From 942ef4b002838842344344d440a701c9a61af378 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Fri, 1 Feb 2019 16:30:42 +1100
Subject: [PATCH] Move `advance_slot` to its own file.

---
 .../beacon_chain/src/canonical_head.rs        | 12 ---------
 beacon_node/beacon_chain/src/lib.rs           |  2 +-
 beacon_node/beacon_chain/src/state.rs         | 25 +++++++++++++++++++
 3 files changed, 26 insertions(+), 13 deletions(-)
 create mode 100644 beacon_node/beacon_chain/src/state.rs

diff --git a/beacon_node/beacon_chain/src/canonical_head.rs b/beacon_node/beacon_chain/src/canonical_head.rs
index 14f1adf81..35262961e 100644
--- a/beacon_node/beacon_chain/src/canonical_head.rs
+++ b/beacon_node/beacon_chain/src/canonical_head.rs
@@ -4,7 +4,6 @@ use types::{beacon_state::SlotProcessingError, BeaconBlock, BeaconState, Hash256
 
 #[derive(Debug, PartialEq)]
 pub enum Error {
-    PastSlot,
     SlotProcessingError(SlotProcessingError),
 }
 
@@ -32,17 +31,6 @@ where
     pub fn head(&self) -> RwLockReadGuard<CheckPoint> {
         self.canonical_head.read()
     }
-
-    pub fn advance_state(&self, slot: u64) -> Result<(), SlotProcessingError> {
-        let state_slot = self.state.read().slot;
-        let head_block_root = self.head().beacon_block_root;
-        for _ in state_slot..slot {
-            self.state
-                .write()
-                .per_slot_processing(head_block_root.clone(), &self.spec)?;
-        }
-        Ok(())
-    }
 }
 
 impl From<SlotProcessingError> for Error {
diff --git a/beacon_node/beacon_chain/src/lib.rs b/beacon_node/beacon_chain/src/lib.rs
index 5c719dddd..314f18dc9 100644
--- a/beacon_node/beacon_chain/src/lib.rs
+++ b/beacon_node/beacon_chain/src/lib.rs
@@ -10,7 +10,7 @@ pub mod dump;
 mod finalized_head;
 mod info;
 mod lmd_ghost;
-// mod state_transition;
+mod state;
 
 use self::attestation_targets::AttestationTargets;
 use self::block_graph::BlockGraph;
diff --git a/beacon_node/beacon_chain/src/state.rs b/beacon_node/beacon_chain/src/state.rs
new file mode 100644
index 000000000..33e50ce35
--- /dev/null
+++ b/beacon_node/beacon_chain/src/state.rs
@@ -0,0 +1,25 @@
+use crate::{BeaconChain, ClientDB, SlotClock};
+use types::beacon_state::SlotProcessingError;
+
+impl<T, U> BeaconChain<T, U>
+where
+    T: ClientDB,
+    U: SlotClock,
+{
+    /// Advance the `self.state` `BeaconState` to the supplied slot.
+    ///
+    /// This will perform per_slot and per_epoch processing as required.
+    ///
+    /// The `previous_block_root` will be set to the root of the current head block (as determined
+    /// by the fork-choice rule).
+    pub fn advance_state(&self, slot: u64) -> Result<(), SlotProcessingError> {
+        let state_slot = self.state.read().slot;
+        let head_block_root = self.head().beacon_block_root;
+        for _ in state_slot..slot {
+            self.state
+                .write()
+                .per_slot_processing(head_block_root.clone(), &self.spec)?;
+        }
+        Ok(())
+    }
+}