Revert "Merge pull request #200 from sigp/new-structure"

This reverts commit d7a3545be1, reversing
changes made to 1da06c156c.

eth2/fork_choice/src/lib.rs

@@ -0,0 +1,118 @@
+// Copyright 2019 Sigma Prime Pty Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+//! This crate stores the various implementations of fork-choice rules that can be used for the
+//! beacon blockchain.
+//!
+//! There are four implementations. One is the naive longest chain rule (primarily for testing
+//! purposes). The other three are proposed implementations of the LMD-GHOST fork-choice rule with various forms of optimisation.
+//!
+//! The current implementations are:
+//! - [`longest-chain`]: Simplistic longest-chain fork choice - primarily for testing, **not for
+//! production**.
+//! - [`slow_lmd_ghost`]: This is a simple and very inefficient implementation given in the ethereum 2.0
+//! specifications (https://github.com/ethereum/eth2.0-specs/blob/v0.1/specs/core/0_beacon-chain.md#get_block_root).
+//! - [`optimised_lmd_ghost`]: This is an optimised version of the naive implementation as proposed
+//! by Vitalik. The reference implementation can be found at: https://github.com/ethereum/research/blob/master/ghost/ghost.py
+//! - [`protolambda_lmd_ghost`]: Another optimised version of LMD-GHOST designed by @protolambda.
+//! The go implementation can be found here: https://github.com/protolambda/lmd-ghost.
+//!
+//! [`slow_lmd_ghost`]: struct.SlowLmdGhost.html
+//! [`optimised_lmd_ghost`]: struct.OptimisedLmdGhost.html
+//! [`protolambda_lmd_ghost`]: struct.ProtolambdaLmdGhost.html
+
+extern crate db;
+extern crate ssz;
+extern crate types;
+
+pub mod longest_chain;
+pub mod optimised_lmd_ghost;
+pub mod protolambda_lmd_ghost;
+pub mod slow_lmd_ghost;
+
+use db::stores::BeaconBlockAtSlotError;
+use db::DBError;
+use types::{BeaconBlock, Hash256};
+
+/// Defines the interface for Fork Choices. Each Fork choice will define their own data structures
+/// which can be built in block processing through the `add_block` and `add_attestation` functions.
+/// The main fork choice algorithm is specified in `find_head
+pub trait ForkChoice: Send + Sync {
+    /// Called when a block has been added. Allows generic block-level data structures to be
+    /// built for a given fork-choice.
+    fn add_block(
+        &mut self,
+        block: &BeaconBlock,
+        block_hash: &Hash256,
+    ) -> Result<(), ForkChoiceError>;
+    /// Called when an attestation has been added. Allows generic attestation-level data structures to be built for a given fork choice.
+    // This can be generalised to a full attestation if required later.
+    fn add_attestation(
+        &mut self,
+        validator_index: u64,
+        target_block_hash: &Hash256,
+    ) -> Result<(), ForkChoiceError>;
+    /// The fork-choice algorithm to find the current canonical head of the chain.
+    // TODO: Remove the justified_start_block parameter and make it internal
+    fn find_head(&mut self, justified_start_block: &Hash256) -> Result<Hash256, ForkChoiceError>;
+}
+
+/// Possible fork choice errors that can occur.
+#[derive(Debug, PartialEq)]
+pub enum ForkChoiceError {
+    MissingBeaconBlock(Hash256),
+    MissingBeaconState(Hash256),
+    IncorrectBeaconState(Hash256),
+    CannotFindBestChild,
+    ChildrenNotFound,
+    StorageError(String),
+}
+
+impl From<DBError> for ForkChoiceError {
+    fn from(e: DBError) -> ForkChoiceError {
+        ForkChoiceError::StorageError(e.message)
+    }
+}
+
+impl From<BeaconBlockAtSlotError> for ForkChoiceError {
+    fn from(e: BeaconBlockAtSlotError) -> ForkChoiceError {
+        match e {
+            BeaconBlockAtSlotError::UnknownBeaconBlock(hash) => {
+                ForkChoiceError::MissingBeaconBlock(hash)
+            }
+            BeaconBlockAtSlotError::InvalidBeaconBlock(hash) => {
+                ForkChoiceError::MissingBeaconBlock(hash)
+            }
+            BeaconBlockAtSlotError::DBError(string) => ForkChoiceError::StorageError(string),
+        }
+    }
+}
+
+/// Fork choice options that are currently implemented.
