ``GENESIS_EPOCH``).
    """
    current_epoch = get_current_epoch(state)
    return GENESIS_EPOCH if current_epoch == GENESIS_EPOCH else Epoch(current_epoch - 1)

get_block_root

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def get_block_root(state: BeaconState, epoch: Epoch) -> Root:
    """
    Return the block root at the start of a recent ``epoch``.
    """
    return get_block_root_at_slot(state, compute_start_slot_at_epoch(epoch))

get_block_root_at_slot

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def get_block_root_at_slot(state: BeaconState, slot: Slot) -> Root:
    """
    Return the block root at a recent ``slot``.
    """
    assert slot < state.slot <= slot + SLOTS_PER_HISTORICAL_ROOT
    return state.block_roots[slot % SLOTS_PER_HISTORICAL_ROOT]

get_randao_mix

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def get_randao_mix(state: BeaconState, epoch: Epoch) -> Bytes32:
    """
    Return the randao mix at a recent ``epoch``.
    """
    return state.randao_mixes[epoch % EPOCHS_PER_HISTORICAL_VECTOR]

get_active_validator_indices

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def get_active_validator_indices(state: BeaconState, epoch: Epoch) -> Sequence[ValidatorIndex]:
    """
    Return the sequence of active validator indices at ``epoch``.
    """
    return [ValidatorIndex(i) for i, v in enumerate(state.validators) if is_active_validator(v, epoch)]

get_validator_churn_limit

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def get_validator_churn_limit(state: BeaconState) -> uint64:
    """
    Return the validator churn limit for the current epoch.
    """
    active_validator_indices = get_active_validator_indices(state, get_current_epoch(state))
    return max(MIN_PER_EPOCH_CHURN_LIMIT, uint64(len(active_validator_indices)) // CHURN_LIMIT_QUOTIENT)

get_seed

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def get_seed(state: BeaconState, epoch: Epoch, domain_type: DomainType) -> Bytes32:
    """
    Return the seed at ``epoch``.
    """
    mix = get_randao_mix(state, Epoch(epoch + EPOCHS_PER_HISTORICAL_VECTOR - MIN_SEED_LOOKAHEAD - 1))  # Avoid underflow
    return hash(domain_type + uint_to_bytes(epoch) + mix)

get_committee_count_per_slot

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def get_committee_count_per_slot(state: BeaconState, epoch: Epoch) -> uint64:
    """
    Return the number of committees in each slot for the given ``epoch``.
    """
    return max(uint64(1), min(
        MAX_COMMITTEES_PER_SLOT,
        uint64(len(get_active_validator_indices(state, epoch))) // SLOTS_PER_EPOCH // TARGET_COMMITTEE_SIZE,
    ))

get_beacon_committee

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def get_beacon_committee(state: BeaconState, slot: Slot, index: CommitteeIndex) -> Sequence[ValidatorIndex]:
    """
    Return the beacon committee at ``slot`` for ``index``.
    """
    epoch = compute_epoch_at_slot(slot)
    committees_per_slot = get_committee_count_per_slot(state, epoch)
    return compute_committee(
        indices=get_active_validator_indices(state, epoch),
        seed=get_seed(state, epoch, DOMAIN_BEACON_ATTESTER),
        index=(slot % SLOTS_PER_EPOCH) * committees_per_slot + index,
        count=committees_per_slot * SLOTS_PER_EPOCH,
    )

get_beacon_proposer_index

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def get_beacon_proposer_index(state: BeaconState) -> ValidatorIndex:
    """
    Return the beacon proposer index at the current slot.
    """
    epoch = get_current_epoch(state)
    seed = hash(get_seed(state, epoch, DOMAIN_BEACON_PROPOSER) + uint_to_bytes(state.slot))
    indices = get_active_validator_indices(state, epoch)
    return compute_proposer_index(state, indices, seed)

get_total_balance

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def get_total_balance(state: BeaconState, indices: Set[ValidatorIndex]) -> Gwei:
    """
    Return the combined effective balance of the ``indices``.
    ``EFFECTIVE_BALANCE_INCREMENT`` Gwei minimum to avoid divisions by zero.
    Math safe up to ~10B ETH, after which this overflows uint64.
    """
    return Gwei(max(EFFECTIVE_BALANCE_INCREMENT, sum([state.validators[index].effective_balance for index in indices])))

get_total_active_balance

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def get_total_active_balance(state: BeaconState) -> Gwei:
    """
    Return the combined effective balance of the active validators.
    Note: ``get_total_balance`` returns ``EFFECTIVE_BALANCE_INCREMENT`` Gwei minimum to avoid divisions by zero.
    """
    return get_total_balance(state, set(get_active_validator_indices(state, get_current_epoch(state))))

get_domain

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def get_domain(state: BeaconState, domain_type: DomainType, epoch: Epoch=None) -> Domain:
    """
    Return the signature domain (fork version concatenated with domain type) of a message.
    """
    epoch = get_current_epoch(state) if epoch is None else epoch
    fork_version = state.fork.previous_version if epoch < state.fork.epoch else state.fork.current_version
    return compute_domain(domain_type, fork_version, state.genesis_validators_root)

