state transition function

src/blockchain/CMakeLists.txt

@@ -5,15 +5,16 @@
 #
 
 add_library(blockchain
-    impl/storage_util.cpp
-    impl/block_storage_impl.cpp
     impl/block_storage_error.cpp
+    impl/block_storage_impl.cpp
     impl/block_storage_initializer.cpp
     impl/block_tree_error.cpp
     impl/block_tree_impl.cpp
     impl/block_tree_initializer.cpp
     impl/cached_tree.cpp
     impl/genesis_block_header_impl.cpp
+    impl/storage_util.cpp
+    state_transition_function.cpp
 )
 target_link_libraries(blockchain
     Boost::boost

src/blockchain/is_justifiable_slot.hpp

@@ -0,0 +1,25 @@
+/**
+ * Copyright Quadrivium LLC
+ * All Rights Reserved
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#include <cmath>
+
+#include <boost/assert.hpp>
+
+#include "types/slot.hpp"
+
+namespace lean {
+  inline bool isJustifiableSlot(Slot finalized_slot, Slot candidate) {
+    BOOST_ASSERT(candidate >= finalized_slot);
+    auto delta = candidate - finalized_slot;
+    return delta <= 5
+        // any x^2
+        or fmod(sqrt(delta), 1) == 0
+        // any x^2+x
+        or fmod(sqrt(delta + 0.25), 1) == 0.5;
+  }
+}  // namespace lean

