add more epoch processing

* document several parts
* almost complete epoch state processing

beacon_chain/spec/beaconstate.nim

@@ -57,11 +57,11 @@ func on_startup*(initial_validator_entries: openArray[InitialValidator],
     shard_and_committee_for_slots[i] = n
     shard_and_committee_for_slots[EPOCH_LENGTH + i] = n
 
-  # The spec says to use validators, but it's actually indices..
-  let validator_indices = get_active_validator_indices(validators)
+  # TODO validators vs indices
+  let active_validator_indices = get_active_validator_indices(validators)
 
   let persistent_committees = split(shuffle(
-    validator_indices, ZERO_HASH), SHARD_COUNT)
+    active_validator_indices, ZERO_HASH), SHARD_COUNT)
 
   BeaconState(
     validator_registry: validators,
@@ -112,9 +112,9 @@ func append_to_recent_block_hashes*(old_block_hashes: seq[Eth2Digest],
   result = old_block_hashes
   result.add repeat(parent_hash, d)
 
-proc get_attestation_participants*(state: BeaconState,
+func get_attestation_participants*(state: BeaconState,
                                    attestation_data: AttestationData,
-                                   participation_bitfield: seq[byte]): seq[int] =
+                                   participation_bitfield: seq[byte]): seq[Uint24] =
   ## Attestation participants in the attestation data are called out in a
   ## bit field that corresponds to the committee of the shard at the time - this
   ## function converts it to list of indices in to BeaconState.validators
@@ -138,3 +138,16 @@ proc get_attestation_participants*(state: BeaconState,
       if bit == 1:
           result.add(vindex)
     return # found the shard, we're done
+
+func change_validators*(state: var BeaconState,
+                        current_slot: uint64) =
+  ## Change validator registry.
+
+  let res = get_changed_validators(
+    state.validator_registry,
+    state.latest_penalized_exit_balances,
+    state.validator_registry_delta_chain_tip,
+    current_slot
+  )
+  state.validator_registry = res.validators
+  state.latest_penalized_exit_balances = res.latest_penalized_exit_balances

beacon_chain/spec/datatypes.nim

@@ -44,7 +44,7 @@ const
   SLOT_DURATION*                            = 6    # seconds
   MIN_ATTESTATION_INCLUSION_DELAY*          = 4    # slots (~25 minutes)
   EPOCH_LENGTH*                             = 64   # slots (~6.4 minutes)
-  MIN_VALIDATOR_SET_CHANGE_INTERVAL*        = 2^8  # slots (~25.6 minutes)
+  MIN_VALIDATOR_REGISTRY_CHANGE_INTERVAL*   = 2^8  # slots (~25.6 minutes)
   POW_RECEIPT_ROOT_VOTING_PERIOD*           = 2^10 # slots (~1.7 hours)
   SHARD_PERSISTENT_COMMITTEE_CHANGE_PERIOD* = 2^17 # slots (~9 days)
   SQRT_E_DROP_TIME*                         = 2^17 # slots (~9 days); amount of time it takes for the
@@ -182,7 +182,7 @@ type
     slot_included*: uint64                        # Slot in which it was included
 
   ValidatorStatusCodes* {.pure.} = enum
-    PENDING_ACITVATION = 0
+    PENDING_ACTIVATION = 0
     ACTIVE = 1
     EXITED_WITHOUT_PENALTY = 2
     EXITED_WITH_PENALTY = 3

beacon_chain/spec/helpers.nim

@@ -104,7 +104,7 @@ proc get_shard_and_committees_for_slot*(
   let index = state.get_shard_and_committees_index(slot)
   state.shard_and_committee_for_slots[index]
 
-func get_beacon_proposer_index*(state: BeaconState, slot: uint64): uint64 =
+func get_beacon_proposer_index*(state: BeaconState, slot: uint64): Uint24 =
   ## From Casper RPJ mini-spec:
   ## When slot i begins, validator Vidx is expected
   ## to create ("propose") a block, which contains a pointer to some parent block
@@ -133,3 +133,5 @@ func get_fork_version*(fork_data: ForkData, slot: uint64): uint64 =
 func get_domain*(fork_data: ForkData, slot: uint64, domain_type: uint64): uint64 =
   # TODO Slot overflow? Or is slot 32 bits for all intents and purposes?
   (get_fork_version(fork_data, slot) shl 32) + domain_type
+
+func is_power_of_2*(v: uint64): bool = discard # TODO

beacon_chain/spec/validator.nim

@@ -7,7 +7,7 @@
 # Helpers and functions pertaining to managing the validator set
 
 import
-  options, nimcrypto,
+  options, nimcrypto, sequtils, math,
   eth_common,
   ../ssz,
   ./crypto, ./datatypes, ./digest, ./helpers
@@ -81,7 +81,7 @@ func get_active_validator_indices*(validators: openArray[ValidatorRecord]): seq[
 
