Integrate fork choice into beacon_chain.

- Adds fork_choice to beacon_chain struct.
- Adds add_attestation inside process_free_attestation.
- Adds add_block inside process_block.
- Shifts core fork-choice logic into lib.rs.

eth2/fork_choice/src/lib.rs

@@ -23,8 +23,51 @@ pub mod longest_chain;
 pub mod optimised_lmd_ghost;
 pub mod protolambda_lmd_ghost;
 
+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 {
+    /// 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)
+    }
+}
+
 /// Fork choice options that are currently implemented.
-pub enum ForkChoice {
+pub enum ForkChoiceAlgorithms {
     /// Chooses the longest chain becomes the head. Not for production.
     LongestChain,
     /// A simple and highly inefficient implementation of LMD ghost.