src/blockchain/state_transition_function.cpp

@@ -0,0 +1,278 @@
+/**
+ * Copyright Quadrivium LLC
+ * All Rights Reserved
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include "blockchain/state_transition_function.hpp"
+
+#include <boost/assert.hpp>
+
+#include "blockchain/is_justifiable_slot.hpp"
+#include "types/signed_block.hpp"
+#include "types/state.hpp"
+
+namespace lean {
+  constexpr BlockHash kZeroHash;
+
+  inline bool getBit(const std::vector<bool> &bits, size_t i) {
+    return i < bits.size() and bits.at(i);
+  }
+
+  // TODO(turuslan): spec doesn't rotate history
+  inline void setBit(std::vector<bool> &bits, size_t i) {
+    if (bits.size() <= i) {
+      bits.resize(i + 1);
+    }
+    bits.at(i) = true;
+  }
+
+  using Justifications = std::map<BlockHash, std::vector<bool>>;
+
+  /**
+   * Returns a map of `root -> justifications` constructed from the flattened
+   * data in the state.
+   */
+  inline Justifications getJustifications(const State &state) {
+    auto &roots = state.justifications_roots.data();
+    auto &validators = state.justifications_validators.data();
+    Justifications justifications;
+    size_t offset = 0;
+    BOOST_ASSERT(validators.size() == roots.size() * VALIDATOR_REGISTRY_LIMIT);
+    for (auto &root : roots) {
+      auto next_offset = offset + VALIDATOR_REGISTRY_LIMIT;
+      std::vector<bool> bits{
+          validators.begin() + offset,
+          validators.begin() + next_offset,
+      };
+      justifications[root] = std::move(bits);
+      offset = next_offset;
+    }
+    return justifications;
+  }
+
+  /**
+   * Saves a map of `root -> justifications` back into the state's flattened
+   * data structure.
+   */
+  inline void setJustifications(State &state,
+                                const Justifications &justifications) {
+    auto &roots = state.justifications_roots.data();
+    auto &validators = state.justifications_validators.data();
+    roots.resize(0);
+    roots.reserve(justifications.size());
+    validators.resize(0);
+    validators.reserve(justifications.size() * VALIDATOR_REGISTRY_LIMIT);
+    for (auto &[root, bits] : justifications) {
+      BOOST_ASSERT(bits.size() == VALIDATOR_REGISTRY_LIMIT);
+      roots.push_back(root);
+      validators.insert(validators.end(), bits.begin(), bits.end());
+    }
+  }
+
+  State StateTransitionFunction::generateGenesisState(
+      const Config &config) const {
+    BlockHeader header;
+    header.body_root = sszHash(BlockBody{});
+    return State{
+        .config = config,
+        .latest_block_header = header,
+    };
+  }
+
+  Block StateTransitionFunction::genesisBlock(const State &state) const {
+    return Block{.state_root = sszHash(state)};
+  }
+
+  outcome::result<State> StateTransitionFunction::stateTransition(
+      const SignedBlock &signed_block,
+      const State &parent_state,
+      bool check_state_root) const {
+    auto &block = signed_block.message;
+    auto state = parent_state;
+    // Process slots (including those with no blocks) since block
+    OUTCOME_TRY(processSlots(state, block.slot));
+    // Process block
+    OUTCOME_TRY(processBlock(state, block));
+    // Verify state root
+    if (check_state_root) {
+      auto state_root = sszHash(state);
+      if (block.state_root != state_root) {
+        return Error::STATE_ROOT_DOESNT_MATCH;
+      }
+    }
+    return std::move(state);
+  }
+
+  outcome::result<void> StateTransitionFunction::processSlots(State &state,
+                                                              Slot slot) const {
+    if (state.slot >= slot) {
+      return Error::INVALID_SLOT;
+    }
+    while (state.slot < slot) {
+      processSlot(state);
+      ++state.slot;
+    }
+    return outcome::success();
+  }
+
+  void StateTransitionFunction::processSlot(State &state) const {
+    // Cache latest block header state root
+    if (state.latest_block_header.state_root == kZeroHash) {
+      state.latest_block_header.state_root = sszHash(state);
+    }
+  }
+
+  outcome::result<void> StateTransitionFunction::processBlock(
+      State &state, const Block &block) const {
+    OUTCOME_TRY(processBlockHeader(state, block));
+    OUTCOME_TRY(processOperations(state, block.body));
+    return outcome::success();
+  }
+
+  outcome::result<void> StateTransitionFunction::processBlockHeader(
+      State &state, const Block &block) const {
+    // Verify that the slots match
+    if (block.slot != state.slot) {
+      return Error::INVALID_SLOT;
+    }
+    // Verify that the block is newer than latest block header
+    if (block.slot <= state.latest_block_header.slot) {
+      return Error::INVALID_SLOT;
+    }
+    // Verify that proposer index is the correct index
+    if (block.proposer_index != block.slot % state.config.num_validators) {
+      return Error::INVALID_PROPOSER;
+    }
+    // Verify that the parent matches
+    state.latest_block_header.updateHash();
+    if (block.parent_root != state.latest_block_header.hash()) {
+      return Error::PARENT_ROOT_DOESNT_MATCH;
+    }
+
+    // If this was first block post genesis, 3sf mini special treatment is
+    // required to correctly set genesis block root as already justified and
+    // finalized. This is not possible at the time of genesis state generation
