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Minimum slot number, simplify excessive anti-underflow logic 

Adds a minimum slot number large enough that integer underflows involving epochs and slots will not happen; simplifies some logic that was more complex to handle them.
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vbuterin 2019-01-27 23:12:37 -06:00 committed by GitHub
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specs/core/0_beacon-chain.md
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@ -190,7 +190,7 @@ Code snippets appearing in `this style` are to be interpreted as Python code. Be
| Name | Value | | Name | Value |
| - | - | | - | - |
| `GENESIS_FORK_VERSION` | `0` | | `GENESIS_FORK_VERSION` | `0` |
| `GENESIS_SLOT` | `0` | | `GENESIS_SLOT` | `2**19` |
| `GENESIS_EPOCH` | `slot_to_epoch(GENESIS_SLOT)` | | `GENESIS_EPOCH` | `slot_to_epoch(GENESIS_SLOT)` |
| `GENESIS_START_SHARD` | `0` | | `GENESIS_START_SHARD` | `0` |
| `FAR_FUTURE_EPOCH` | `2**64 - 1` | | `FAR_FUTURE_EPOCH` | `2**64 - 1` |
@ -198,6 +198,8 @@ Code snippets appearing in `this style` are to be interpreted as Python code. Be
| `EMPTY_SIGNATURE` | `int_to_bytes96(0)` | | `EMPTY_SIGNATURE` | `int_to_bytes96(0)` |
| `BLS_WITHDRAWAL_PREFIX_BYTE` | `int_to_bytes1(0)` | | `BLS_WITHDRAWAL_PREFIX_BYTE` | `int_to_bytes1(0)` |
* `GENESIS_SLOT` should be at least as large in terms of time as the largest of the time parameters or state list lengths below (ie. it should be at least as large as any value measured in slots, and at least `EPOCH_LENGTH` times as large as any value measured in epochs).
### Time parameters ### Time parameters
| Name | Value | Unit | Duration | | Name | Value | Unit | Duration |
@ -212,11 +214,12 @@ Code snippets appearing in `this style` are to be interpreted as Python code. Be
### State list lengths ### State list lengths
| Name | Value | Unit | | Name | Value | Unit | Duration |
| `LATEST_BLOCK_ROOTS_LENGTH` | `2**13` (= 8,192) | slots | | - | - | :-: | :-: |
| `LATEST_RANDAO_MIXES_LENGTH` | `2**13` (= 8,192) | epochs | | `LATEST_BLOCK_ROOTS_LENGTH` | `2**13` (= 8,192) | slots | ~13 hours |
| `LATEST_INDEX_ROOTS_LENGTH` | `2**13` (= 8,192) | epochs | | `LATEST_RANDAO_MIXES_LENGTH` | `2**13` (= 8,192) | epochs | ~36 days |
| `LATEST_PENALIZED_EXIT_LENGTH` | `2**13` (= 8,192) | epochs | | `LATEST_INDEX_ROOTS_LENGTH` | `2**13` (= 8,192) | epochs | ~36 days |
| `LATEST_PENALIZED_EXIT_LENGTH` | `2**13` (= 8,192) | epochs | ~36 days |
### Reward and penalty quotients ### Reward and penalty quotients
@ -1036,8 +1039,7 @@ def get_randao_mix(state: BeaconState,
""" """
Returns the randao mix at a recent ``slot``. Returns the randao mix at a recent ``slot``.
""" """
assert get_current_epoch(state) < epoch + LATEST_RANDAO_MIXES_LENGTH assert get_current_epoch(state) - LATEST_RANDAO_MIXES_LENGTH < epoch <= get_current_epoch(state)
assert epoch <= get_current_epoch(state)
return state.latest_randao_mixes[epoch % LATEST_RANDAO_MIXES_LENGTH] return state.latest_randao_mixes[epoch % LATEST_RANDAO_MIXES_LENGTH]
``` ```
@ -1049,8 +1051,7 @@ def get_active_index_root(state: BeaconState,
""" """
Returns the index root at a recent ``epoch``. Returns the index root at a recent ``epoch``.
""" """
assert get_current_epoch(state) < epoch + LATEST_INDEX_ROOTS_LENGTH assert get_current_epoch(state) - LATEST_INDEX_ROOTS_LENGTH < epoch <= get_current_epoch(state)
assert epoch <= get_current_epoch(state)
return state.latest_index_roots[epoch % LATEST_INDEX_ROOTS_LENGTH] return state.latest_index_roots[epoch % LATEST_INDEX_ROOTS_LENGTH]
``` ```
@ -1062,10 +1063,9 @@ def generate_seed(state: BeaconState,
""" """
Generate a seed for the given ``epoch``. Generate a seed for the given ``epoch``.
""" """
randao_mix_epoch = epoch + LATEST_RANDAO_MIXES_LENGTH - SEED_LOOKAHEAD
return hash( return hash(
get_randao_mix(state, randao_mix_epoch) + get_randao_mix(state, epoch - SEED_LOOKAHEAD) +
get_active_index_root(state, epoch) get_active_index_root(state, epoch)
) )
``` ```