get_indexed_attestation

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def get_indexed_attestation(state: BeaconState, attestation: Attestation) -> IndexedAttestation:
    """
    Return the indexed attestation corresponding to ``attestation``.
    """
    attesting_indices = get_attesting_indices(state, attestation)

    return IndexedAttestation(
        attesting_indices=sorted(attesting_indices),
        data=attestation.data,
        signature=attestation.signature,
    )

get_attesting_indices

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def get_attesting_indices(state: BeaconState, attestation: Attestation) -> Set[ValidatorIndex]:
    """
    Return the set of attesting indices corresponding to ``data`` and ``bits``.
    """
    committee = get_beacon_committee(state, attestation.data.slot, attestation.data.index)
    return set(index for i, index in enumerate(committee) if attestation.aggregation_bits[i])

Beacon state mutators

increase_balance

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def increase_balance(state: BeaconState, index: ValidatorIndex, delta: Gwei) -> None:
    """
    Increase the validator balance at index ``index`` by ``delta``.
    """
    state.balances[index] += delta

decrease_balance

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def decrease_balance(state: BeaconState, index: ValidatorIndex, delta: Gwei) -> None:
    """
    Decrease the validator balance at index ``index`` by ``delta``, with underflow protection.
    """
    state.balances[index] = 0 if delta > state.balances[index] else state.balances[index] - delta

initiate_validator_exit

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def initiate_validator_exit(state: BeaconState, index: ValidatorIndex) -> None:
    """
    Initiate the exit of the validator with index ``index``.
    """
    # Return if validator already initiated exit
    validator = state.validators[index]
    if validator.exit_epoch != FAR_FUTURE_EPOCH:
        return

    # Compute exit queue epoch
    exit_epochs = [v.exit_epoch for v in state.validators if v.exit_epoch != FAR_FUTURE_EPOCH]
    exit_queue_epoch = max(exit_epochs + [compute_activation_exit_epoch(get_current_epoch(state))])
    exit_queue_churn = len([v for v in state.validators if v.exit_epoch == exit_queue_epoch])
    if exit_queue_churn >= get_validator_churn_limit(state):
        exit_queue_epoch += Epoch(1)

    # Set validator exit epoch and withdrawable epoch
    validator.exit_epoch = exit_queue_epoch
    validator.withdrawable_epoch = Epoch(validator.exit_epoch + MIN_VALIDATOR_WITHDRAWABILITY_DELAY)

slash_validator

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def slash_validator(state: BeaconState,
                    slashed_index: ValidatorIndex,
                    whistleblower_index: ValidatorIndex=None) -> None:
    """
    Slash the validator with index ``slashed_index``.
    """
    epoch = get_current_epoch(state)
    initiate_validator_exit(state, slashed_index)
    validator = state.validators[slashed_index]
    validator.slashed = True
    validator.withdrawable_epoch = max(validator.withdrawable_epoch, Epoch(epoch + EPOCHS_PER_SLASHINGS_VECTOR))
    state.slashings[epoch % EPOCHS_PER_SLASHINGS_VECTOR] += validator.effective_balance
    decrease_balance(state, slashed_index, validator.effective_balance // MIN_SLASHING_PENALTY_QUOTIENT)

    # Apply proposer and whistleblower rewards
    proposer_index = get_beacon_proposer_index(state)
    if whistleblower_index is None:
        whistleblower_index = proposer_index
    whistleblower_reward = Gwei(validator.effective_balance // WHISTLEBLOWER_REWARD_QUOTIENT)
    proposer_reward = Gwei(whistleblower_reward // PROPOSER_REWARD_QUOTIENT)
    increase_balance(state, proposer_index, proposer_reward)
    increase_balance(state, whistleblower_index, Gwei(whistleblower_reward - proposer_reward))

Genesis

Before the Ethereum beacon chain genesis has been triggered, and for every Ethereum proof-of-work block, let candidate_state = initialize_beacon_state_from_eth1(eth1_block_hash, eth1_timestamp, deposits) where:

  • eth1_block_hash is the hash of the Ethereum proof-of-work block
  • eth1_timestamp is the Unix timestamp corresponding to eth1_block_hash
  • deposits is the sequence of all deposits, ordered chronologically, up to (and including) the block with hash eth1_block_hash

Proof-of-work blocks must only be considered once they are at least SECONDS_PER_ETH1_BLOCK * ETH1_FOLLOW_DISTANCE seconds old (i.e. eth1_timestamp + SECONDS_PER_ETH1_BLOCK * ETH1_FOLLOW_DISTANCE <= current_unix_time). Due to this constraint, if GENESIS_DELAY < SECONDS_PER_ETH1_BLOCK * ETH1_FOLLOW_DISTANCE, then the genesis_time can happen before the time/state is first known. Values should be configured to avoid this case.

