Pull basic LMD Ghost into fork choice crate.

- Moves the basic implementation into the fork choice crate.
- Builds the option of fork choices into beacon_struct.

eth2/fork_choice/src/basic_lmd_ghost.rs

@@ -0,0 +1,167 @@
+extern crate db;
+
+// TODO: Pull out the dependency on self and beacon_chain
+
+use db::{
+    stores::{BeaconBlockAtSlotError, BeaconBlockStore},
+    ClientDB, DBError,
+};
+use slot_clock::{SlotClock, TestingSlotClockError};
+use std::collections::HashSet;
+use std::sync::Arc;
+use types::{
+    readers::{BeaconBlockReader, BeaconStateReader},
+    validator_registry::get_active_validator_indices,
+    Hash256,
+};
+
+#[derive(Debug, PartialEq)]
+pub enum Error {
+    DBError(String),
+    MissingBeaconState(Hash256),
+    InvalidBeaconState(Hash256),
+    MissingBeaconBlock(Hash256),
+    InvalidBeaconBlock(Hash256),
+}
+
+
+    /// A very inefficient implementation of LMD ghost.
+    pub fn slow_lmd_ghost(&self, start_hash: &Hash256) -> Result<Hash256, Error> {
+        let start = self
+            .block_store
+            .get_reader(&start_hash)?
+            .ok_or(Error::MissingBeaconBlock(*start_hash))?;
+
+        let start_state_root = start.state_root();
+
+        let state = self
+            .state_store
+            .get_reader(&start_state_root)?
+            .ok_or(Error::MissingBeaconState(start_state_root))?
+            .into_beacon_state()
+            .ok_or(Error::InvalidBeaconState(start_state_root))?;
+
+        let active_validator_indices =
+            get_active_validator_indices(&state.validator_registry, start.slot());
+
+        let mut attestation_targets = Vec::with_capacity(active_validator_indices.len());
+        for i in active_validator_indices {
+            if let Some(target) = self.get_latest_attestation_target(i as u64) {
+                attestation_targets.push(target);
+            }
+        }
+
+        let mut head_hash = Hash256::zero();
+        let mut head_vote_count = 0;
+
+        loop {
+            let child_hashes_and_slots = get_child_hashes_and_slots(
+                &self.block_store,
+                &head_hash,
+                &self.block_graph.leaves(),
+            )?;
+
+            if child_hashes_and_slots.len() == 0 {
+                break;
+            }
+
+            for (child_hash, child_slot) in child_hashes_and_slots {
+                let vote_count = get_vote_count(
+                    &self.block_store,
+                    &attestation_targets[..],
+                    &child_hash,
+                    child_slot,
+                )?;
+
+                if vote_count > head_vote_count {
+                    head_hash = child_hash;
+                    head_vote_count = vote_count;
+                }
+            }
+        }
+
+        Ok(head_hash)
+    }
+/// Get the total number of votes for some given block root.
+///
+/// The vote count is incrememented each time an attestation target votes for a block root.
+fn get_vote_count<T: ClientDB>(
+    block_store: &Arc<BeaconBlockStore<T>>,
+    attestation_targets: &[Hash256],
+    block_root: &Hash256,
+    slot: u64,
+) -> Result<u64, Error> {
+    let mut count = 0;
+    for target in attestation_targets {
+        let (root_at_slot, _) = block_store
+            .block_at_slot(&block_root, slot)?
+            .ok_or(Error::MissingBeaconBlock(*block_root))?;
+        if root_at_slot == *target {
+            count += 1;
+        }
+    }
+    Ok(count)
+}
+
+/// Starting from some `leaf_hashes`, recurse back down each branch until the `root_hash`, adding
+/// each `block_root` and `slot` to a HashSet.
+fn get_child_hashes_and_slots<T: ClientDB>(
+    block_store: &Arc<BeaconBlockStore<T>>,
+    root_hash: &Hash256,
+    leaf_hashes: &HashSet<Hash256>,
+) -> Result<HashSet<(Hash256, u64)>, Error> {
+    let mut hash_set = HashSet::new();
+
+    for leaf_hash in leaf_hashes {
+        let mut current_hash = *leaf_hash;
+
+        loop {
+            if let Some(block_reader) = block_store.get_reader(&current_hash)? {
+                let parent_root = block_reader.parent_root();
+
+                let new_hash = hash_set.insert((current_hash, block_reader.slot()));
+
+                // If the hash just added was already in the set, break the loop.
+                //
+                // In such a case, the present branch has merged with a branch that is already in
+                // the set.
+                if !new_hash {
+                    break;
+                }
+
+                // The branch is exhausted if the parent of this block is the root_hash.
+                if parent_root == *root_hash {
+                    break;
+                }
+
+                current_hash = parent_root.clone();
+            } else {
+                return Err(Error::MissingBeaconBlock(current_hash));
+            }
+        }
+    }
+
+    Ok(hash_set)
+}
+
+impl From<DBError> for Error {
+    fn from(e: DBError) -> Error {
+        Error::DBError(e.message)
+    }
+}
+
+impl From<BeaconBlockAtSlotError> for Error {
+    fn from(e: BeaconBlockAtSlotError) -> Error {
+        match e {
+            BeaconBlockAtSlotError::UnknownBeaconBlock(h) => Error::MissingBeaconBlock(h),
+            BeaconBlockAtSlotError::InvalidBeaconBlock(h) => Error::InvalidBeaconBlock(h),
+            BeaconBlockAtSlotError::DBError(msg) => Error::DBError(msg),
+        }
+    }
+}
+
+impl From<TestingSlotClockError> for Error {
+    fn from(_: TestingSlotClockError) -> Error {
+        unreachable!(); // Testing clock never throws an error.
+    }
+}

eth2/fork_choice/src/lib.rs

@@ -22,3 +22,15 @@ pub mod basic_lmd_ghost;
 pub mod longest_chain;
 pub mod optimised_lmd_ghost;
 pub mod protolambda_lmd_ghost;
+
+/// Fork choice options that are currently implemented.
+pub enum ForkChoice {
+    /// Chooses the longest chain becomes the head. Not for production.
+    LongestChain,
+    /// A simple and highly inefficient implementation of LMD ghost.
+    BasicLMDGhost,
+    /// An optimised version of LMD-GHOST by Vitalik.
+    OptimmisedLMDGhost,
+    /// An optimised version of LMD-GHOST by Protolambda.
+    ProtoLMDGhost,
+}