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)
2020-12-11 08:10:50 +00:00
- [Dependency calculation](#dependency-calculation)
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## Introduction
This document is the beacon chain fork choice spec for part of Ethereum 2.0 Phase 1. The only change that we add from phase 0 is that we add a concept of "data dependencies"; a block is only eligible for consideration in the fork choice after a data availability test has been successfully completed for all dependencies. The "root" of a shard block for data dependency purposes is considered to be a DataCommitment object, which is a pair of a Kate commitment and a length.
## Dependency calculation
```python
def get_new_dependencies(state: BeaconState) -> Set[DataCommitment]:
return set(
# Already confirmed during this epoch
[c.commitment for c in state.current_epoch_pending_headers if c.confirmed] +
# Already confirmed during previous epoch
[c.commitment for c in state.previous_epoch_pending_headers if c.confirmed] +
# Confirmed in the epoch before the previous
[c for c in shard for shard in state.most_recent_confirmed_commitments if c != DataCommitment()]
)
```
```python
def get_all_dependencies(store: Store, block: BeaconBlock) -> Set[DataCommitment]:
if block.slot < SHARDING_FORK_SLOT:
return set()
else:
latest = get_new_dependencies(store.block_states[hash_tree_root(block)])
older = get_all_dependencies(store, store.blocks[block.parent_root])
return latest.union(older)
```