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def initialize_beacon_state_from_eth1(eth1_block_hash: Hash32,
                                      eth1_timestamp: uint64,
                                      deposits: Sequence[Deposit]) -> BeaconState:
    fork = Fork(
        previous_version=GENESIS_FORK_VERSION,
        current_version=GENESIS_FORK_VERSION,
        epoch=GENESIS_EPOCH,
    )
    state = BeaconState(
        genesis_time=eth1_timestamp + GENESIS_DELAY,
        fork=fork,
        eth1_data=Eth1Data(block_hash=eth1_block_hash, deposit_count=uint64(len(deposits))),
        latest_block_header=BeaconBlockHeader(body_root=hash_tree_root(BeaconBlockBody())),
        randao_mixes=[eth1_block_hash] * EPOCHS_PER_HISTORICAL_VECTOR,  # Seed RANDAO with Eth1 entropy
    )

    # Process deposits
    leaves = list(map(lambda deposit: deposit.data, deposits))
    for index, deposit in enumerate(deposits):
        deposit_data_list = List[DepositData, 2**DEPOSIT_CONTRACT_TREE_DEPTH](*leaves[:index + 1])
        state.eth1_data.deposit_root = hash_tree_root(deposit_data_list)
        process_deposit(state, deposit)

    # Process activations
    for index, validator in enumerate(state.validators):
        balance = state.balances[index]
        validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)
        if validator.effective_balance == MAX_EFFECTIVE_BALANCE:
            validator.activation_eligibility_epoch = GENESIS_EPOCH
            validator.activation_epoch = GENESIS_EPOCH

    # Set genesis validators root for domain separation and chain versioning
    state.genesis_validators_root = hash_tree_root(state.validators)

    return state

Note: The ETH1 block with eth1_timestamp meeting the minimum genesis active validator count criteria can also occur before MIN_GENESIS_TIME.

Genesis state

Let genesis_state = candidate_state whenever is_valid_genesis_state(candidate_state) is True for the first time.

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def is_valid_genesis_state(state: BeaconState) -> bool:
    if state.genesis_time < MIN_GENESIS_TIME:
        return False
    if len(get_active_validator_indices(state, GENESIS_EPOCH)) < MIN_GENESIS_ACTIVE_VALIDATOR_COUNT:
        return False
    return True

Genesis block

Let genesis_block = BeaconBlock(state_root=hash_tree_root(genesis_state)).

Beacon chain state transition function

The post-state corresponding to a pre-state state and a signed block signed_block is defined as state_transition(state, signed_block). State transitions that trigger an unhandled exception (e.g. a failed assert or an out-of-range list access) are considered invalid. State transitions that cause a uint64 overflow or underflow are also considered invalid.

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def state_transition(state: BeaconState, signed_block: SignedBeaconBlock, validate_result: bool=True) -> None:
    block = signed_block.message
    # Process slots (including those with no blocks) since block
    process_slots(state, block.slot)
    # Verify signature
    if validate_result:
        assert verify_block_signature(state, signed_block)
    # Process block
    process_block(state, block)
    # Verify state root
    if validate_result:
        assert block.state_root == hash_tree_root(state)

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def verify_block_signature(state: BeaconState, signed_block: SignedBeaconBlock) -> bool:
    proposer = state.validators[signed_block.message.proposer_index]
    signing_root = compute_signing_root(signed_block.message, get_domain(state, DOMAIN_BEACON_PROPOSER))
    return bls.Verify(proposer.pubkey, signing_root, signed_block.signature)

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def process_slots(state: BeaconState, slot: Slot) -> None:
    assert state.slot < slot
    while state.slot < slot:
        process_slot(state)
        # Process epoch on the start slot of the next epoch
        if (state.slot + 1) % SLOTS_PER_EPOCH == 0:
            process_epoch(state)
        state.slot = Slot(state.slot + 1)

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def process_slot(state: BeaconState) -> None:
    # Cache state root
    previous_state_root = hash_tree_root(state)
    state.state_roots[state.slot % SLOTS_PER_HISTORICAL_ROOT] = previous_state_root
    # Cache latest block header state root
    if state.latest_block_header.state_root == Bytes32():
        state.latest_block_header.state_root = previous_state_root
    # Cache block root
    previous_block_root = hash_tree_root(state.latest_block_header)
    state.block_roots[state.slot % SLOTS_PER_HISTORICAL_ROOT] = previous_block_root

Epoch processing

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def process_epoch(state: BeaconState) -> None:
    process_justification_and_finalization(state)
    process_rewards_and_penalties(state)
    process_registry_updates(state)
    process_slashings(state)
    process_eth1_data_reset(state)
    process_effective_balance_updates(state)
    process_slashings_reset(state)
    process_randao_mixes_reset(state)
    process_historical_roots_update(state)
    process_participation_record_updates(state)

Helper functions

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def get_matching_source_attestations(state: BeaconState, epoch: Epoch) -> Sequence[PendingAttestation]:
    assert epoch in (get_previous_epoch(state), get_current_epoch(state))
    return state.current_epoch_attestations if epoch == get_current_epoch(state) else state.previous_epoch_attestations

