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# Altair -- Fork Logic
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## Table of contents
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- [Introduction ](#introduction )
- [Configuration ](#configuration )
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- [Helper functions ](#helper-functions )
- [Misc ](#misc )
- [`compute_fork_version` ](#compute_fork_version )
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- [Fork to Altair ](#fork-to-altair )
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- [Fork trigger ](#fork-trigger )
- [Upgrading the state ](#upgrading-the-state )
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## Introduction
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This document describes the process of the first upgrade of the beacon chain: the Altair hard fork, introducing light client support and other improvements.
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## Configuration
| Name | Value |
| - | - |
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| `ALTAIR_FORK_VERSION` | `Version('0x01000000')` |
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| `ALTAIR_FORK_EPOCH` | `Epoch(74240)` (Oct 27, 2021, 10:56:23am UTC) |
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## Helper functions
### Misc
#### `compute_fork_version`
```python
def compute_fork_version(epoch: Epoch) -> Version:
"""
Return the fork version at the given ``epoch``.
"""
if epoch >= ALTAIR_FORK_EPOCH:
return ALTAIR_FORK_VERSION
return GENESIS_FORK_VERSION
```
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## Fork to Altair
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### Fork trigger
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The fork is triggered at epoch `ALTAIR_FORK_EPOCH` .
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Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since it starts with Altair version logic.
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### Upgrading the state
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If `state.slot % SLOTS_PER_EPOCH == 0` and `compute_epoch_at_slot(state.slot) == ALTAIR_FORK_EPOCH` , an irregular state change is made to upgrade to Altair.
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The upgrade occurs after the completion of the inner loop of `process_slots` that sets `state.slot` equal to `ALTAIR_FORK_EPOCH * SLOTS_PER_EPOCH` .
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Care must be taken when transitioning through the fork boundary as implementations will need a modified [state transition function ](../phase0/beacon-chain.md#beacon-chain-state-transition-function ) that deviates from the Phase 0 document.
In particular, the outer `state_transition` function defined in the Phase 0 document will not expose the precise fork slot to execute the upgrade in the presence of skipped slots at the fork boundary. Instead the logic must be within `process_slots` .
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```python
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def translate_participation(state: BeaconState, pending_attestations: Sequence[phase0.PendingAttestation]) -> None:
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for attestation in pending_attestations:
data = attestation.data
inclusion_delay = attestation.inclusion_delay
# Translate attestation inclusion info to flag indices
participation_flag_indices = get_attestation_participation_flag_indices(state, data, inclusion_delay)
# Apply flags to all attesting validators
epoch_participation = state.previous_epoch_participation
for index in get_attesting_indices(state, data, attestation.aggregation_bits):
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for flag_index in participation_flag_indices:
epoch_participation[index] = add_flag(epoch_participation[index], flag_index)
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def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
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epoch = phase0.get_current_epoch(pre)
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post = BeaconState(
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# Versioning
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genesis_time=pre.genesis_time,
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genesis_validators_root=pre.genesis_validators_root,
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slot=pre.slot,
fork=Fork(
previous_version=pre.fork.current_version,
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current_version=ALTAIR_FORK_VERSION,
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epoch=epoch,
),
# History
latest_block_header=pre.latest_block_header,
block_roots=pre.block_roots,
state_roots=pre.state_roots,
historical_roots=pre.historical_roots,
# Eth1
eth1_data=pre.eth1_data,
eth1_data_votes=pre.eth1_data_votes,
eth1_deposit_index=pre.eth1_deposit_index,
# Registry
validators=pre.validators,
balances=pre.balances,
# Randomness
randao_mixes=pre.randao_mixes,
# Slashings
slashings=pre.slashings,
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# Participation
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previous_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
current_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
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# Finality
justification_bits=pre.justification_bits,
previous_justified_checkpoint=pre.previous_justified_checkpoint,
current_justified_checkpoint=pre.current_justified_checkpoint,
finalized_checkpoint=pre.finalized_checkpoint,
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# Inactivity
Typing problems fixes (#2271)
* Typing problem fixed: `process_block_header` passes `Bytes32()` to `state_root` of `BeaconBlockHeader`, which type is `Root`
* Typing problem fixed in `initialize_beacon_state_from_eth1`: `len` returns an `int` value, while `deposit_count=uint64` of `Eth1Data` has type `uint64`
* Typing problem fixed in `process_rewards_and_penalties`: `numerator` of type `int` passed to `weight` parameter of `get_flag_index_deltas`, which has type `uint64`
* Typing problem fixed in `process_attestation`; `False` passes as `crosslink_success` parameter of `PendingAttestation`, which has type `boolean`. `False` is an instance of `(python.)bool` and is not an instance of `(ssz.)boolean`
* Typing problem fixed: `shard_data_roots` of `ShardTransition` has type `List[Bytes32]`, but its elements are used as if they were `Root` values, e.g. in `process_chunk_challenge` method: passed to `data_root` of `CustodyChunkChallengeRecord` which has type `Root`
* Typing problem fixed in `process_custody_final_updates`: `index` has type `int`, while `validator_indices_in_records` has type `Set[ValidatorIndex]`, so tesing whether `index in validator_indices_in_records` can be risky, depending on implementation details. `ValidatorIndex(index) in validator_indices_in_records` is a safer variant.
* Typing problem fixed: `slashed` parameter of `pack_compact_validator` has type `(python.)bool`, however in `committee_to_compact_committee` a value of `(ssz.)boolean` is passed as a value of the parameter
* Typing problem fixed: `inactivity_scores` is a `List[uint64,...]`, while it is intialized/appended with values of `(python.)int` type
* fixed according to @protolambda suggestions
* changed types of _WEIGHT constants and appropriate variables/parameters, according to @protolambda suggestions
* revert code formatting back
* Introduced ZERO_ROOT according to @protolambda 's suggestion
* Reverted back to , according to @protolambda comments
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inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
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)
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# Fill in previous epoch participation from the pre state's pending attestations
translate_participation(post, pre.previous_epoch_attestations)
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# Fill in sync committees
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# Note: A duplicate committee is assigned for the current and next committee at the fork boundary
post.current_sync_committee = get_next_sync_committee(post)
post.next_sync_committee = get_next_sync_committee(post)
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return post
```