+    // and are set at zero bytes because genesis block is calculated using
+    // genesis state causing a circular dependency
+    if (state.latest_block_header.slot == 0) {
+      // block.parent_root is the genesis root
+      state.latest_justified.root = block.parent_root;
+      state.latest_finalized.root = block.parent_root;
+    }
+
+    // now that we can vote on parent, push it at its correct slot index in the
+    // structures
+    state.historical_block_hashes.push_back(block.parent_root);
+    // genesis block is always justified
+    state.justified_slots.push_back(state.latest_block_header.slot == 0);
+
+    // if there were empty slots, push zero hash for those ancestors
+    for (auto num_empty_slots = block.slot - state.latest_block_header.slot - 1;
+         num_empty_slots > 0;
+         --num_empty_slots) {
+      state.historical_block_hashes.push_back(kZeroHash);
+      state.justified_slots.push_back(false);
+    }
+
+    // Cache current block as the new latest block
+    state.latest_block_header = block.getHeader();
+    // Overwritten in the next process_slot call
+    state.latest_block_header.state_root = kZeroHash;
+    return outcome::success();
+  }
+
+  outcome::result<void> StateTransitionFunction::processOperations(
+      State &state, const BlockBody &body) const {
+    // process attestations
+    OUTCOME_TRY(processAttestations(state, body.attestations.data()));
+    // other operations will get added as the functionality evolves
+    return outcome::success();
+  }
+
+  outcome::result<void> StateTransitionFunction::processAttestations(
+      State &state, const std::vector<SignedVote> &attestations) const {
+    // get justifications, justified slots and historical block hashes are
+    // already upto date as per the processing in process_block_header
+    auto justifications = getJustifications(state);
+
+    // From 3sf-mini/consensus.py - apply votes
+    for (auto &signed_vote : attestations) {
+      auto &vote = signed_vote.data;
+      if (vote.source.slot >= state.historical_block_hashes.size()) {
+        return Error::INVALID_VOTE_SOURCE_SLOT;
+      }
+      if (vote.target.slot >= state.historical_block_hashes.size()) {
+        return Error::INVALID_VOTE_TARGET_SLOT;
+      }
+      // Ignore votes whose source is not already justified,
+      // or whose target is not in the history, or whose target is not a
+      // valid justifiable slot
+      if (not getBit(state.justified_slots.data(), vote.source.slot)
+          // This condition is missing in 3sf mini but has been added here
+          // because we don't want to re-introduce the target again for
+          // remaining votes if the slot is already justified and its tracking
+          // already cleared out from justifications map
+          or getBit(state.justified_slots.data(), vote.target.slot)
+          or vote.source.root
+                 != state.historical_block_hashes.data().at(vote.source.slot)
+          or vote.target.root
+                 != state.historical_block_hashes.data().at(vote.target.slot)
+          or vote.target.slot <= vote.source.slot
+          or not isJustifiableSlot(state.latest_finalized.slot,
+                                   vote.target.slot)) {
+        continue;
+      }
+
+      auto justifications_it = justifications.find(vote.target.root);
+      // Track attempts to justify new hashes
+      if (justifications_it == justifications.end()) {
+        justifications_it =
+            justifications.emplace(vote.target.root, std::vector<bool>{}).first;
+        justifications_it->second.resize(VALIDATOR_REGISTRY_LIMIT);
+      }
+
+      if (vote.validator_id >= justifications_it->second.size()) {
+        return Error::INVALID_VOTER;
+      }
+      justifications_it->second.at(vote.validator_id) = true;
+
+      size_t count = 0;
+      for (auto &&bit : justifications_it->second) {
+        if (bit) {
+          ++count;
+        }
+      }
+
+      // If 2/3 voted for the same new valid hash to justify
+      // in 3sf mini this is strict equality, but we have updated it to >=
+      // also have modified it from count >= (2 * state.config.num_validators)
+      // / 3 to prevent integer division which could lead to less than 2/3 of
+      // validators justifying specially if the num_validators is low in testing
+      // scenarios
+      if (3 * count >= 2 * state.config.num_validators) {
+        state.latest_justified = vote.target;
+        setBit(state.justified_slots.data(), vote.target.slot);
+        justifications.erase(vote.target.root);
+
+        // Finalization: if the target is the next valid justifiable hash after
+        // the source
+        auto any = false;
+        for (auto slot = vote.source.slot + 1; slot < vote.target.slot;
+             ++slot) {
+          if (isJustifiableSlot(state.latest_finalized.slot, slot)) {
+            any = true;
+            break;
+          }
+        }
+        if (not any) {
+          state.latest_finalized = vote.source;
+        }
+      }
+    }
+
+    // flatten and set updated justifications back to the state
+    setJustifications(state, justifications);
+    return outcome::success();
+  }
+}  // namespace lean