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get_previous_epoch as a helper #565

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Feb 4, 2019
Merged

get_previous_epoch as a helper #565

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e4c4c04
Update 0_beacon-chain.md
terencechain Feb 3, 2019
3a6da98
Added a note for underflow. Thanks Hsiao Wei! : )
hwwhww Feb 3, 2019
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18 changes: 16 additions & 2 deletions specs/core/0_beacon-chain.md
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Original file line number Diff line number Diff line change
Expand Up @@ -54,6 +54,7 @@
- [`hash`](#hash)
- [`hash_tree_root`](#hash_tree_root)
- [`slot_to_epoch`](#slot_to_epoch)
- [`get_previous_epoch`](#get_previous_epoch)
- [`get_current_epoch`](#get_current_epoch)
- [`get_epoch_start_slot`](#get_epoch_start_slot)
- [`is_active_validator`](#is_active_validator)
Expand Down Expand Up @@ -639,6 +640,19 @@ def slot_to_epoch(slot: SlotNumber) -> EpochNumber:
return slot // EPOCH_LENGTH
```

### `get_previous_epoch`

```python
def get_previous_epoch(state: BeaconState) -> EpochNumber:
"""`
Return the previous epoch of the given ``state``.
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If the current epoch is ``GENESIS_EPOCH``, return ``GENESIS_EPOCH``.
"""
if slot_to_epoch(state.slot) > GENESIS_EPOCH:
return slot_to_epoch(state.slot) - 1
return slot_to_epoch(state.slot)
```

### `get_current_epoch`

```python
Expand Down Expand Up @@ -844,7 +858,7 @@ def get_crosslink_committees_at_slot(state: BeaconState,
"""
epoch = slot_to_epoch(slot)
current_epoch = get_current_epoch(state)
previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else current_epoch
previous_epoch = get_previous_epoch(state)
next_epoch = current_epoch + 1

assert previous_epoch <= epoch <= next_epoch
Expand Down Expand Up @@ -1827,7 +1841,7 @@ The steps below happen when `(state.slot + 1) % EPOCH_LENGTH == 0`.
#### Helpers

* Let `current_epoch = get_current_epoch(state)`.
* Let `previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else current_epoch`.
* Let `previous_epoch = get_previous_epoch(state)`.
* Let `next_epoch = current_epoch + 1`.

[Validators](#dfn-Validator) attesting during the current epoch:
Expand Down