 func get_new_shuffling*(seed: Eth2Digest,
                         validators: openArray[ValidatorRecord],
-                        crosslinking_start_shard: int
+                        crosslinking_start_shard: uint64
                         ): array[EPOCH_LENGTH, seq[ShardAndCommittee]] =
   ## Split up validators into groups at the start of every epoch,
   ## determining at what height they can make attestations and what shard they are making crosslinks for
@@ -91,7 +91,7 @@ func get_new_shuffling*(seed: Eth2Digest,
     active_validators = get_active_validator_indices(validators)
     committees_per_slot = clamp(
       len(active_validators) div EPOCH_LENGTH div TARGET_COMMITTEE_SIZE,
-      1, SHARD_COUNT div EPOCH_LENGTH)
+      1, SHARD_COUNT div EPOCH_LENGTH).uint64
     # Shuffle with seed
     shuffled_active_validator_indices = shuffle(active_validators, seed)
     # Split the shuffled list into cycle_length pieces
@@ -102,12 +102,13 @@ func get_new_shuffling*(seed: Eth2Digest,
   for slot, slot_indices in validators_per_slot:
     let
       shard_indices = split(slot_indices, committees_per_slot)
-      shard_id_start = crosslinking_start_shard + slot * committees_per_slot
+      shard_id_start =
+        crosslinking_start_shard + slot.uint64 * committees_per_slot
 
     var committees = newSeq[ShardAndCommittee](shard_indices.len)
     for shard_position, indices in shard_indices:
       committees[shard_position].shard =
-        uint64(shard_id_start + shard_position) mod SHARD_COUNT
+        uint64(shard_id_start + shard_position.uint64) mod SHARD_COUNT.uint64
       committees[shard_position].committee = indices
 
     result[slot] = committees
@@ -173,3 +174,75 @@ func exit_validator*(index: Uint24,
       validator.pubkey,
       EXIT,
   )
+
+func get_changed_validators*(validators: seq[ValidatorRecord],
+                             latest_penalized_exit_balances: seq[uint64],
+                             validator_registry_delta_chain_tip: Eth2Digest,
+                             current_slot: uint64):
+      tuple[
+        validators: seq[ValidatorRecord],
+        latest_penalized_exit_balances: seq[uint64],
+        validator_registry_delta_chain_tip: Eth2Digest] =
+  ## Return changed validator registry and `latest_penalized_exit_balances`,
+  ## `validator_registry_delta_chain_tip`.
+
+  # TODO inefficient
+  var validators = validators
+
+  # The active validators
+  let active_validator_indices = get_active_validator_indices(validators)
+  # The total balance of active validators
+  # TODO strange spec code
+  let total_balance = sum(mapIt(active_validator_indices, get_effective_balance(validators[it])))
+  # The maximum total Gwei that can be deposited and withdrawn
+  let max_allowable_change = max(
+      uint64(2 * MAX_DEPOSIT * GWEI_PER_ETH),
+      total_balance div MAX_CHURN_QUOTIENT.uint64
+  )
+
+  # Go through the list start to end, depositing and withdrawing as many as possible
+  var total_changed: uint64 = 0
+  var validator_registry_delta_chain_tip = validator_registry_delta_chain_tip
+  for i in 0..<validators.len:
+    if validators[i].status == PENDING_ACTIVATION:
+      validators[i].status = ACTIVE
+      total_changed.inc(get_effective_balance(validators[i]).int)
+      let validator_registry_delta_chain_tip =
+        get_new_validator_registry_delta_chain_tip(
+          validator_registry_delta_chain_tip,
+          i.Uint24,
+          validators[i].pubkey,
+          ACTIVATION,
+      )
+    elif validators[i].status == EXITED_WITHOUT_PENALTY:
+      validators[i].latest_status_change_slot = current_slot
+      total_changed.inc(get_effective_balance(validators[i]).int)
+      validator_registry_delta_chain_tip =
+        get_new_validator_registry_delta_chain_tip(
+          validator_registry_delta_chain_tip,
+          i.Uint24,
+          validators[i].pubkey,
+          EXIT,
+      )
+    if total_changed >= max_allowable_change:
+        break
+
+  # Calculate the total ETH that has been penalized in the last ~2-3 withdrawal periods
+  let period_index =
+    (current_slot div COLLECTIVE_PENALTY_CALCULATION_PERIOD.uint64).int
+  let total_penalties = (
+    (latest_penalized_exit_balances[period_index]) +
+    (if period_index >= 1: latest_penalized_exit_balances[period_index - 1] else: 0) +
+    (if period_index >= 2: latest_penalized_exit_balances[period_index - 2] else: 0)
+  )
+
+  # Calculate penalties for slashed validators
+  func to_penalize(v: ValidatorRecord): bool =
+      v.status == EXITED_WITH_PENALTY
+  var validators_to_penalize = filter(validators, to_penalize)
+  for v in validators_to_penalize.mitems():
+    v.balance.dec(
+      (get_effective_balance(v) * min(total_penalties * 3'u64, total_balance) div
+        total_balance).int)
+
+  (validators, latest_penalized_exit_balances, validator_registry_delta_chain_tip)