+pub enum ForkChoiceAlgorithms {
+    /// Chooses the longest chain becomes the head. Not for production.
+    LongestChain,
+    /// A simple and highly inefficient implementation of LMD ghost.
+    SlowLMDGhost,
+    /// An optimised version of LMD-GHOST by Vitalik.
+    OptimisedLMDGhost,
+    /// An optimised version of LMD-GHOST by Protolambda.
+    ProtoLMDGhost,
+}

eth2/fork_choice/src/longest_chain.rs

@@ -0,0 +1,93 @@
+use db::stores::BeaconBlockStore;
+use db::{ClientDB, DBError};
+use ssz::{Decodable, DecodeError};
+use std::sync::Arc;
+use types::{BeaconBlock, Hash256, Slot};
+
+pub enum ForkChoiceError {
+    BadSszInDatabase,
+    MissingBlock,
+    DBError(String),
+}
+
+pub fn longest_chain<T>(
+    head_block_hashes: &[Hash256],
+    block_store: &Arc<BeaconBlockStore<T>>,
+) -> Result<Option<usize>, ForkChoiceError>
+where
+    T: ClientDB + Sized,
+{
+    let mut head_blocks: Vec<(usize, BeaconBlock)> = vec![];
+
+    /*
+     * Load all the head_block hashes from the DB as SszBeaconBlocks.
+     */
+    for (index, block_hash) in head_block_hashes.iter().enumerate() {
+        let ssz = block_store
+            .get(&block_hash)?
+            .ok_or(ForkChoiceError::MissingBlock)?;
+        let (block, _) = BeaconBlock::ssz_decode(&ssz, 0)?;
+        head_blocks.push((index, block));
+    }
+
+    /*
+     * Loop through all the head blocks and find the highest slot.
+     */
+    let highest_slot: Option<Slot> = None;
+    for (_, block) in &head_blocks {
+        let slot = block.slot;
+
+        match highest_slot {
+            None => Some(slot),
+            Some(winning_slot) => {
+                if slot > winning_slot {
+                    Some(slot)
+                } else {
+                    Some(winning_slot)
+                }
+            }
+        };
+    }
+
+    /*
+     * Loop through all the highest blocks and sort them by highest hash.
+     *
+     * Ultimately, the index of the head_block hash with the highest slot and highest block
+     * hash will be the winner.
+     */
+    match highest_slot {
+        None => Ok(None),
+        Some(highest_slot) => {
+            let mut highest_blocks = vec![];
+            for (index, block) in head_blocks {
+                if block.slot == highest_slot {
+                    highest_blocks.push((index, block))
+                }
+            }
+
+            highest_blocks.sort_by(|a, b| head_block_hashes[a.0].cmp(&head_block_hashes[b.0]));
+            let (index, _) = highest_blocks[0];
+            Ok(Some(index))
+        }
+    }
+}
+
+impl From<DecodeError> for ForkChoiceError {
+    fn from(_: DecodeError) -> Self {
+        ForkChoiceError::BadSszInDatabase
+    }
+}
+
+impl From<DBError> for ForkChoiceError {
+    fn from(e: DBError) -> Self {
+        ForkChoiceError::DBError(e.message)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_naive_fork_choice() {
+        assert_eq!(2 + 2, 4);
+    }
+}

eth2/fork_choice/src/optimised_lmd_ghost.rs

@@ -0,0 +1,443 @@
+// Copyright 2019 Sigma Prime Pty Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+extern crate byteorder;
+extern crate fast_math;
+use crate::{ForkChoice, ForkChoiceError};
+use byteorder::{BigEndian, ByteOrder};
+use db::{
+    stores::{BeaconBlockStore, BeaconStateStore},
+    ClientDB,
+};
+use fast_math::log2_raw;
+use std::collections::HashMap;
+use std::sync::Arc;
+use types::{
+    readers::BeaconBlockReader,
+    slot_epoch_height::{Height, Slot},
+    validator_registry::get_active_validator_indices,
+    BeaconBlock, Hash256,
+};
+
+//TODO: Pruning - Children
+//TODO: Handle Syncing
+
+//TODO: Sort out global constants
+const GENESIS_SLOT: u64 = 0;
+const FORK_CHOICE_BALANCE_INCREMENT: u64 = 1e9 as u64;
+const MAX_DEPOSIT_AMOUNT: u64 = 32e9 as u64;
+const EPOCH_LENGTH: u64 = 64;
+
+/// The optimised LMD-GHOST fork choice rule.
