More polishing to fork choice rule
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@ -337,30 +337,29 @@ Beacon block production is significantly different because of the proof of stake
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### Beacon chain fork choice rule
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The beacon chain fork choice rule is a hybrid that combines justification and finality with Latest Message Driven (LMD) GHOST. At any point in time a validator `v` calculates the head as follows.
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The beacon chain fork choice rule is a hybrid that combines justification and finality with Latest Message Driven (LMD) Greediest Heaviest Observed SubTree (GHOST). At any point in time a validator `v` calculates the beacon chain head as follows.
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* Let `store` be the set of all attestations and blocks that the validator `v` has verified (in particular, ancestors must be recursively verified), including attestations not included in any chain.
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* Let `store` be the set of attestations and blocks that the validator `v` has observed and verified (in particular, block ancestors must be recursively verified). Attestations not part of any chain are still included in `store`.
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* Let `finalized_head` be the finalized block with the highest slot number. (A block `B` is finalized if there is a descendant of `B` in `store` the processing of which sets `B` as finalized.)
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* Let `justified_head` be the descendant of `finalized_head` with the highest slot number that has been justified for at least `CYCLE_LENGTH` slots. (A block `B` is justified is there is a descendant of `B` in `store` the processing of which sets `B` as justified.) If no such descendant exists set `justified_head` to `finalized_head`.
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* Let `get_most_recent_attestation(store, validator)` be the attestation in `store` from `validator` with the highest slot number. If several such attestations exist use the one the validator `v` verified first.
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* Let `get_most_recent_attestation_target(store, validator)` be the target block in `get_most_recent_attestation(store, validator)`.
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* Let `get_ancestor(store, block, slot)` be the ancestor of `block` with slot number `slot`. The `get_ancestor` function can be defined recursively as `def get_ancestor(store, block, slot): return block if block.slot == slot else get_ancestor(store, store.get_parent(block), slot)`.
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* Let `get_latest_attestation(store, validator)` be the attestation in `store` from `validator` with the highest slot number. If several such attestations exist use the one the validator `v` verified first.
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* Let `get_latest_attestation_target(store, validator)` be the target block in the attestation `get_latest_attestation(store, validator)`.
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* The head is `lmd_ghost(store, justified_head)` where the function `lmd_ghost` is defined below. Note that the implementation below is suboptimal; there are implementations that compute the head in logarithmic time.
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```python
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def lmd_ghost(store, start):
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validators = [start.state.validators[i] for i in range(get_active_validators(start.state.validators, start.slot))]
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latest_message_targets = [get_most_recent_attestation_target(store, v) for v in validators]
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active_validators = [start.state.validators[i] for i in range(get_active_validators(start.state.validators, start.slot))]
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latest_attestation_targets = [get_latest_attestation_target(store, validator) for validator in active_validators]
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def get_vote_count(block):
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return len([target for target in latest_attestation_targets if get_ancestor(store, target, block.slot) == block])
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head = start
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while 1:
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c = get_children(head)
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if len(c) == 0:
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return head
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def get_vote_count(block):
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return len([t for t in latest_message_targets if get_ancestor(store, t, block.slot) == block])
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head = max(c, key=get_vote_count)
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children = get_children(head)
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if len(children) == 0:
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return head
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head = max(children, key=get_vote_count)
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```
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## Beacon chain state transition function
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@ -1187,7 +1186,6 @@ Note: This spec is ~65% complete.
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**Possible modifications and additions**
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* [ ] Replace the IMD fork choice rule with LMD
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* [ ] Homogenise types to `uint64` ([PR 36](https://github.com/ethereum/eth2.0-specs/pull/36))
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* [ ] Reduce the slot duration to 8 seconds
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* [ ] Allow for the delayed inclusion of aggregated signatures
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