beacon_chain/state_transition.nim

@@ -5,27 +5,49 @@
 #   * Apache v2 license (license terms in the root directory or at http://www.apache.org/licenses/LICENSE-2.0).
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
-# A imcomplete implementation of the state transition function, as described
-# under "Per-block processing" in https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md
+# State transition, as described in
+# https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function
 #
-# The code is here mainly to verify the data types and get an idea about
-# missing pieces - needs testing throughout
+# The purpose of this code right is primarily educational, to help piece
+# together the mechanics of the beacon state and to discover potential problem
+# areas.
+#
+# General notes about the code (TODO):
+# * It's inefficient - we quadratically copy, allocate and iterate when there
+#   are faster options
+# * Weird styling - the sections taken from the spec use python styling while
+#   the others use NEP-1 - helps grepping identifiers in spec
+# * We mix procedural and functional styles for no good reason, except that the
+#   spec does so also.
+# * There are no tests, and likely lots of bugs.
+# * For validators, sometimes indices are used and sometimes instances - this
+#   causes unnecessary friction in some sections
+# * For indices, we get a mix of uint64, Uint24 and int - this is currently
+#   swept under the rug with casts
+# * The spec uses uint64 for data types, but functions in the spec often assume
+#   signed bigint semantics - under- and overflows ensue
+# * Sane error handling is missing in most cases (yay, we'll get the chance to
+#   debate exceptions again!)
+#
+# When updating the code, add TODO sections to mark where there are clear
+# improvements to be made - other than that, keep things similar to spec for
+# now.
 
 import
   math, options, sequtils,
-  ./extras,
+  ./extras, ./ssz,
   ./spec/[beaconstate, crypto, datatypes, digest, helpers, validator],
-  ./ssz,
-  milagro_crypto # nimble install https://github.com/status-im/nim-milagro-crypto@#master
+  milagro_crypto
 
-# TODO there's an ugly mix of functional and procedural styles here that
-#      is due to how the spec is mixed as well - once we're past the prototype
-#      stage, this will need clearing up and unification.
-
-func checkAttestations(state: BeaconState,
-                       blck: BeaconBlock,
-                       parent_slot: uint64): Option[seq[PendingAttestationRecord]] =
-  # TODO perf improvement potential..
+func processAttestations(state: var BeaconState,
+                         blck: BeaconBlock,
+                         parent_slot: uint64): bool =
+  # Each block includes a number of attestations that the proposer chose. Each
+  # attestation represents an update to a specific shard and is signed by a
+  # committee of validators.
+  # Here we make sanity checks for each attestation and it to the state - most
+  # updates will happen at the epoch boundary where state updates happen in
+  # bulk.
   if blck.attestations.len > MAX_ATTESTATIONS_PER_BLOCK:
     return
 
@@ -33,7 +55,8 @@ func checkAttestations(state: BeaconState,
   for attestation in blck.attestations:
     if attestation.data.slot <= blck.slot - MIN_ATTESTATION_INCLUSION_DELAY:
       return
-    # TODO unsigned undeflow in spec
+
+    # TODO spec - unsigned underflow
     if attestation.data.slot >= max(parent_slot.int - EPOCH_LENGTH + 1, 0).uint64:
       return
 
@@ -42,6 +65,7 @@ func checkAttestations(state: BeaconState,
         state.justified_slot
       else:
         state.previous_justified_slot
+
     if attestation.data.justified_slot != expected_justified_slot:
       return
 
@@ -77,14 +101,16 @@ func checkAttestations(state: BeaconState,
     debugEcho "Aggregate sig verify message: ",
       attestation.aggregate_sig.verifyMessage(msg, agg_pubkey)
 
-    res.add PendingAttestationRecord(
+    # All checks passed - update state
+    # TODO no rollback in case of errors
+    state.latest_attestations.add PendingAttestationRecord(
       data: attestation.data,
       participation_bitfield: attestation.participation_bitfield,
       custody_bitfield: attestation.custody_bitfield,
       slot_included: blck.slot
     )
 
-  some(res)
+  true
 
 func verifyProposerSignature(state: BeaconState, blck: BeaconBlock): bool =
   var blck_without_sig = blck
@@ -109,9 +135,9 @@ func processRandaoReveal(state: var BeaconState,
     let proposer_index = get_beacon_proposer_index(state, slot)
     state.validator_registry[proposer_index.int].randao_skips.inc()
 
-  var
+  let
     proposer_index = get_beacon_proposer_index(state, blck.slot)
-    proposer = state.validator_registry[proposer_index.int]
+    proposer = state.validator_registry[proposer_index.int].addr
 
   # Check that proposer commit and reveal match
   if repeat_hash(blck.randao_reveal, proposer.randao_skips + 1) !=
@@ -143,31 +169,25 @@ func processSpecials(state: var BeaconState, blck: BeaconBlock): bool =
   # TODO incoming spec changes here..
   true
 