+/// NOTE: This uses u32 to represent difference between block heights. Thus this is only
+/// applicable for block height differences in the range of a u32.
+/// This can potentially be parallelized in some parts.
+// we use fast log2, a log2 lookup table is implemented in Vitaliks code, potentially do
+// the comparison. Log2_raw takes 2ns according to the documentation.
+#[inline]
+fn log2_int(x: u32) -> u32 {
+    log2_raw(x as f32) as u32
+}
+
+fn power_of_2_below(x: u32) -> u32 {
+    2u32.pow(log2_int(x))
+}
+
+/// Stores the necessary data structures to run the optimised lmd ghost algorithm.
+pub struct OptimisedLMDGhost<T: ClientDB + Sized> {
+    /// A cache of known ancestors at given heights for a specific block.
+    //TODO: Consider FnvHashMap
+    cache: HashMap<CacheKey<u32>, Hash256>,
+    /// Log lookup table for blocks to their ancestors.
+    //TODO: Verify we only want/need a size 16 log lookup
+    ancestors: Vec<HashMap<Hash256, Hash256>>,
+    /// Stores the children for any given parent.
+    children: HashMap<Hash256, Vec<Hash256>>,
+    /// The latest attestation targets as a map of validator index to block hash.
+    //TODO: Could this be a fixed size vec
+    latest_attestation_targets: HashMap<u64, Hash256>,
+    /// Block storage access.
+    block_store: Arc<BeaconBlockStore<T>>,
+    /// State storage access.
+    state_store: Arc<BeaconStateStore<T>>,
+    max_known_height: Height,
+}
+
+impl<T> OptimisedLMDGhost<T>
+where
+    T: ClientDB + Sized,
+{
+    pub fn new(
+        block_store: Arc<BeaconBlockStore<T>>,
+        state_store: Arc<BeaconStateStore<T>>,
+    ) -> Self {
+        OptimisedLMDGhost {
+            cache: HashMap::new(),
+            ancestors: vec![HashMap::new(); 16],
+            latest_attestation_targets: HashMap::new(),
+            children: HashMap::new(),
+            max_known_height: Height::new(0),
+            block_store,
+            state_store,
+        }
+    }
+
+    /// Finds the latest votes weighted by validator balance. Returns a hashmap of block_hash to
+    /// weighted votes.
+    pub fn get_latest_votes(
+        &self,
+        state_root: &Hash256,
+        block_slot: Slot,
+    ) -> Result<HashMap<Hash256, u64>, ForkChoiceError> {
+        // get latest votes
+        // Note: Votes are weighted by min(balance, MAX_DEPOSIT_AMOUNT) //
+        // FORK_CHOICE_BALANCE_INCREMENT
+        // build a hashmap of block_hash to weighted votes
+        let mut latest_votes: HashMap<Hash256, u64> = HashMap::new();
+        // gets the current weighted votes
+        let current_state = self
+            .state_store
+            .get_deserialized(&state_root)?
+            .ok_or_else(|| ForkChoiceError::MissingBeaconState(*state_root))?;
+
+        let active_validator_indices = get_active_validator_indices(
+            &current_state.validator_registry,
+            block_slot.epoch(EPOCH_LENGTH),
+        );
+
+        for index in active_validator_indices {
+            let balance =
+                std::cmp::min(current_state.validator_balances[index], MAX_DEPOSIT_AMOUNT)
+                    / FORK_CHOICE_BALANCE_INCREMENT;
+            if balance > 0 {
+                if let Some(target) = self.latest_attestation_targets.get(&(index as u64)) {
+                    *latest_votes.entry(*target).or_insert_with(|| 0) += balance;
+                }
+            }
+        }
+
+        Ok(latest_votes)
+    }
+
+    /// Gets the ancestor at a given height `at_height` of a block specified by `block_hash`.
+    fn get_ancestor(&mut self, block_hash: Hash256, at_height: Height) -> Option<Hash256> {
+        // return None if we can't get the block from the db.
