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

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# Ethereum 2.0 Light Client Support -- From Phase 0 to Light Client Patch
**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 Light-client patch](#fork-to-light-client-patch)
- [Fork trigger](#fork-trigger)
- [Upgrading the state](#upgrading-the-state)
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## Introduction
This document describes the process of moving from Phase 0 to Phase 1 of Ethereum 2.0.
## Configuration
Warning: this configuration is not definitive.
| Name | Value |
| - | - |
| `ALTAIR_FORK_VERSION` | `Version('0x01000000')` |
| `ALTAIR_FORK_SLOT` | `Slot(0)` **TBD** |
## Fork to Light-client patch
### 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 slot `ALTAIR_FORK_SLOT`, where `ALTAIR_FORK_SLOT % SLOTS_PER_EPOCH == 0`.
### Upgrading the state
After `process_slots` of Phase 0 finishes, if `state.slot == ALTAIR_FORK_SLOT`, an irregular state change is made to upgrade to light-client patch.
```python
def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
epoch = get_current_epoch(pre)
post = BeaconState(
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(0) for _ in range(len(pre.validators))],
current_epoch_participation=[ParticipationFlags(0) 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,
)
# Fill in sync committees
post.current_sync_committee = get_sync_committee(post, get_current_epoch(post))
post.next_sync_committee = get_sync_committee(post, get_current_epoch(post) + EPOCHS_PER_SYNC_COMMITTEE_PERIOD)
return post
```