-func process_block*(state: BeaconState, blck: BeaconBlock): Option[BeaconState] =
+func processBlock(state: var BeaconState, blck: BeaconBlock): bool =
   ## When a new block is received, all participants must verify that the block
   ## makes sense and update their state accordingly. This function will return
   ## the new state, unless something breaks along the way
-
-  # TODO: simplistic way to be able to rollback state
-  var state = state
+  # TODO state not rolled back in case of failure
 
   let
     parent_hash = blck.ancestor_hashes[0]
     slot = blck.slot
     parent_slot = slot - 1 # TODO Not!! can skip slots...
   # TODO actually get parent block, which means fixing up BeaconState refs above;
-  # there's no distinction between active/crystallized state anymore, etc.
 
   state.latest_block_hashes =
     append_to_recent_block_hashes(state.latest_block_hashes, parent_slot, slot,
       parent_hash)
 
-  let processed_attestations = checkAttestations(state, blck, parent_slot)
-  if processed_attestations.isNone:
+  if not processAttestations(state, blck, parent_slot):
     return
 
-  state.latest_attestations.add processed_attestations.get()
-
   if not verifyProposerSignature(state, blck):
     return
 
@@ -180,46 +200,69 @@ func process_block*(state: BeaconState, blck: BeaconBlock): Option[BeaconState]
   if not processSpecials(state, blck):
     return
 
-  some(state) # Looks ok - move on with the updated state
+  true
 
 func flatten[T](v: openArray[seq[T]]): seq[T] =
+  # TODO not in nim - doh.
   for x in v: result.add x
 
 func get_epoch_boundary_attesters(
     state: BeaconState,
-    attestations: openArray[PendingAttestationRecord]): seq[int] =
+    attestations: openArray[PendingAttestationRecord]): seq[Uint24] =
+  # TODO spec - add as helper?
   deduplicate(flatten(mapIt(attestations,
     get_attestation_participants(state, it.data, it.participation_bitfield))))
 
 func adjust_for_inclusion_distance[T](magnitude: T, dist: T): T =
   magnitude div 2 + (magnitude div 2) * MIN_ATTESTATION_INCLUSION_DELAY div dist
 
-func processEpoch*(state: BeaconState, blck: BeaconBlock): Option[BeaconState] =
+func boundary_attestations(
+    state: BeaconState, boundary_hash: Eth2Digest,
+    attestations: openArray[PendingAttestationRecord]
+    ): seq[PendingAttestationRecord] =
+  # TODO spec - add as helper?
+  filterIt(attestations,
+    it.data.epoch_boundary_hash == boundary_hash and
+    it.data.justified_slot == state.justified_slot)
+
+func sum_effective_balances(
+    state: BeaconState, validator_indices: openArray[Uint24]): uint64 =
+    # TODO spec - add as helper?
+  sum(mapIt(
+    validator_indices, get_effective_balance(state.validator_registry[it]))
+  )
+
+func lowerThan(candidate, current: Eth2Digest): bool =
+  # return true iff candidate is "lower" than current, per spec rule:
+  # "ties broken by favoring lower `shard_block_hash` values"
+  # TODO spec - clarify hash ordering..
+  for i, v in current.data:
+    if v > candidate.data[i]: return true
+  return false
+
+func processEpoch(state: var BeaconState, blck: BeaconBlock): bool =
   ## Epoch processing happens every time we've passed EPOCH_LENGTH blocks.
   ## Because some slots may be skipped, it may happen that we go through the
   ## loop more than once - each time the latest_state_recalculation_slot will be
   ## increased by EPOCH_LENGTH.
 
-  # TODO: simplistic way to be able to rollback state
-  var state = state
-
-  # Precomputation
-
   while blck.slot >= EPOCH_LENGTH.uint64 + state.latest_state_recalculation_slot:
+    # Convenience shortcut, from spec
     let s = state.latest_state_recalculation_slot
 
+    # Precomputation
     let
-      active_validators =
-        mapIt(get_active_validator_indices(state.validator_registry),
-          state.validator_registry[it])
-
-      total_balance = sum(mapIt(active_validators, get_effective_balance(it)))
-
+      active_validator_indices =
+        get_active_validator_indices(state.validator_registry)
+      total_balance = sum_effective_balances(state, active_validator_indices)
       total_balance_in_eth = total_balance.int div GWEI_PER_ETH
 
       # The per-slot maximum interest rate is `2/reward_quotient`.)
       reward_quotient = BASE_REWARD_QUOTIENT * int_sqrt(total_balance_in_eth)
 
+      # TODO not in spec, convenient
+      epoch_boundary_hash = get_block_hash(state, blck, s)
+
     proc base_reward(v: ValidatorRecord): uint64 =
       get_effective_balance(v) div reward_quotient.uint64
 
@@ -229,128 +272,268 @@ func processEpoch*(state: BeaconState, blck: BeaconBlock): Option[BeaconState] =
       this_epoch_attestations = filterIt(state.latest_attestations,
         s <= it.data.slot and it.data.slot < s + EPOCH_LENGTH)
 