+        let block_height = {
+            let block_slot = self
+                .block_store
+                .get_deserialized(&block_hash)
+                .ok()?
+                .expect("Should have returned already if None")
+                .slot;
+
+            block_slot.height(Slot::from(GENESIS_SLOT))
+        };
+
+        // verify we haven't exceeded the block height
+        if at_height >= block_height {
+            if at_height > block_height {
+                return None;
+            } else {
+                return Some(block_hash);
+            }
+        }
+        // check if the result is stored in our cache
+        let cache_key = CacheKey::new(&block_hash, at_height.as_u32());
+        if let Some(ancestor) = self.cache.get(&cache_key) {
+            return Some(*ancestor);
+        }
+
+        // not in the cache recursively search for ancestors using a log-lookup
+
+        if let Some(ancestor) = {
+            let ancestor_lookup = self.ancestors
+                [log2_int((block_height - at_height - 1u64).as_u32()) as usize]
+                .get(&block_hash)
+                //TODO: Panic if we can't lookup and fork choice fails
+                .expect("All blocks should be added to the ancestor log lookup table");
+            self.get_ancestor(*ancestor_lookup, at_height)
+        } {
+            // add the result to the cache
+            self.cache.insert(cache_key, ancestor);
+            return Some(ancestor);
+        }
+
+        None
+    }
+
+    // looks for an obvious block winner given the latest votes for a specific height
+    fn get_clear_winner(
+        &mut self,
+        latest_votes: &HashMap<Hash256, u64>,
+        block_height: Height,
+    ) -> Option<Hash256> {
+        // map of vote counts for every hash at this height
+        let mut current_votes: HashMap<Hash256, u64> = HashMap::new();
+        let mut total_vote_count = 0;
+
+        // loop through the latest votes and count all votes
+        // these have already been weighted by balance
+        for (hash, votes) in latest_votes.iter() {
+            if let Some(ancestor) = self.get_ancestor(*hash, block_height) {
+                let current_vote_value = current_votes.get(&ancestor).unwrap_or_else(|| &0);
+                current_votes.insert(ancestor, current_vote_value + *votes);
+                total_vote_count += votes;
+            }
+        }
+        // Check if there is a clear block winner at this height. If so return it.
+        for (hash, votes) in current_votes.iter() {
+            if *votes >= total_vote_count / 2 {
+                // we have a clear winner, return it
+                return Some(*hash);
+            }
+        }
+        // didn't find a clear winner
+        None
+    }
+
+    // Finds the best child, splitting children into a binary tree, based on their hashes
+    fn choose_best_child(&self, votes: &HashMap<Hash256, u64>) -> Option<Hash256> {
+        let mut bitmask = 0;
+        for bit in (0..=255).rev() {
+            let mut zero_votes = 0;
+            let mut one_votes = 0;
+            let mut single_candidate = None;
+
+            for (candidate, votes) in votes.iter() {
+                let candidate_uint = BigEndian::read_u32(candidate);
+                if candidate_uint >> (bit + 1) != bitmask {
+                    continue;
+                }
+                if (candidate_uint >> bit) % 2 == 0 {
+                    zero_votes += votes;
+                } else {
+                    one_votes += votes;
+                }
+
+                if single_candidate.is_none() {
+                    single_candidate = Some(candidate);
+                } else {
+                    single_candidate = None;
+                }
+            }
+            bitmask = (bitmask * 2) + {
+                if one_votes > zero_votes {
+                    1
+                } else {
+                    0
+                }
+            };
+            if let Some(candidate) = single_candidate {
+                return Some(*candidate);
+            }
+            //TODO Remove this during benchmark after testing
+            assert!(bit >= 1);
+        }
+        // should never reach here
+        None
+    }
+}
+
+impl<T: ClientDB + Sized> ForkChoice for OptimisedLMDGhost<T> {
+    fn add_block(
+        &mut self,
+        block: &BeaconBlock,
+        block_hash: &Hash256,
+    ) -> Result<(), ForkChoiceError> {
+        // get the height of the parent
+        let parent_height = self
+            .block_store
+            .get_deserialized(&block.parent_root)?
+            .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(block.parent_root))?
