Incorporate state_transition.py into state transition spec

specs/core/0_beacon-chain.md

@@ -97,16 +97,15 @@
     - [On genesis](#on-genesis)
     - [Beacon chain state transition function](#beacon-chain-state-transition-function)
         - [State caching](#state-caching)
-        - [Per-epoch processing](#per-epoch-processing)
+        - [Epoch processing](#epoch-processing)
             - [Helper functions](#helper-functions-1)
             - [Justification and finalization](#justification-and-finalization)
             - [Crosslinks](#crosslinks)
             - [Rewards and penalties](#rewards-and-penalties)
             - [Registry updates](#registry-updates)
             - [Slashings](#slashings)
             - [Final updates](#final-updates)
-        - [Per-slot processing](#per-slot-processing)
-        - [Per-block processing](#per-block-processing)
+        - [Block processing](#block-processing)
             - [Block header](#block-header)
             - [RANDAO](#randao)
             - [Eth1 data](#eth1-data)
@@ -1264,50 +1263,59 @@ def get_genesis_beacon_state(genesis_validator_deposits: List[Deposit],
 
 ## Beacon chain state transition function
 
-We now define the state transition function. At a high level, the state transition is made up of four parts:
-
-1. State caching, which happens at the start of every slot.
-2. The per-epoch transitions, which happens at the start of the first slot of every epoch.
-3. The per-slot transitions, which happens at every slot.
-4. The per-block transitions, which happens at every block.
-
-Transition section notes:
-* The state caching caches the state root of the previous slot and updates block and state roots records.
-* The per-epoch transitions focus on the [validator](#dfn-validator) registry, including adjusting balances and activating and exiting [validators](#dfn-validator), as well as processing crosslinks and managing block justification/finalization.
-* The per-slot transitions focus on the slot counter.
-* The per-block transitions generally focus on verifying aggregate signatures and saving temporary records relating to the per-block activity in the `BeaconState`.
-
-Beacon blocks that trigger unhandled Python exceptions (e.g. out-of-range list accesses) and failed `assert`s during the state transition are considered invalid.
-
-Note: If there are skipped slots between a block and its parent block, run the steps in the [state-root](#state-caching), [per-epoch](#per-epoch-processing), and [per-slot](#per-slot-processing) sections once for each skipped slot and then once for the slot containing the new block.
+The post-state corresponding to a pre-state `state` and a block `block` is defined as `state_transition(state, block)`. State transitions that trigger an unhandled exception (e.g. a failed `assert` or an out-of-range list access) are considered invalid.
+
+```python
+def state_transition(state: BeaconState, block: BeaconBlock) -> BeaconBlock:
+    # Block must post-date the state
+    assert state.slot < block.slot
+    # Process slots (including those with no blocks) since block
+    while state.slot < block.slot:
+        # Cache state at the start of every slot
+        cache_state(state)
+        # Process epoch at the start of the first slot of every epoch
+        if (state.slot + 1) % SLOTS_PER_EPOCH == 0:
+            process_epoch(state)
+        # Increment slot number
+        state.slot += 1
+    # Process block
+    process_block(state, block)
+    # Return post-state
+    return state
+```
 
 ### State caching
 
-At every `slot > GENESIS_SLOT` run the following function:
-
 ```python
 def cache_state(state: BeaconState) -> None:
-    # Cache latest known state root (for previous slot)
-    latest_state_root = hash_tree_root(state)
-    state.latest_state_roots[state.slot % SLOTS_PER_HISTORICAL_ROOT] = latest_state_root
+    # Cache state root of previous slot
+    previous_state_root = hash_tree_root(state)
+    state.latest_state_roots[state.slot % SLOTS_PER_HISTORICAL_ROOT] = previous_state_root
 
-    # Store latest known state root (for previous slot) in latest_block_header if it is empty
+    # Cache previous state root in latest_block_header, if empty
     if state.latest_block_header.state_root == ZERO_HASH:
-        state.latest_block_header.state_root = latest_state_root
+        state.latest_block_header.state_root = previous_state_root
+
+    # Cache block root of previous slot
+    previous_block_root = signing_root(state.latest_block_header)
+    state.latest_block_roots[state.slot % SLOTS_PER_HISTORICAL_ROOT] = previous_block_root
 
-    # Cache latest known block root (for previous slot)
-    latest_block_root = signing_root(state.latest_block_header)
-    state.latest_block_roots[state.slot % SLOTS_PER_HISTORICAL_ROOT] = latest_block_root
 ```
 
