rework to do virtual block transitions

specs/core/0_beacon-chain.md

@@ -637,7 +637,7 @@ Processing the beacon chain is similar to processing the Ethereum 1.0 chain. Cli
 
 For a beacon chain block, `block`, to be processed by a node, the following conditions must be met:
 
-* The parent block (possibly a skip block) with hash `block.ancestor_hashes[0]` has been processed and accepted.
+* The parent block with hash `block.ancestor_hashes[0]` has been processed and accepted.
 * The Ethereum 1.0 block pointed to by the `state.processed_pow_receipt_root` has been processed and accepted.
 * The node's local clock time is greater than or equal to `state.genesis_time + block.slot * SLOT_DURATION`.
 
@@ -1192,7 +1192,7 @@ def exit_validator(index: int,
 
 Below are the processing steps that happen at every slot.
 
-First we define the following helpers.
+First we define the following helper.
 
 ```python
 def get_updated_ancestor_hashes(parent: BeaconBlock,
@@ -1202,39 +1202,23 @@ def get_updated_ancestor_hashes(parent: BeaconBlock,
         if parent.slot % 2**i == 0:
             new_ancestor_hashes[i] = parent_hash
     return new_ancestor_hashes
-
-
-def get_skip_block(slot: int,
-                   parent: BeaconBlock,
-                   parent_hash: Hash32) -> BeaconBlock:
-    return BeaconBlock(
-        slot=state.slot,
-        randao_reveal=ZERO_HASH,
-        candidate_pow_receipt_root=ZERO_HASH,
-        ancestor_hashes=get_updated_ancestor_hashes(parent, parent_hash),
-        state_root=ZERO_HASH,  # filled in with post-state-transition root
-        attestations=[],
-        specials=[],
-        proposer_signature=[0, 0]
-    )
 ```
 
-* Let `parent_hash` be the hash of the beacon block at the immediate previous slot.
+* Let `parent_hash` be the hash of the latest beacon block that was processed in the chain.
 * Let `parent` be the `BeaconBlock` with the hash `parent_hash`.
+* Set `state.latest_block_hashes = state.latest_block_hashes + [parent_hash]`. (The output of `get_block_hash` should not change, except that it will no longer throw for `state.slot - 1`).
 
 If there is a block from the proposer for `state.slot`, we process that incoming block:
+* Let `skip_block = False`
 * Let `block` be that associated incoming block.
+* Verify that `block.ancestor_hashes` equals `get_updated_ancestor_hashes(parent, parent_hash)`.
 
-If there is not a block from the proposer for `state.slot`, a "skip block" is inserted into the beacon chain:
+If there is not a block from the proposer for `state.slot`, the slot is processed without a block:
-* Let `block` be a skip block defined by `get_skip_block(state.slot, parent, parent_hash)`.
+* Let `skip_block = True`
-
-Verify that `block.ancestor_hashes` equals `get_updated_ancestor_hashes(parent, parent_hash)`.
-
-Set `state.latest_block_hashes = state.latest_block_hashes + [parent_hash]`. (The output of `get_block_hash` should not change, except that it will no longer throw for `state.slot - 1`).
 
 ### Proposer signature
 
-If `block` is not a skip block:
+If not `skip_block`:
 
 * Let `block_hash_without_sig` be the hash of `block` where `proposer_signature` is set to `[0, 0]`.
 * Let `proposal_hash = hash(ProposalSignedData(state.slot, BEACON_CHAIN_SHARD_NUMBER, block_hash_without_sig))`.
@@ -1242,6 +1226,8 @@ If `block` is not a skip block:
 
 ### Attestations
 
+*If `skip_block`, do not process attestations. Move to next section.*
+
 * Verify that `len(block.attestations) <= MAX_ATTESTATIONS_PER_BLOCK`.
 
 For each `attestation` in `block.attestations`:
@@ -1260,11 +1246,11 @@ For each `attestation` in `block.attestations`:
 
 ### RANDAO
 
-If `block` is a skip block:
+If `skip_block`:
 
 * Let `state.validator_registry[get_beacon_proposer_index(state, state.slot)].randao_skips += 1`.
 
-If `block` is not a skip block:
+If not `skip_block`:
 
 * Let `repeat_hash(x, n) = x if n == 0 else repeat_hash(hash(x), n-1)`.
 * Let `proposer = state.validator_registry[get_beacon_proposer_index(state, state.slot)]`.
@@ -1275,13 +1261,16 @@ If `block` is not a skip block:
 
 ### PoW receipt root
 
-If `block` is not a skip block:
+*If `skip_block`, do not process PoW receipt root. Move to next section.*
+
 
 * If `block.candidate_pow_receipt_root` is `x.candidate_pow_receipt_root` for some `x` in `state.candidate_pow_receipt_roots`, set `x.votes += 1`.
 * Otherwise, append to `state.candidate_pow_receipt_roots` a new `CandidatePoWReceiptRootRecord(candidate_pow_receipt_root=block.candidate_pow_receipt_root, votes=1)`.
 
 ### Special objects
 
+*If `skip_block`, do not process special objects. Move to next section.*
+
 * Verify that the quantity of each type of object in `block.specials` is less than or equal to its maximum (see table at the top).
 * Verify that objects are sorted in order of `kind`. That is, `block.specials[i+1].kind >= block.specials[i].kind` for `0 <= i < len(block.specials-1)`.
 
@@ -1615,11 +1604,9 @@ while len(state.persistent_committee_reassignments) > 0 and state.persistent_com
 
 ## State root processing
 
-If `block` is not a skip block:
+*If `skip_block`, do not process state root. Move to next section.*
-* Verify `block.state_root == hash(state)`
 
-If `block` is a skip block:
+Verify `block.state_root == hash(state)`
-* Set `block.state_root = hash(state)`
 
 # Appendix
 ## Appendix A - Hash function