-      this_epoch_boundary_attestations = filterIt(this_epoch_attestations,
-        it.data.epoch_boundary_hash == get_block_hash(state, blck, s) and
-          it.data.justified_slot == state.justified_slot)
+      this_epoch_boundary_attestations =
+        boundary_attestations(state, epoch_boundary_hash,
+          this_epoch_attestations)
 
       this_epoch_boundary_attesters =
         get_epoch_boundary_attesters(state, this_epoch_attestations)
 
-      this_epoch_boundary_attesting_balance = sum(
-        mapIt(this_epoch_boundary_attesters,
-          get_effective_balance(state.validator_registry[it]))
-      )
+      this_epoch_boundary_attesting_balance =
+        sum_effective_balances(state, this_epoch_boundary_attesters)
 
     let
       previous_epoch_attestations = filterIt(state.latest_attestations,
         s <= it.data.slot + EPOCH_LENGTH and it.data.slot < s)
-      previous_epoch_boundary_attestations = filterIt(previous_epoch_attestations,
-        it.data.epoch_boundary_hash == get_block_hash(state, blck, s) and
-          it.data.justified_slot == state.justified_slot)
+
+      previous_epoch_boundary_attestations =
+        boundary_attestations(state, epoch_boundary_hash,
+          previous_epoch_attestations)
+
       previous_epoch_boundary_attesters =
         get_epoch_boundary_attesters(state, previous_epoch_boundary_attestations)
-      previous_epoch_boundary_attesting_balance = sum(
-        mapIt(previous_epoch_boundary_attesters,
-          get_effective_balance(state.validator_registry[it]))
-      )
 
-    # TODO gets pretty hairy here
+      previous_epoch_boundary_attesting_balance =
+        sum_effective_balances(state, this_epoch_boundary_attesters)
+
+    # TODO this is really hairy - we cannot capture `state` directly, but we
+    #      can capture a pointer to it - this is safe because we don't leak
+    #      these closures outside this scope, but still..
+    let statePtr = state.addr
     func attesting_validators(
-        obj: ShardAndCommittee, shard_block_hash: Eth2Digest): seq[int] =
+        obj: ShardAndCommittee, shard_block_hash: Eth2Digest): seq[Uint24] =
       flatten(
         mapIt(
           filterIt(concat(this_epoch_attestations, previous_epoch_attestations),
             it.data.shard == obj.shard and
               it.data.shard_block_hash == shard_block_hash),
-          get_attestation_participants(state, it.data, it.participation_bitfield)))
+          get_attestation_participants(statePtr[], it.data, it.participation_bitfield)))
 
-    # TODO which shard_block_hash:es?
-    # * Let `attesting_validators(obj)` be equal to `attesting_validators(obj, shard_block_hash)` for the value of `shard_block_hash` such that `sum([get_effective_balance(v) for v in attesting_validators(obj, shard_block_hash)])` is maximized (ties broken by favoring lower `shard_block_hash` values).
-    # * Let `total_attesting_balance(obj)` be the sum of the balances-at-stake of `attesting_validators(obj)`.
-    # * Let `winning_hash(obj)` be the winning `shard_block_hash` value.
-    # * Let `total_balance(obj) = sum([get_effective_balance(v) for v in obj.committee])`.
+    func winning_hash(obj: ShardAndCommittee): Eth2Digest =
+      # * Let `winning_hash(obj)` be the winning `shard_block_hash` value.
+      # ... such that `sum([get_effective_balance(v) for v in attesting_validators(obj, shard_block_hash)])`
+      # is maximized (ties broken by favoring lower `shard_block_hash` values).
+      let candidates =
+        mapIt(
+          filterIt(concat(this_epoch_attestations, previous_epoch_attestations),
+            it.data.shard == obj.shard),
+          it.data.shard_block_hash)
 
-    # Let `inclusion_slot(v)` equal `a.slot_included` for the attestation `a` where `v` is in `get_attestation_participants(state, a.data, a.participation_bitfield)`, and `inclusion_distance(v) = a.slot_included - a.data.slot` for the same attestation. We define a function `adjust_for_inclusion_distance(magnitude, distance)` which adjusts the reward of an attestation based on how long it took to get included (the longer, the lower the reward). Returns a value between 0 and `magnitude`.
+      var max_hash = candidates[0]
+      var max_val =
+        sum_effective_balances(statePtr[], attesting_validators(obj, max_hash))
+      for candidate in candidates[1..^1]:
+        let val = sum_effective_balances(statePtr[], attesting_validators(obj, candidate))
+        if val > max_val or (val == max_val and candidate.lowerThan(max_hash)):
+          max_hash = candidate
+          max_val = val
+      max_hash
 
-    # Adjust justified slots and crosslink status
+    func attesting_validators(obj: ShardAndCommittee): seq[Uint24] =
+      attesting_validators(obj, winning_hash(obj))
 
