GENESIS_SLOT == 0

[djrtwo]

• Move genesis slot and epoch to 0
• ensure get_previous_epoch cannot underflow and that usage of this function during GENESIS_EPOCH is correct
• remove check for greater than GENESIS_SLOT in process_attestation because there is nothing less than GENESIS_SLOT
• add blockers to the following functions
  • update_justification_and_finalization at and before GENESIS_EPOCH+1
  • apply_rewards at GENESIS_EPOCH
• add sanity test for processing finality through first 5 epochs

Currently blocked by #920

[hwwhww]

I remember that @vbuterin chose to use the flag as a cleaner solution (#374 (comment)). :p I'm fine either way!

[vbuterin]

I'm fine either way. But if we are going to have the Genesis slot be 0 why not just remove the constant?

[djrtwo]

Which, GENESIS_EPOCH?

[protolambda]

+1 for removing the constants gensis-slot and genesis-epoch. 0 is perfectly clear.

[hwwhww]

After merging with #925, one of the new tests assert state.current_justified_root == prev_state.current_justified_root fails

https://github.com/ethereum/eth2.0-specs/blob/3c8d1b23a52c8be12c01628d8546a4443a217ca7/tests/phase0/test_sanity.py#L65-L66

Because

1. state.current_justified_root was updated in https://github.com/ethereum/eth2.0-specs/blob/a93d34b8e4c46196a52afce3047897fa0bea1b86/specs/core/0_beacon-chain.md#justification:

       state.current_justified_epoch = get_current_epoch(state)
       state.current_justified_root = get_block_root(state, get_epoch_start_slot(state.current_justified_epoch))

2. state.current_justified_epoch is 0, but state.current_justified_root NOT not initial value ZERO_HASH.

I think it does make sense to say when state.current_justified_epoch == 0, the current_justified_root is the genesis_block_root. However, it's no way to set it in genesis_state since genesis_block_root is built with genesis_state_root...
The easy solution might be either:

1. Take it as a special case: add some condition check if in update_justification_and_finalization; if it's genesis epoch, don't use get_block_root to update current_justified_root.
2. As before Decouple justification and finalization processing #925, only call get_block_root when state.current_justified_epoc does change.

[djrtwo]

@hwwhww I simply made the update_justification_and_finalization exit if if get_current_epoch(state) <= GENESIS_EPOCH + 1: rather than just on GENESIS_EPOCH. This avoids updating justification to GENESIS_EPOCH and finalization to GENESIS_EPOCH as well. The first epoch for either than can be written to state is epoch 1

[hwwhww]

LGTM!