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# Ethereum 2.0 Altair fork
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**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
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- [Introduction ](#introduction )
- [Configuration ](#configuration )
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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 Ethereum 2.0: the Altair hard fork, introducing light client support and other improvements.
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## Configuration
Warning: this configuration is not definitive.
| Name | Value |
| - | - |
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| `ALTAIR_FORK_VERSION` | `Version('0x01000000')` |
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| `ALTAIR_FORK_EPOCH` | `Epoch(18446744073709551615)` **TBD** |
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## Fork to Altair
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### Fork trigger
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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` .
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Note that for the pure Altair testnets, 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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After `process_slots` of Phase 0 finishes, 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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```python
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def upgrade_to_altair(pre: phase0.BeaconState) -> BeaconState:
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epoch = get_current_epoch(pre)
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 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)
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return post
```