-    var new_justified_slot: Option[uint64]
-    # overflow intentional!
-    state.justified_slot_bitfield = state.justified_slot_bitfield * 2
+    func total_attesting_balance(obj: ShardAndCommittee): uint64 =
+      sum_effective_balances(statePtr[], attesting_validators(obj))
 
-    if 3'u64 * previous_epoch_boundary_attesting_balance >= 2'u64 * total_balance:
-      # TODO spec says "flip the second lowest bit to 1" and does "AND", wrong?
-      state.justified_slot_bitfield = state.justified_slot_bitfield or 2
-      new_justified_slot = some(s - EPOCH_LENGTH)
+    func total_balance_sac(obj: ShardAndCommittee): uint64 =
+      sum_effective_balances(statePtr[], obj.committee)
 
-    if 3'u64 * this_epoch_boundary_attesting_balance >= 2'u64 * total_balance:
-      # TODO spec says "flip the second lowest bit to 1" and does "AND", wrong?
-      state.justified_slot_bitfield = state.justified_slot_bitfield or 1
-      new_justified_slot = some(s)
+    func inclusion_slot(v: Uint24): uint64 =
+      for a in statePtr[].latest_attestations:
+        if v in get_attestation_participants(statePtr[], a.data, a.participation_bitfield):
+          return a.slot_included
+      assert false # shouldn't happen..
 
-    if state.justified_slot == s - EPOCH_LENGTH and
-        state.justified_slot_bitfield mod 4 == 3:
-      state.finalized_slot = state.justified_slot
-    if state.justified_slot == s - EPOCH_LENGTH - EPOCH_LENGTH and
-        state.justified_slot_bitfield mod 8 == 7:
-      state.finalized_slot = state.justified_slot
+    func inclusion_distance(v: Uint24): uint64 =
+      for a in statePtr[].latest_attestations:
+        if v in get_attestation_participants(statePtr[], a.data, a.participation_bitfield):
+          return a.slot_included - a.data.slot
+      assert false # shouldn't happen..
 
-    if state.justified_slot == s - EPOCH_LENGTH - 2 * EPOCH_LENGTH and
-        state.justified_slot_bitfield mod 16 in [15'u64, 14]:
-      state.finalized_slot = state.justified_slot
+    block: # Adjust justified slots and crosslink status
+      var new_justified_slot: Option[uint64]
+      # TODO where's that bitfield type when you need it?
+      # TODO what happens with the bits that drop off..?
+      state.justified_slot_bitfield = state.justified_slot_bitfield shl 1
 
-    state.previous_justified_slot = state.justified_slot
+      if 3'u64 * previous_epoch_boundary_attesting_balance >= 2'u64 * total_balance:
+        # TODO spec says "flip the second lowest bit to 1" and does "AND", wrong?
+        state.justified_slot_bitfield = state.justified_slot_bitfield or 2
+        new_justified_slot = some(s - EPOCH_LENGTH)
 
-    if new_justified_slot.isSome():
-      state.justified_slot = new_justified_slot.get()
+      if 3'u64 * this_epoch_boundary_attesting_balance >= 2'u64 * total_balance:
+        # TODO spec says "flip the second lowest bit to 1" and does "AND", wrong?
+        state.justified_slot_bitfield = state.justified_slot_bitfield or 1
+        new_justified_slot = some(s)
 
-    # for obj in state.shard_and_committee_for_slots:
-    #   3 * total_attesting_balance(obj) >= 2 * total_balance(obj):
-    #     state.crosslinks[shard] = CrosslinkRecord(
-    #       slot: latest_state_recalculation_slot + EPOCH_LENGTH,
-    #       hash: winning_hash(obj))
+      if state.justified_slot == s - EPOCH_LENGTH and
+          state.justified_slot_bitfield mod 4 == 3:
+        state.finalized_slot = state.justified_slot
+      if state.justified_slot == s - EPOCH_LENGTH - EPOCH_LENGTH and
+          state.justified_slot_bitfield mod 8 == 7:
+        state.finalized_slot = state.justified_slot
 
-    # Balance recalculations related to FFG rewards
-    let
-      # The portion lost by offline [validators](#dfn-validator) after `D`
-      # epochs is about `D*D/2/inactivity_penalty_quotient`.
-      inactivity_penalty_quotient = SQRT_E_DROP_TIME^2
-      time_since_finality = blck.slot - state.finalized_slot
+      if state.justified_slot == s - EPOCH_LENGTH - 2 * EPOCH_LENGTH and
+          state.justified_slot_bitfield mod 16 in [15'u64, 14]:
+        state.finalized_slot = state.justified_slot
 
-    if time_since_finality <= 4'u64 * EPOCH_LENGTH:
-      # for v in previous_epoch_boundary_attesters:
-      #   state.validators[v].balance.inc(adjust_for_inclusion_distance(
-      #     base_reward(state.validators[v]) *
-      #       prev_cycle_boundary_attesting_balance div total_balance,
-      #     inclusion_distance(v)))
+      state.previous_justified_slot = state.justified_slot
 
