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Fix out-of-bounds in get_shuffling #641

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merged 3 commits into from
Feb 18, 2019
Merged

Fix out-of-bounds in get_shuffling #641

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b92cd9b
Add fix to `get_shuffling`
paulhauner Feb 16, 2019
1d95c14
Update 0_beacon-chain.md
JustinDrake Feb 16, 2019
4baa130
Update 0_beacon-chain.md
JustinDrake Feb 16, 2019
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21 changes: 7 additions & 14 deletions specs/core/0_beacon-chain.md
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Original file line number Diff line number Diff line change
Expand Up @@ -795,23 +795,16 @@ def get_shuffling(seed: Bytes32,
validators: List[Validator],
epoch: Epoch) -> List[List[ValidatorIndex]]
"""
Shuffle ``validators`` into crosslink committees seeded by ``seed`` and ``epoch``.
Return a list of ``committees_per_epoch`` committees where each
committee is itself a list of validator indices.
Shuffle active validators and split into crosslink committees.
Return a list of committees (each a list of validator indices).
"""

# Shuffle active validator indices
active_validator_indices = get_active_validator_indices(validators, epoch)
length = len(active_validator_indices)
shuffled_indices = [active_validator_indices[get_permuted_index(i, length, seed)] for i in range(length)]

committees_per_epoch = get_epoch_committee_count(len(active_validator_indices))

# Shuffle
shuffled_active_validator_indices = [
active_validator_indices[get_permuted_index(i, len(active_validator_indices), seed)]
for i in active_validator_indices
]

# Split the shuffled list into committees_per_epoch pieces
return split(shuffled_active_validator_indices, committees_per_epoch)
# Split the shuffled active validator indices
return split(shuffled_indices, get_epoch_committee_count(length))
```

**Invariant**: if `get_shuffling(seed, validators, epoch)` returns some value `x` for some `epoch <= get_current_epoch(state) + ACTIVATION_EXIT_DELAY`, it should return the same value `x` for the same `seed` and `epoch` and possible future modifications of `validators` forever in phase 0, and until the ~1 year deletion delay in phase 2 and in the future.
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