+            .slot()
+            .height(Slot::from(GENESIS_SLOT));
+
+        let parent_hash = &block.parent_root;
+
+        // add the new block to the children of parent
+        (*self
+            .children
+            .entry(block.parent_root)
+            .or_insert_with(|| vec![]))
+        .push(block_hash.clone());
+
+        // build the ancestor data structure
+        for index in 0..16 {
+            if parent_height % (1 << index) == 0 {
+                self.ancestors[index].insert(*block_hash, *parent_hash);
+            } else {
+                // TODO: This is unsafe. Will panic if parent_hash doesn't exist. Using it for debugging
+                let parent_ancestor = self.ancestors[index][parent_hash];
+                self.ancestors[index].insert(*block_hash, parent_ancestor);
+            }
+        }
+        // update the max height
+        self.max_known_height = std::cmp::max(self.max_known_height, parent_height + 1);
+        Ok(())
+    }
+
+    fn add_attestation(
+        &mut self,
+        validator_index: u64,
+        target_block_root: &Hash256,
+    ) -> Result<(), ForkChoiceError> {
+        // simply add the attestation to the latest_attestation_target if the block_height is
+        // larger
+        let attestation_target = self
+            .latest_attestation_targets
+            .entry(validator_index)
+            .or_insert_with(|| *target_block_root);
+        // if we already have a value
+        if attestation_target != target_block_root {
+            // get the height of the target block
+            let block_height = self
+                .block_store
+                .get_deserialized(&target_block_root)?
+                .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*target_block_root))?
+                .slot()
+                .height(Slot::from(GENESIS_SLOT));
+
+            // get the height of the past target block
+            let past_block_height = self
+                .block_store
+                .get_deserialized(&attestation_target)?
+                .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*attestation_target))?
+                .slot()
+                .height(Slot::from(GENESIS_SLOT));
+            // update the attestation only if the new target is higher
+            if past_block_height < block_height {
+                *attestation_target = *target_block_root;
+            }
+        }
+        Ok(())
+    }
+
+    /// Perform lmd_ghost on the current chain to find the head.
+    fn find_head(&mut self, justified_block_start: &Hash256) -> Result<Hash256, ForkChoiceError> {
+        let block = self
+            .block_store
+            .get_deserialized(&justified_block_start)?
+            .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*justified_block_start))?;
+
+        let block_slot = block.slot();
+        let block_height = block_slot.height(Slot::from(GENESIS_SLOT));
+        let state_root = block.state_root();
+
+        let mut current_head = *justified_block_start;
+
+        let mut latest_votes = self.get_latest_votes(&state_root, block_slot)?;
+
+        // remove any votes that don't relate to our current head.
+        latest_votes.retain(|hash, _| self.get_ancestor(*hash, block_height) == Some(current_head));
+
+        // begin searching for the head
+        loop {
+            // if there are no children, we are done, return the current_head
+            let children = match self.children.get(&current_head) {
+                Some(children) => children.clone(),
+                None => return Ok(current_head),
+            };
+
+            // logarithmic lookup blocks to see if there are obvious winners, if so,
+            // progress to the next iteration.
+            let mut step =
+                power_of_2_below(self.max_known_height.saturating_sub(block_height).as_u32()) / 2;
+            while step > 0 {
+                if let Some(clear_winner) = self.get_clear_winner(
+                    &latest_votes,
+                    block_height - (block_height % u64::from(step)) + u64::from(step),
+                ) {
+                    current_head = clear_winner;
+                    break;
+                }
+                step /= 2;
+            }
+            if step > 0 {
+            }
+            // if our skip lookup failed and we only have one child, progress to that child
+            else if children.len() == 1 {
+                current_head = children[0];
+            }
+            // we need to find the best child path to progress down.
+            else {
+                let mut child_votes = HashMap::new();
+                for (voted_hash, vote) in latest_votes.iter() {
+                    // if the latest votes correspond to a child
+                    if let Some(child) = self.get_ancestor(*voted_hash, block_height + 1) {
+                        // add up the votes for each child
+                        *child_votes.entry(child).or_insert_with(|| 0) += vote;
+                    }
+                }
+                // given the votes on the children, find the best child
+                current_head = self
+                    .choose_best_child(&child_votes)
+                    .ok_or(ForkChoiceError::CannotFindBestChild)?;
+            }
+
+            // No head was found, re-iterate
+
+            // update the block height for the next iteration
+            let block_height = self
+                .block_store
+                .get_deserialized(&current_head)?
