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Merge pull request #482 from ethereum/get_shuffing-invariant 

clarify get_shuffling invariant
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Hsiao-Wei Wang 2019-01-25 15:11:04 +08:00 committed by GitHub
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specs/core/0_beacon-chain.md
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@ -905,7 +905,7 @@ def get_shuffling(seed: Bytes32,
return split(shuffled_active_validator_indices, committees_per_slot * EPOCH_LENGTH)
```
**Invariant**: if `get_shuffling(seed, validators, slot)` returns some value `x`, it should return the same value `x` for the same `seed` and `slot` and possible future modifications of `validators` forever in phase 0, and until the ~1 year deletion delay in phase 2 and in the future.
**Invariant**: if `get_shuffling(seed, validators, slot)` returns some value `x` for some `slot <= state.slot + ENTRY_EXIT_DELAY`, it should return the same value `x` for the same `seed` and `slot` and possible future modifications of `validators` forever in phase 0, and until the ~1 year deletion delay in phase 2 and in the future.
**Note**: this definition and the next few definitions make heavy use of repetitive computing. Production implementations are expected to appropriately use caching/memoization to avoid redoing work.