-      for v in get_active_validator_indices(state.validator_registry):
-        if v notin previous_epoch_boundary_attesters:
-          state.validator_registry[v].balance.dec(
-            base_reward(state.validator_registry[v]).int)
-    else:
-      # Any validator in `prev_cycle_boundary_attesters` sees their balance
-      # unchanged.
-      # Others might get penalized:
-      for vindex, v in state.validator_registry.mpairs():
-        if (v.status == ACTIVE and vindex notin previous_epoch_boundary_attesters) or
-            v.status == EXITED_WITH_PENALTY:
-          v.balance.dec(
-            (base_reward(v) + get_effective_balance(v) * time_since_finality div
-              inactivity_penalty_quotient.uint64).int)
+      if new_justified_slot.isSome():
+        state.justified_slot = new_justified_slot.get()
 
-      # For each `v` in `prev_cycle_boundary_attesters`, we determine the proposer `proposer_index = get_beacon_proposer_index(state, inclusion_slot(v))` and set `state.validators[proposer_index].balance += base_reward(v) // INCLUDER_REWARD_SHARE_QUOTIENT`.
+      for sac in state.shard_and_committee_for_slots:
+        # TODO or just state.shard_and_committee_for_slots[s]?
+        for obj in sac:
+          if 3'u64 * total_attesting_balance(obj) >= 2'u64 * total_balance_sac(obj):
+            state.latest_crosslinks[obj.shard] = CrosslinkRecord(
+              slot: state.latest_state_recalculation_slot + EPOCH_LENGTH,
+              shard_block_hash: winning_hash(obj))
 
-    # Balance recalculations related to crosslink rewards
+    block: # Balance recalculations related to FFG rewards
+      let
+        # The portion lost by offline [validators](#dfn-validator) after `D`
+        # epochs is about `D*D/2/inactivity_penalty_quotient`.
+        inactivity_penalty_quotient = SQRT_E_DROP_TIME^2
+        time_since_finality = blck.slot - state.finalized_slot
 
-    # Ethereum 1.0 chain related rules
+      if time_since_finality <= 4'u64 * EPOCH_LENGTH:
+        for v in previous_epoch_boundary_attesters:
+          state.validator_registry[v].balance.inc(adjust_for_inclusion_distance(
+            base_reward(state.validator_registry[v]) *
+            previous_epoch_boundary_attesting_balance div total_balance,
+            inclusion_distance(v)).int)
 
-    # Validator registry change
+        for v in active_validator_indices:
+          if v notin previous_epoch_boundary_attesters:
+            state.validator_registry[v].balance.dec(
+              base_reward(state.validator_registry[v]).int)
+      else:
+        # Any validator in `prev_cycle_boundary_attesters` sees their balance
+        # unchanged.
+        # Others might get penalized:
+        for vindex, v in state.validator_registry.mpairs():
+          if (v.status == ACTIVE and
+                vindex.Uint24 notin previous_epoch_boundary_attesters) or
+              v.status == EXITED_WITH_PENALTY:
+            v.balance.dec(
+              (base_reward(v) + get_effective_balance(v) * time_since_finality div
+                inactivity_penalty_quotient.uint64).int)
 
-    # If a validator registry change does NOT happen
+        for v in previous_epoch_boundary_attesters:
+          let proposer_index = get_beacon_proposer_index(state, inclusion_slot(v))
+          state.validator_registry[proposer_index].balance.inc(
+            (base_reward(state.validator_registry[v]) div INCLUDER_REWARD_QUOTIENT.uint64).int)
 
-    # Proposer reshuffling
+    block: # Balance recalculations related to crosslink rewards
+      for sac in state.shard_and_committee_for_slots[0 ..< EPOCH_LENGTH]:
+        for obj in sac:
+          for vindex in obj.committee:
+            let v = state.validator_registry[vindex].addr
 
-    # Finally...
+            if vindex in attesting_validators(obj):
+              v.balance.inc(adjust_for_inclusion_distance(
+                base_reward(v[]) * total_attesting_balance(obj) div total_balance_sac(obj),
+                inclusion_distance(vindex)).int)
+            else:
+              v.balance.dec(base_reward(v[]).int)
 