-### Per-epoch processing
+### Epoch processing
 
-The steps below happen when `state.slot > GENESIS_SLOT and (state.slot + 1) % SLOTS_PER_EPOCH == 0`.
+```python
+def process_epoch(state: BeaconState) -> None:
+    process_justification_and_finalization(state)
+    process_crosslinks(state)
+    process_rewards_and_penalties(state)
+    process_registry_updates(state)
+    process_slashings(state)
+    process_final_updates(state)
+```
 
 #### Helper functions
 
-We define epoch transition helper functions:
-
 ```python
 def get_total_active_balance(state: BeaconState) -> Gwei:
     return get_total_balance(state, get_active_validator_indices(state, get_current_epoch(state)))
@@ -1387,8 +1395,6 @@ def get_earliest_attestation(state: BeaconState, attestations: List[PendingAttes
 
 #### Justification and finalization
 
-Run the following function:
-
 ```python
 def process_justification_and_finalization(state: BeaconState) -> None:
     if get_current_epoch(state) <= GENESIS_EPOCH + 1:
@@ -1436,8 +1442,6 @@ def process_justification_and_finalization(state: BeaconState) -> None:
 
 #### Crosslinks
 
-Run the following function:
-
 ```python
 def process_crosslinks(state: BeaconState) -> None:
     state.previous_crosslinks = [c for c in state.current_crosslinks]
@@ -1453,8 +1457,6 @@ def process_crosslinks(state: BeaconState) -> None:
 
 #### Rewards and penalties
 
-First, we define additional helpers:
-
 ```python
 def get_base_reward(state: BeaconState, index: ValidatorIndex) -> Gwei:
     adjusted_quotient = integer_squareroot(get_total_active_balance(state)) // BASE_REWARD_QUOTIENT
@@ -1526,8 +1528,6 @@ def get_crosslink_deltas(state: BeaconState) -> Tuple[List[Gwei], List[Gwei]]:
     return [rewards, penalties]
 ```
 
-Run the following function:
-
 ```python
 def process_rewards_and_penalties(state: BeaconState) -> None:
     if get_current_epoch(state) == GENESIS_EPOCH:
@@ -1542,8 +1542,6 @@ def process_rewards_and_penalties(state: BeaconState) -> None:
 
 #### Registry updates
 
-Run the following function:
-
 ```python
 def process_registry_updates(state: BeaconState) -> None:
     # Process activation eligibility and ejections
@@ -1568,8 +1566,6 @@ def process_registry_updates(state: BeaconState) -> None:
 
 #### Slashings
 
-Run the following function:
-
 ```python
 def process_slashings(state: BeaconState) -> None:
     current_epoch = get_current_epoch(state)
@@ -1592,8 +1588,6 @@ def process_slashings(state: BeaconState) -> None:
 
 #### Final updates
 
-Run the following function:
-
 ```python
 def process_final_updates(state: BeaconState) -> None:
     current_epoch = get_current_epoch(state)
@@ -1632,19 +1626,17 @@ def process_final_updates(state: BeaconState) -> None:
     state.current_epoch_attestations = []
 ```
 
-### Per-slot processing
-
-At every `slot > GENESIS_SLOT` run the following function:
+### Block processing
 
 ```python
-def advance_slot(state: BeaconState) -> None:
-    state.slot += 1
+def process_block(state: BeaconState, block: BeaconBlock) -> None:
+    process_block_header(state, block)
+    process_randao(state, block)
+    process_eth1_data(state, block)
+    process_operations(state, block.body)
+    # verify_block_state_root(state, block)
 ```
 
