eth2.0-specs/specs/phase1/fork-choice.md

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# Ethereum 2.0 Phase 1 -- Beacon Chain Fork Choice
**Notice**: This document is a work-in-progress for researchers and implementers.
## Table of contents
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- [Introduction](#introduction)
- [Helpers](#helpers)
- [Extended `LatestMessage`](#extended-latestmessage)
- [Updated `update_latest_messages`](#updated-update_latest_messages)
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## Introduction
This document is the beacon chain fork choice spec for part of Ethereum 2.0 Phase 1.
### Helpers
#### Extended `LatestMessage`
```python
@dataclass(eq=True, frozen=True)
class LatestMessage(object):
epoch: Epoch
root: Root
shard: Shard
shard_root: Root
```
#### Updated `update_latest_messages`
```python
def update_latest_messages(store: Store, attesting_indices: Sequence[ValidatorIndex], attestation: Attestation) -> None:
target = attestation.data.target
beacon_block_root = attestation.data.beacon_block_root
# TODO: separate shard chain vote
shard = attestation.data.shard
for i in attesting_indices:
if i not in store.latest_messages or target.epoch > store.latest_messages[i].epoch:
store.latest_messages[i] = LatestMessage(
epoch=target.epoch, root=beacon_block_root, shard=shard, shard_root=attestation.data.shard_head_root
)
```