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def get_matching_target_attestations(state: BeaconState, epoch: Epoch) -> Sequence[PendingAttestation]:
    return [
        a for a in get_matching_source_attestations(state, epoch)
        if a.data.target.root == get_block_root(state, epoch)
    ]

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def get_matching_head_attestations(state: BeaconState, epoch: Epoch) -> Sequence[PendingAttestation]:
    return [
        a for a in get_matching_target_attestations(state, epoch)
        if a.data.beacon_block_root == get_block_root_at_slot(state, a.data.slot)
    ]

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def get_unslashed_attesting_indices(state: BeaconState,
                                    attestations: Sequence[PendingAttestation]) -> Set[ValidatorIndex]:
    output = set()  # type: Set[ValidatorIndex]
    for a in attestations:
        output = output.union(get_attesting_indices(state, a))
    return set(filter(lambda index: not state.validators[index].slashed, output))

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def get_attesting_balance(state: BeaconState, attestations: Sequence[PendingAttestation]) -> Gwei:
    """
    Return the combined effective balance of the set of unslashed validators participating in ``attestations``.
    Note: ``get_total_balance`` returns ``EFFECTIVE_BALANCE_INCREMENT`` Gwei minimum to avoid divisions by zero.
    """
    return get_total_balance(state, get_unslashed_attesting_indices(state, attestations))

Justification and finalization

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def process_justification_and_finalization(state: BeaconState) -> None:
    # Initial FFG checkpoint values have a `0x00` stub for `root`.
    # Skip FFG updates in the first two epochs to avoid corner cases that might result in modifying this stub.
    if get_current_epoch(state) <= GENESIS_EPOCH + 1:
        return
    previous_attestations = get_matching_target_attestations(state, get_previous_epoch(state))
    current_attestations = get_matching_target_attestations(state, get_current_epoch(state))
    total_active_balance = get_total_active_balance(state)
    previous_target_balance = get_attesting_balance(state, previous_attestations)
    current_target_balance = get_attesting_balance(state, current_attestations)
    weigh_justification_and_finalization(state, total_active_balance, previous_target_balance, current_target_balance)

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def weigh_justification_and_finalization(state: BeaconState,
                                         total_active_balance: Gwei,
                                         previous_epoch_target_balance: Gwei,
                                         current_epoch_target_balance: Gwei) -> None:
    previous_epoch = get_previous_epoch(state)
    current_epoch = get_current_epoch(state)
    old_previous_justified_checkpoint = state.previous_justified_checkpoint
    old_current_justified_checkpoint = state.current_justified_checkpoint

    # Process justifications
    state.previous_justified_checkpoint = state.current_justified_checkpoint
    state.justification_bits[1:] = state.justification_bits[:JUSTIFICATION_BITS_LENGTH - 1]
    state.justification_bits[0] = 0b0
    if previous_epoch_target_balance * 3 >= total_active_balance * 2:
        state.current_justified_checkpoint = Checkpoint(epoch=previous_epoch,
                                                        root=get_block_root(state, previous_epoch))
        state.justification_bits[1] = 0b1
    if current_epoch_target_balance * 3 >= total_active_balance * 2:
        state.current_justified_checkpoint = Checkpoint(epoch=current_epoch,
                                                        root=get_block_root(state, current_epoch))
        state.justification_bits[0] = 0b1

    # Process finalizations
    bits = state.justification_bits
    # The 2nd/3rd/4th most recent epochs are justified, the 2nd using the 4th as source
    if all(bits[1:4]) and old_previous_justified_checkpoint.epoch + 3 == current_epoch:
        state.finalized_checkpoint = old_previous_justified_checkpoint
    # The 2nd/3rd most recent epochs are justified, the 2nd using the 3rd as source
    if all(bits[1:3]) and old_previous_justified_checkpoint.epoch + 2 == current_epoch:
        state.finalized_checkpoint = old_previous_justified_checkpoint
    # The 1st/2nd/3rd most recent epochs are justified, the 1st using the 3rd as source
    if all(bits[0:3]) and old_current_justified_checkpoint.epoch + 2 == current_epoch:
        state.finalized_checkpoint = old_current_justified_checkpoint
    # The 1st/2nd most recent epochs are justified, the 1st using the 2nd as source
    if all(bits[0:2]) and old_current_justified_checkpoint.epoch + 1 == current_epoch:
        state.finalized_checkpoint = old_current_justified_checkpoint

Rewards and penalties

Helpers
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def get_base_reward(state: BeaconState, index: ValidatorIndex) -> Gwei:
    total_balance = get_total_active_balance(state)
    effective_balance = state.validators[index].effective_balance
    return Gwei(effective_balance * BASE_REWARD_FACTOR // integer_squareroot(total_balance) // BASE_REWARDS_PER_EPOCH)