-### Per-block processing
-
-For every `block` except the genesis block, run `process_block_header(state, block)`, `process_randao(state, block)` and `process_eth1_data(state, block)`.
-
 #### Block header
 
 ```python
@@ -1691,11 +1683,25 @@ def process_eth1_data(state: BeaconState, block: BeaconBlock) -> None:
 
 #### Operations
 
-##### Proposer slashings
+```python
+def process_operations(state: BeaconState, body: BeaconBlockBody) -> None:
+    assert len(body.deposits) == min(MAX_DEPOSITS, state.latest_eth1_data.deposit_count - state.deposit_index)
+    assert len(body.transfers) == len(set(body.transfers))
 
-Verify that `len(block.body.proposer_slashings) <= MAX_PROPOSER_SLASHINGS`.
+    for operations, max_operations, function in (
+        (body.proposer_slashings, MAX_PROPOSER_SLASHINGS, process_proposer_slashing),
+        (body.attester_slashings, MAX_ATTESTER_SLASHINGS, process_attester_slashing),
+        (body.attestations, MAX_ATTESTATIONS, process_attestation),
+        (body.deposits, MAX_DEPOSITS, process_deposit),
+        (body.voluntary_exits, MAX_VOLUNTARY_EXITS, process_voluntary_exit),
+        (body.transfers, MAX_TRANSFERS, process_transfer),
+    ):
+        assert len(operations) <= max_operations
+        for operation in operations:
+            function(state, operation)
+```
 
-For each `proposer_slashing` in `block.body.proposer_slashings`, run the following function:
+##### Proposer slashings
 
 ```python
 def process_proposer_slashing(state: BeaconState,
@@ -1721,10 +1727,6 @@ def process_proposer_slashing(state: BeaconState,
 
 ##### Attester slashings
 
-Verify that `len(block.body.attester_slashings) <= MAX_ATTESTER_SLASHINGS`.
-
-For each `attester_slashing` in `block.body.attester_slashings`, run the following function:
-
 ```python
 def process_attester_slashing(state: BeaconState,
                               attester_slashing: AttesterSlashing) -> None:
@@ -1759,10 +1761,6 @@ def process_attester_slashing(state: BeaconState,
 
 ##### Attestations
 
-Verify that `len(block.body.attestations) <= MAX_ATTESTATIONS`.
-
-For each `attestation` in `block.body.attestations`, run the following function:
-
 ```python
 def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     """
@@ -1801,10 +1799,6 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
 
 ##### Deposits
 
-Verify that `len(block.body.deposits) == min(MAX_DEPOSITS, state.latest_eth1_data.deposit_count - state.deposit_index)`.
-
-For each `deposit` in `block.body.deposits`, run the following function:
-
 ```python
 def process_deposit(state: BeaconState, deposit: Deposit) -> None:
     """
@@ -1851,10 +1845,6 @@ def process_deposit(state: BeaconState, deposit: Deposit) -> None:
 
 ##### Voluntary exits
 
-Verify that `len(block.body.voluntary_exits) <= MAX_VOLUNTARY_EXITS`.
-
-For each `exit` in `block.body.voluntary_exits`, run the following function:
-
 ```python
 def process_voluntary_exit(state: BeaconState, exit: VoluntaryExit) -> None:
     """
@@ -1879,10 +1869,6 @@ def process_voluntary_exit(state: BeaconState, exit: VoluntaryExit) -> None:
 
 ##### Transfers
 
-Verify that `len(block.body.transfers) <= MAX_TRANSFERS` and that all transfers are distinct.
-
-For each `transfer` in `block.body.transfers`, run the following function:
-
 ```python
 def process_transfer(state: BeaconState, transfer: Transfer) -> None:
     """
@@ -1917,8 +1903,6 @@ def process_transfer(state: BeaconState, transfer: Transfer) -> None:
 
 #### State root verification
 
-Verify the block's `state_root` by running the following function:
-
 ```python
 def verify_block_state_root(state: BeaconState, block: BeaconBlock) -> None:
     assert block.state_root == hash_tree_root(state)