+    block: # Ethereum 1.0 chain related rules
+      if state.latest_state_recalculation_slot mod
+          POW_RECEIPT_ROOT_VOTING_PERIOD.uint64 == 0:
+        for x in state.candidate_pow_receipt_roots:
+          if x.votes * 2 >= POW_RECEIPT_ROOT_VOTING_PERIOD.uint64:
+            state.processed_pow_receipt_root = x.candidate_pow_receipt_root
+            break
+        state.candidate_pow_receipt_roots = @[]
+
+    block: # Validator registry change
+      if state.finalized_slot > state.validator_registry_latest_change_slot and
+          allIt(state.shard_and_committee_for_slots,
+            allIt(it,
+              state.latest_crosslinks[it.shard].slot >
+                state.validator_registry_latest_change_slot)):
+        state.change_validators(s)
+        state.validator_registry_latest_change_slot = s + EPOCH_LENGTH
+        for i in 0..<EPOCH_LENGTH:
+          state.shard_and_committee_for_slots[i] =
+            state.shard_and_committee_for_slots[EPOCH_LENGTH + i]
+        # https://github.com/ethereum/eth2.0-specs/issues/223
+        let next_start_shard = (state.shard_and_committee_for_slots[^1][^1].shard + 1) mod SHARD_COUNT
+        for i, v in get_new_shuffling(
+            state.next_seed, state.validator_registry, next_start_shard):
+          state.shard_and_committee_for_slots[i + EPOCH_LENGTH] = v
+        state.next_seed = state.randao_mix
+      else:
+        # If a validator registry change does NOT happen
+        for i in 0..<EPOCH_LENGTH:
+          state.shard_and_committee_for_slots[i] =
+            state.shard_and_committee_for_slots[EPOCH_LENGTH + i]
+        let time_since_finality = blck.slot - state.validator_registry_latest_change_slot
+        let start_shard = state.shard_and_committee_for_slots[0][0].shard
+        if time_since_finality * EPOCH_LENGTH <= MIN_VALIDATOR_REGISTRY_CHANGE_INTERVAL.uint64 or
+            is_power_of_2(time_since_finality):
+          for i, v in get_new_shuffling(
+              state.next_seed, state.validator_registry, start_shard):
+            state.shard_and_committee_for_slots[i + EPOCH_LENGTH] = v
+          state.next_seed = state.randao_mix
+          # Note that `start_shard` is not changed from the last epoch.
+
+    block: # Proposer reshuffling
+      let active_validator_indices = get_active_validator_indices(state.validator_registry)
+      let num_validators_to_reshuffle = len(active_validator_indices) div SHARD_PERSISTENT_COMMITTEE_CHANGE_PERIOD
+      for i in 0..<num_validators_to_reshuffle:
+        # Multiplying i to 2 to ensure we have different input to all the required hashes in the shuffling
+        # and none of the hashes used for entropy in this loop will be the same
+        let validator_index = 0.Uint24 # active_validator_indices[hash(state.randao_mix + bytes8(i * 2)) mod len(active_validator_indices)]
+        let new_shard = 0'u64 # hash(state.randao_mix + bytes8(i * 2 + 1)) mod SHARD_COUNT
+        let shard_reassignment_record = ShardReassignmentRecord(
+            validator_index: validator_index,
+            shard: new_shard,
+            slot: s + SHARD_PERSISTENT_COMMITTEE_CHANGE_PERIOD.uint64
+        )
+        state.persistent_committee_reassignments.add(shard_reassignment_record)
+
+      while len(state.persistent_committee_reassignments) > 0 and
+          state.persistent_committee_reassignments[0].slot <= s:
+        let reassignment = state.persistent_committee_reassignments[0]
+        state.persistent_committee_reassignments.delete(0)
+        for committee in state.persistent_committees.mitems():
+          if reassignment.validator_index in committee:
+            committee.delete(committee.find(reassignment.validator_index))
+        state.persistent_committees[reassignment.shard.int].add(
+          reassignment.validator_index)
+
+    block: # Finally...
+      # Remove all attestation records older than slot `s`.
+      for i, v in state.validator_registry:
+        if v.balance < MIN_BALANCE.uint64 and v.status == ACTIVE:
+          exit_validator(i.Uint24, state, penalize=false, current_slot=blck.slot)
+      state.latest_block_hashes = state.latest_block_hashes[EPOCH_LENGTH..^1]
+      state.latest_state_recalculation_slot.inc(EPOCH_LENGTH)
+
+  true
+
+func updateState*(state: BeaconState, blck: BeaconBlock): Option[BeaconState] =
+  ## Adjust `state` according to the information in `blck`.
+  ## Returns the new state, or `none` if the block is invalid.
+
+  # TODO check to which extent this copy can be avoided (considering forks etc),
+  #      for now, it serves as a reminder that we need to handle invalid blocks
+  #      somewhere..
+  # TODO many functions will mutate `state` partially without rolling back
+  #      the changes in case of failure (look out for `var BeaconState` and
+  #      bool return values...)
+  var state = state
+
+  # Block processing is split up into two phases - lightweight updates done
+  # for each block, and bigger updates done for each epoch.
+
+  # Lightweight updates that happen for every block
+  if not processBlock(state, blck): return
+
+  # Heavy updates that happen for every epoch
+  if not processEpoch(state, blck): return
+
+  # All good, we can return the new state
   some(state)

tests/test_block_processing.nim

@@ -14,13 +14,13 @@ import
 suite "Block processing":
   ## For now just test that we can compile and execute block processing with mock data.
 
-  test "Mock process_block":
+  test "Mock state update":
     let
       state = on_startup(makeInitialValidators(), 0, Eth2Digest())
       blck = BeaconBlock(
         slot: 1,
         ancestor_hashes: @[Eth2Digest()]
       )
-      newState = process_block(state, blck)
+      newState = updateState(state, blck)
     check:
       newState.isNone() # Broken block, should fail processing