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def get_proposer_reward(state: BeaconState, attesting_index: ValidatorIndex) -> Gwei:
    return Gwei(get_base_reward(state, attesting_index) // PROPOSER_REWARD_QUOTIENT)

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def get_finality_delay(state: BeaconState) -> uint64:
    return get_previous_epoch(state) - state.finalized_checkpoint.epoch

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def is_in_inactivity_leak(state: BeaconState) -> bool:
    return get_finality_delay(state) > MIN_EPOCHS_TO_INACTIVITY_PENALTY

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def get_eligible_validator_indices(state: BeaconState) -> Sequence[ValidatorIndex]:
    previous_epoch = get_previous_epoch(state)
    return [
        ValidatorIndex(index) for index, v in enumerate(state.validators)
        if is_active_validator(v, previous_epoch) or (v.slashed and previous_epoch + 1 < v.withdrawable_epoch)
    ]

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def get_attestation_component_deltas(state: BeaconState,
                                     attestations: Sequence[PendingAttestation]
                                     ) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Helper with shared logic for use by get source, target, and head deltas functions
    """
    rewards = [Gwei(0)] * len(state.validators)
    penalties = [Gwei(0)] * len(state.validators)
    total_balance = get_total_active_balance(state)
    unslashed_attesting_indices = get_unslashed_attesting_indices(state, attestations)
    attesting_balance = get_total_balance(state, unslashed_attesting_indices)
    for index in get_eligible_validator_indices(state):
        if index in unslashed_attesting_indices:
            increment = EFFECTIVE_BALANCE_INCREMENT  # Factored out from balance totals to avoid uint64 overflow
            if is_in_inactivity_leak(state):
                # Since full base reward will be canceled out by inactivity penalty deltas,
                # optimal participation receives full base reward compensation here.
                rewards[index] += get_base_reward(state, index)
            else:
                reward_numerator = get_base_reward(state, index) * (attesting_balance // increment)
                rewards[index] += reward_numerator // (total_balance // increment)
        else:
            penalties[index] += get_base_reward(state, index)
    return rewards, penalties
Components of attestation deltas
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def get_source_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return attester micro-rewards/penalties for source-vote for each validator.
    """
    matching_source_attestations = get_matching_source_attestations(state, get_previous_epoch(state))
    return get_attestation_component_deltas(state, matching_source_attestations)

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def get_target_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return attester micro-rewards/penalties for target-vote for each validator.
    """
    matching_target_attestations = get_matching_target_attestations(state, get_previous_epoch(state))
    return get_attestation_component_deltas(state, matching_target_attestations)

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def get_head_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return attester micro-rewards/penalties for head-vote for each validator.
    """
    matching_head_attestations = get_matching_head_attestations(state, get_previous_epoch(state))
    return get_attestation_component_deltas(state, matching_head_attestations)

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def get_inclusion_delay_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return proposer and inclusion delay micro-rewards/penalties for each validator.
    """
    rewards = [Gwei(0) for _ in range(len(state.validators))]
    matching_source_attestations = get_matching_source_attestations(state, get_previous_epoch(state))
    for index in get_unslashed_attesting_indices(state, matching_source_attestations):
        attestation = min([
            a for a in matching_source_attestations
            if index in get_attesting_indices(state, a)
        ], key=lambda a: a.inclusion_delay)
        rewards[attestation.proposer_index] += get_proposer_reward(state, index)
        max_attester_reward = Gwei(get_base_reward(state, index) - get_proposer_reward(state, index))
        rewards[index] += Gwei(max_attester_reward // attestation.inclusion_delay)

    # No penalties associated with inclusion delay
    penalties = [Gwei(0) for _ in range(len(state.validators))]
    return rewards, penalties

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def get_inactivity_penalty_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return inactivity reward/penalty deltas for each validator.
    """
    penalties = [Gwei(0) for _ in range(len(state.validators))]
    if is_in_inactivity_leak(state):
        matching_target_attestations = get_matching_target_attestations(state, get_previous_epoch(state))
        matching_target_attesting_indices = get_unslashed_attesting_indices(state, matching_target_attestations)
        for index in get_eligible_validator_indices(state):
            # If validator is performing optimally this cancels all rewards for a neutral balance
            base_reward = get_base_reward(state, index)
            penalties[index] += Gwei(BASE_REWARDS_PER_EPOCH * base_reward - get_proposer_reward(state, index))
            if index not in matching_target_attesting_indices:
                effective_balance = state.validators[index].effective_balance
                penalties[index] += Gwei(effective_balance * get_finality_delay(state) // INACTIVITY_PENALTY_QUOTIENT)