+                .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*justified_block_start))?
+                .slot()
+                .height(Slot::from(GENESIS_SLOT));
+
+            // prune the latest votes for votes that are not part of current chosen chain
+            // more specifically, only keep votes that have head as an ancestor
+            latest_votes
+                .retain(|hash, _| self.get_ancestor(*hash, block_height) == Some(current_head));
+        }
+    }
+}
+
+/// Type for storing blocks in a memory cache. Key is comprised of block-hash plus the height.
+#[derive(PartialEq, Eq, Hash)]
+pub struct CacheKey<T> {
+    block_hash: Hash256,
+    block_height: T,
+}
+
+impl<T> CacheKey<T> {
+    pub fn new(block_hash: &Hash256, block_height: T) -> Self {
+        CacheKey {
+            block_hash: *block_hash,
+            block_height,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    pub fn test_power_of_2_below() {
+        println!("{:?}", std::f32::MAX);
+        assert_eq!(power_of_2_below(4), 4);
+        assert_eq!(power_of_2_below(5), 4);
+        assert_eq!(power_of_2_below(7), 4);
+        assert_eq!(power_of_2_below(24), 16);
+        assert_eq!(power_of_2_below(32), 32);
+        assert_eq!(power_of_2_below(33), 32);
+        assert_eq!(power_of_2_below(63), 32);
+    }
+}

eth2/fork_choice/src/slow_lmd_ghost.rs

@@ -0,0 +1,223 @@
+// Copyright 2019 Sigma Prime Pty Ltd.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+extern crate db;
+
+use crate::{ForkChoice, ForkChoiceError};
+use db::{
+    stores::{BeaconBlockStore, BeaconStateStore},
+    ClientDB,
+};
+use std::collections::HashMap;
+use std::sync::Arc;
+use types::{
+    readers::{BeaconBlockReader, BeaconStateReader},
+    slot_epoch_height::Slot,
+    validator_registry::get_active_validator_indices,
+    BeaconBlock, Hash256,
+};
+
+//TODO: Pruning and syncing
+
+//TODO: Sort out global constants
+const GENESIS_SLOT: u64 = 0;
+const FORK_CHOICE_BALANCE_INCREMENT: u64 = 1e9 as u64;
+const MAX_DEPOSIT_AMOUNT: u64 = 32e9 as u64;
+const EPOCH_LENGTH: u64 = 64;
+
+pub struct SlowLMDGhost<T: ClientDB + Sized> {
+    /// The latest attestation targets as a map of validator index to block hash.
+    //TODO: Could this be a fixed size vec
+    latest_attestation_targets: HashMap<u64, Hash256>,
+    /// Stores the children for any given parent.
+    children: HashMap<Hash256, Vec<Hash256>>,
+    /// Block storage access.
+    block_store: Arc<BeaconBlockStore<T>>,
+    /// State storage access.
+    state_store: Arc<BeaconStateStore<T>>,
+}
+
+impl<T> SlowLMDGhost<T>
+where
+    T: ClientDB + Sized,
+{
+    pub fn new(block_store: BeaconBlockStore<T>, state_store: BeaconStateStore<T>) -> Self {
+        SlowLMDGhost {
+            latest_attestation_targets: HashMap::new(),
+            children: HashMap::new(),
+            block_store: Arc::new(block_store),
+            state_store: Arc::new(state_store),
+        }
+    }
+
+    /// Finds the latest votes weighted by validator balance. Returns a hashmap of block_hash to
+    /// weighted votes.
+    pub fn get_latest_votes(
+        &self,
+        state_root: &Hash256,
+        block_slot: Slot,
+    ) -> Result<HashMap<Hash256, u64>, ForkChoiceError> {
+        // get latest votes
+        // Note: Votes are weighted by min(balance, MAX_DEPOSIT_AMOUNT) //
+        // FORK_CHOICE_BALANCE_INCREMENT
+        // build a hashmap of block_hash to weighted votes
+        let mut latest_votes: HashMap<Hash256, u64> = HashMap::new();
+        // gets the current weighted votes
+        let current_state = self
+            .state_store
+            .get_deserialized(&state_root)?
