eth2.0-specs/specs/altair/fork.md

# Altair -- Fork Logic

**Notice**: This document is a work-in-progress for researchers and implementers.

## Table of contents

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- [Introduction](#introduction)
- [Configuration](#configuration)
- [Fork to Altair](#fork-to-altair)
  - [Fork trigger](#fork-trigger)
  - [Upgrading the state](#upgrading-the-state)

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## Introduction

This document describes the process of the first upgrade of the beacon chain: the Altair hard fork, introducing light client support and other improvements.

## Configuration

Warning: this configuration is not definitive.

| Name | Value |
| - | - |
| `ALTAIR_FORK_VERSION` | `Version('0x01000000')` |
| `ALTAIR_FORK_EPOCH` | `Epoch(18446744073709551615)` **TBD** |

## Fork to Altair

### Fork trigger

TBD. Social consensus, along with state conditions such as epoch boundary, finality, deposits, active validator count, etc. may be part of the decision process to trigger the fork. For now we assume the condition will be triggered at epoch `ALTAIR_FORK_EPOCH`.

Note that for the pure Altair networks, we don't apply `upgrade_to_altair` since it starts with Altair version logic.

### Upgrading the state

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.

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`.
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`.

```python
def translate_participation(state: BeaconState, pending_attestations: Sequence[phase0.PendingAttestation]) -> None:
    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):
            for flag_index in participation_flag_indices:
                epoch_participation[index] = add_flag(epoch_participation[index], flag_index)


def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
    epoch = phase0.get_current_epoch(pre)
    post = BeaconState(
        # Versioning
        genesis_time=pre.genesis_time,
        genesis_validators_root=pre.genesis_validators_root,
        slot=pre.slot,
        fork=Fork(
            previous_version=pre.fork.current_version,
            current_version=ALTAIR_FORK_VERSION,
            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,
        # Participation
        previous_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
        current_epoch_participation=[ParticipationFlags(0b0000_0000) for _ in range(len(pre.validators))],
        # 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,
        # Inactivity
        inactivity_scores=[uint64(0) for _ in range(len(pre.validators))],
    )
    # Fill in previous epoch participation from the pre state's pending attestations
    translate_participation(post, pre.previous_epoch_attestations)

    # Fill in sync committees
    # 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)
    return post
```