    # No rewards associated with inactivity penalties
    rewards = [Gwei(0) for _ in range(len(state.validators))]
    return rewards, penalties
get_attestation_deltas
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def get_attestation_deltas(state: BeaconState) -> Tuple[Sequence[Gwei], Sequence[Gwei]]:
    """
    Return attestation reward/penalty deltas for each validator.
    """
    source_rewards, source_penalties = get_source_deltas(state)
    target_rewards, target_penalties = get_target_deltas(state)
    head_rewards, head_penalties = get_head_deltas(state)
    inclusion_delay_rewards, _ = get_inclusion_delay_deltas(state)
    _, inactivity_penalties = get_inactivity_penalty_deltas(state)

    rewards = [
        source_rewards[i] + target_rewards[i] + head_rewards[i] + inclusion_delay_rewards[i]
        for i in range(len(state.validators))
    ]

    penalties = [
        source_penalties[i] + target_penalties[i] + head_penalties[i] + inactivity_penalties[i]
        for i in range(len(state.validators))
    ]

    return rewards, penalties
process_rewards_and_penalties
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def process_rewards_and_penalties(state: BeaconState) -> None:
    # No rewards are applied at the end of `GENESIS_EPOCH` because rewards are for work done in the previous epoch
    if get_current_epoch(state) == GENESIS_EPOCH:
        return

    rewards, penalties = get_attestation_deltas(state)
    for index in range(len(state.validators)):
        increase_balance(state, ValidatorIndex(index), rewards[index])
        decrease_balance(state, ValidatorIndex(index), penalties[index])

Registry updates

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def process_registry_updates(state: BeaconState) -> None:
    # Process activation eligibility and ejections
    for index, validator in enumerate(state.validators):
        if is_eligible_for_activation_queue(validator):
            validator.activation_eligibility_epoch = get_current_epoch(state) + 1

        if (
            is_active_validator(validator, get_current_epoch(state))
            and validator.effective_balance <= EJECTION_BALANCE
        ):
            initiate_validator_exit(state, ValidatorIndex(index))

    # Queue validators eligible for activation and not yet dequeued for activation
    activation_queue = sorted([
        index for index, validator in enumerate(state.validators)
        if is_eligible_for_activation(state, validator)
        # Order by the sequence of activation_eligibility_epoch setting and then index
    ], key=lambda index: (state.validators[index].activation_eligibility_epoch, index))
    # Dequeued validators for activation up to churn limit
    for index in activation_queue[:get_validator_churn_limit(state)]:
        validator = state.validators[index]
        validator.activation_epoch = compute_activation_exit_epoch(get_current_epoch(state))

Slashings

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def process_slashings(state: BeaconState) -> None:
    epoch = get_current_epoch(state)
    total_balance = get_total_active_balance(state)
    adjusted_total_slashing_balance = min(sum(state.slashings) * PROPORTIONAL_SLASHING_MULTIPLIER, total_balance)
    for index, validator in enumerate(state.validators):
        if validator.slashed and epoch + EPOCHS_PER_SLASHINGS_VECTOR // 2 == validator.withdrawable_epoch:
            increment = EFFECTIVE_BALANCE_INCREMENT  # Factored out from penalty numerator to avoid uint64 overflow
            penalty_numerator = validator.effective_balance // increment * adjusted_total_slashing_balance
            penalty = penalty_numerator // total_balance * increment
            decrease_balance(state, ValidatorIndex(index), penalty)

Eth1 data votes updates

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def process_eth1_data_reset(state: BeaconState) -> None:
    next_epoch = Epoch(get_current_epoch(state) + 1)
    # Reset eth1 data votes
    if next_epoch % EPOCHS_PER_ETH1_VOTING_PERIOD == 0:
        state.eth1_data_votes = []

Effective balances updates

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def process_effective_balance_updates(state: BeaconState) -> None:
    # Update effective balances with hysteresis
    for index, validator in enumerate(state.validators):
        balance = state.balances[index]
        HYSTERESIS_INCREMENT = uint64(EFFECTIVE_BALANCE_INCREMENT // HYSTERESIS_QUOTIENT)
        DOWNWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_DOWNWARD_MULTIPLIER
        UPWARD_THRESHOLD = HYSTERESIS_INCREMENT * HYSTERESIS_UPWARD_MULTIPLIER
        if (
            balance + DOWNWARD_THRESHOLD < validator.effective_balance
            or validator.effective_balance + UPWARD_THRESHOLD < balance
        ):
            validator.effective_balance = min(balance - balance % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)

Slashings balances updates

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def process_slashings_reset(state: BeaconState) -> None:
    next_epoch = Epoch(get_current_epoch(state) + 1)
    # Reset slashings
    state.slashings[next_epoch % EPOCHS_PER_SLASHINGS_VECTOR] = Gwei(0)

Randao mixes updates

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def process_randao_mixes_reset(state: BeaconState) -> None:
    current_epoch = get_current_epoch(state)
    next_epoch = Epoch(current_epoch + 1)
    # Set randao mix
    state.randao_mixes[next_epoch % EPOCHS_PER_HISTORICAL_VECTOR] = get_randao_mix(state, current_epoch)

Historical roots updates

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def process_historical_roots_update(state: BeaconState) -> None:
    # Set historical root accumulator
    next_epoch = Epoch(get_current_epoch(state) + 1)
    if next_epoch % (SLOTS_PER_HISTORICAL_ROOT // SLOTS_PER_EPOCH) == 0:
        historical_batch = HistoricalBatch(block_roots=state.block_roots, state_roots=state.state_roots)
        state.historical_roots.append(hash_tree_root(historical_batch))