+            .ok_or_else(|| ForkChoiceError::MissingBeaconState(*state_root))?;
+
+        let active_validator_indices = get_active_validator_indices(
+            &current_state.validator_registry,
+            block_slot.epoch(EPOCH_LENGTH),
+        );
+
+        for index in active_validator_indices {
+            let balance =
+                std::cmp::min(current_state.validator_balances[index], MAX_DEPOSIT_AMOUNT)
+                    / FORK_CHOICE_BALANCE_INCREMENT;
+            if balance > 0 {
+                if let Some(target) = self.latest_attestation_targets.get(&(index as u64)) {
+                    *latest_votes.entry(*target).or_insert_with(|| 0) += balance;
+                }
+            }
+        }
+
+        Ok(latest_votes)
+    }
+
+    /// Get the total number of votes for some given block root.
+    ///
+    /// The vote count is incremented each time an attestation target votes for a block root.
+    fn get_vote_count(
+        &self,
+        latest_votes: &HashMap<Hash256, u64>,
+        block_root: &Hash256,
+    ) -> Result<u64, ForkChoiceError> {
+        let mut count = 0;
+        let block_slot = self
+            .block_store
+            .get_deserialized(&block_root)?
+            .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*block_root))?
+            .slot();
+
+        for (target_hash, votes) in latest_votes.iter() {
+            let (root_at_slot, _) = self
+                .block_store
+                .block_at_slot(&block_root, block_slot)?
+                .ok_or(ForkChoiceError::MissingBeaconBlock(*block_root))?;
+            if root_at_slot == *target_hash {
+                count += votes;
+            }
+        }
+        Ok(count)
+    }
+}
+
+impl<T: ClientDB + Sized> ForkChoice for SlowLMDGhost<T> {
+    /// Process when a block is added
+    fn add_block(
+        &mut self,
+        block: &BeaconBlock,
+        block_hash: &Hash256,
+    ) -> Result<(), ForkChoiceError> {
+        // build the children hashmap
+        // add the new block to the children of parent
+        (*self
+            .children
+            .entry(block.parent_root)
+            .or_insert_with(|| vec![]))
+        .push(block_hash.clone());
+
+        // complete
+        Ok(())
+    }
+
+    fn add_attestation(
+        &mut self,
+        validator_index: u64,
+        target_block_root: &Hash256,
+    ) -> Result<(), ForkChoiceError> {
+        // simply add the attestation to the latest_attestation_target if the block_height is
+        // larger
+        let attestation_target = self
+            .latest_attestation_targets
+            .entry(validator_index)
+            .or_insert_with(|| *target_block_root);
+        // if we already have a value
+        if attestation_target != target_block_root {
+            // get the height of the target block
+            let block_height = self
+                .block_store
+                .get_deserialized(&target_block_root)?
+                .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*target_block_root))?
+                .slot()
+                .height(Slot::from(GENESIS_SLOT));
+
+            // get the height of the past target block
+            let past_block_height = self
+                .block_store
+                .get_deserialized(&attestation_target)?
+                .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*attestation_target))?
+                .slot()
+                .height(Slot::from(GENESIS_SLOT));
+            // update the attestation only if the new target is higher
+            if past_block_height < block_height {
+                *attestation_target = *target_block_root;
+            }
+        }
+        Ok(())
+    }
+
+    /// A very inefficient implementation of LMD ghost.
+    fn find_head(&mut self, justified_block_start: &Hash256) -> Result<Hash256, ForkChoiceError> {
+        let start = self
+            .block_store
+            .get_deserialized(&justified_block_start)?
+            .ok_or_else(|| ForkChoiceError::MissingBeaconBlock(*justified_block_start))?;
+
+        let start_state_root = start.state_root();
+
+        let latest_votes = self.get_latest_votes(&start_state_root, start.slot())?;
+
+        let mut head_hash = Hash256::zero();
+
+        loop {
+            let mut head_vote_count = 0;
+
+            let children = match self.children.get(&head_hash) {
+                Some(children) => children,
+                // we have found the head, exit
+                None => break,
+            };
+
+            for child_hash in children {
+                let vote_count = self.get_vote_count(&latest_votes, &child_hash)?;
+
+                if vote_count > head_vote_count {
+                    head_hash = *child_hash;
+                    head_vote_count = vote_count;
+                }
+            }
+        }
+        Ok(head_hash)
+    }
+}