Participation records rotation

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def process_participation_record_updates(state: BeaconState) -> None:
    # Rotate current/previous epoch attestations
    state.previous_epoch_attestations = state.current_epoch_attestations
    state.current_epoch_attestations = []

Block processing

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def process_block(state: BeaconState, block: BeaconBlock) -> None:
    process_block_header(state, block)
    process_randao(state, block.body)
    process_eth1_data(state, block.body)
    process_operations(state, block.body)

Block header

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def process_block_header(state: BeaconState, block: BeaconBlock) -> None:
    # Verify that the slots match
    assert block.slot == state.slot
    # Verify that the block is newer than latest block header
    assert block.slot > state.latest_block_header.slot
    # Verify that proposer index is the correct index
    assert block.proposer_index == get_beacon_proposer_index(state)
    # Verify that the parent matches
    assert block.parent_root == hash_tree_root(state.latest_block_header)
    # Cache current block as the new latest block
    state.latest_block_header = BeaconBlockHeader(
        slot=block.slot,
        proposer_index=block.proposer_index,
        parent_root=block.parent_root,
        state_root=Bytes32(),  # Overwritten in the next process_slot call
        body_root=hash_tree_root(block.body),
    )

    # Verify proposer is not slashed
    proposer = state.validators[block.proposer_index]
    assert not proposer.slashed

RANDAO

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def process_randao(state: BeaconState, body: BeaconBlockBody) -> None:
    epoch = get_current_epoch(state)
    # Verify RANDAO reveal
    proposer = state.validators[get_beacon_proposer_index(state)]
    signing_root = compute_signing_root(epoch, get_domain(state, DOMAIN_RANDAO))
    assert bls.Verify(proposer.pubkey, signing_root, body.randao_reveal)
    # Mix in RANDAO reveal
    mix = xor(get_randao_mix(state, epoch), hash(body.randao_reveal))
    state.randao_mixes[epoch % EPOCHS_PER_HISTORICAL_VECTOR] = mix

Eth1 data

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def process_eth1_data(state: BeaconState, body: BeaconBlockBody) -> None:
    state.eth1_data_votes.append(body.eth1_data)
    if state.eth1_data_votes.count(body.eth1_data) * 2 > EPOCHS_PER_ETH1_VOTING_PERIOD * SLOTS_PER_EPOCH:
        state.eth1_data = body.eth1_data

Operations

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def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
    # Verify that outstanding deposits are processed up to the maximum number of deposits
    assert len(body.deposits) == min(MAX_DEPOSITS, state.eth1_data.deposit_count - state.eth1_deposit_index)

    def for_ops(operations: Sequence[Any], fn: Callable[[BeaconState, Any], None]) -> None:
        for operation in operations:
            fn(state, operation)

    for_ops(body.proposer_slashings, process_proposer_slashing)
    for_ops(body.attester_slashings, process_attester_slashing)
    for_ops(body.attestations, process_attestation)
    for_ops(body.deposits, process_deposit)
    for_ops(body.voluntary_exits, process_voluntary_exit)
Proposer slashings
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def process_proposer_slashing(state: BeaconState, proposer_slashing: ProposerSlashing) -> None:
    header_1 = proposer_slashing.signed_header_1.message
    header_2 = proposer_slashing.signed_header_2.message

    # Verify header slots match
    assert header_1.slot == header_2.slot
    # Verify header proposer indices match
    assert header_1.proposer_index == header_2.proposer_index
    # Verify the headers are different
    assert header_1 != header_2
    # Verify the proposer is slashable
    proposer = state.validators[header_1.proposer_index]
    assert is_slashable_validator(proposer, get_current_epoch(state))
    # Verify signatures
    for signed_header in (proposer_slashing.signed_header_1, proposer_slashing.signed_header_2):
        domain = get_domain(state, DOMAIN_BEACON_PROPOSER, compute_epoch_at_slot(signed_header.message.slot))
        signing_root = compute_signing_root(signed_header.message, domain)
        assert bls.Verify(proposer.pubkey, signing_root, signed_header.signature)

    slash_validator(state, header_1.proposer_index)
Attester slashings
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def process_attester_slashing(state: BeaconState, attester_slashing: AttesterSlashing) -> None:
    attestation_1 = attester_slashing.attestation_1
    attestation_2 = attester_slashing.attestation_2
    assert is_slashable_attestation_data(attestation_1.data, attestation_2.data)
    assert is_valid_indexed_attestation(state, attestation_1)
    assert is_valid_indexed_attestation(state, attestation_2)

    slashed_any = False
    indices = set(attestation_1.attesting_indices).intersection(attestation_2.attesting_indices)
    for index in sorted(indices):
        if is_slashable_validator(state.validators[index], get_current_epoch(state)):
            slash_validator(state, index)
            slashed_any = True
    assert slashed_any
Attestations
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def process_attestation(state: BeaconState, attestation: Attestation) -> None:
    data = attestation.data
    assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
    assert data.target.epoch == compute_epoch_at_slot(data.slot)
    assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= data.slot + SLOTS_PER_EPOCH
    assert data.index < get_committee_count_per_slot(state, data.target.epoch)

    committee = get_beacon_committee(state, data.slot, data.index)
    assert len(attestation.aggregation_bits) == len(committee)

    pending_attestation = PendingAttestation(
        data=data,
        aggregation_bits=attestation.aggregation_bits,
        inclusion_delay=state.slot - data.slot,
        proposer_index=get_beacon_proposer_index(state),
    )

    if data.target.epoch == get_current_epoch(state):
        assert data.source == state.current_justified_checkpoint
        state.current_epoch_attestations.append(pending_attestation)
    else:
        assert data.source == state.previous_justified_checkpoint
        state.previous_epoch_attestations.append(pending_attestation)

    # Verify signature
    assert is_valid_indexed_attestation(state, get_indexed_attestation(state, attestation))
Deposits
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def get_validator_from_deposit(pubkey: BLSPubkey, withdrawal_credentials: Bytes32, amount: uint64) -> Validator:
    effective_balance = min(amount - amount % EFFECTIVE_BALANCE_INCREMENT, MAX_EFFECTIVE_BALANCE)

    return Validator(
        pubkey=pubkey,
        withdrawal_credentials=withdrawal_credentials,
        activation_eligibility_epoch=FAR_FUTURE_EPOCH,
        activation_epoch=FAR_FUTURE_EPOCH,
        exit_epoch=FAR_FUTURE_EPOCH,
        withdrawable_epoch=FAR_FUTURE_EPOCH,
        effective_balance=effective_balance,
    )

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def add_validator_to_registry(state: BeaconState,
                              pubkey: BLSPubkey,
                              withdrawal_credentials: Bytes32,
                              amount: uint64) -> None:
    state.validators.append(get_validator_from_deposit(pubkey, withdrawal_credentials, amount))
    state.balances.append(amount)

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def apply_deposit(state: BeaconState,
                  pubkey: BLSPubkey,
                  withdrawal_credentials: Bytes32,
                  amount: uint64,
                  signature: BLSSignature) -> None:
    validator_pubkeys = [v.pubkey for v in state.validators]
    if pubkey not in validator_pubkeys:
        # Verify the deposit signature (proof of possession) which is not checked by the deposit contract
        deposit_message = DepositMessage(
            pubkey=pubkey,
            withdrawal_credentials=withdrawal_credentials,
            amount=amount,
        )
        domain = compute_domain(DOMAIN_DEPOSIT)  # Fork-agnostic domain since deposits are valid across forks
        signing_root = compute_signing_root(deposit_message, domain)
        if bls.Verify(pubkey, signing_root, signature):
            add_validator_to_registry(state, pubkey, withdrawal_credentials, amount)
    else:
        # Increase balance by deposit amount
        index = ValidatorIndex(validator_pubkeys.index(pubkey))
        increase_balance(state, index, amount)

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def process_deposit(state: BeaconState, deposit: Deposit) -> None:
    # Verify the Merkle branch
    assert is_valid_merkle_branch(
        leaf=hash_tree_root(deposit.data),
        branch=deposit.proof,
        depth=DEPOSIT_CONTRACT_TREE_DEPTH + 1,  # Add 1 for the List length mix-in
        index=state.eth1_deposit_index,
        root=state.eth1_data.deposit_root,
    )

    # Deposits must be processed in order
    state.eth1_deposit_index += 1

    apply_deposit(
        state=state,
        pubkey=deposit.data.pubkey,
        withdrawal_credentials=deposit.data.withdrawal_credentials,
        amount=deposit.data.amount,
        signature=deposit.data.signature,
    )
Voluntary exits
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def process_voluntary_exit(state: BeaconState, signed_voluntary_exit: SignedVoluntaryExit) -> None:
    voluntary_exit = signed_voluntary_exit.message
    validator = state.validators[voluntary_exit.validator_index]
    # Verify the validator is active
    assert is_active_validator(validator, get_current_epoch(state))
    # Verify exit has not been initiated
    assert validator.exit_epoch == FAR_FUTURE_EPOCH
    # Exits must specify an epoch when they become valid; they are not valid before then
    assert get_current_epoch(state) >= voluntary_exit.epoch
    # Verify the validator has been active long enough
    assert get_current_epoch(state) >= validator.activation_epoch + SHARD_COMMITTEE_PERIOD
    # Verify signature
    domain = get_domain(state, DOMAIN_VOLUNTARY_EXIT, voluntary_exit.epoch)
    signing_root = compute_signing_root(voluntary_exit, domain)
    assert bls.Verify(validator.pubkey, signing_root, signed_voluntary_exit.signature)
    # Initiate exit
    initiate_validator_exit(state, voluntary_exit.validator_index)