From c214bec3446d0a5f8aa7d980d125d8abdff1e441 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Thu, 23 May 2019 16:52:51 +1000
Subject: [PATCH] Add ability to get shuffling from `BeaconState`

---
 eth2/types/src/beacon_state.rs                 | 11 +++++++++++
 eth2/types/src/beacon_state/committee_cache.rs |  9 +++++++++
 2 files changed, 20 insertions(+)

diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index 00b756c1a0..be085c655d 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -343,6 +343,17 @@ impl<T: EthSpec> BeaconState<T> {
         get_active_validator_indices(&self.validator_registry, epoch)
     }
 
+    /// Return the cached active validator indices at some epoch.
+    ///
+    /// Note: the indices are shuffled (i.e., not in ascending order).
+    ///
+    /// Returns an error if that epoch is not cached, or the cache is not initialized.
+    pub fn get_shuffling(&self, relative_epoch: RelativeEpoch) -> Result<&[usize], Error> {
+        let cache = self.cache(relative_epoch)?;
+
+        Ok(cache.shuffling())
+    }
+
     /// Returns the crosslink committees for some slot.
     ///
     /// Note: Utilizes the cache and will fail if the appropriate cache is not initialized.
diff --git a/eth2/types/src/beacon_state/committee_cache.rs b/eth2/types/src/beacon_state/committee_cache.rs
index afefaa6ba9..27374c3391 100644
--- a/eth2/types/src/beacon_state/committee_cache.rs
+++ b/eth2/types/src/beacon_state/committee_cache.rs
@@ -114,6 +114,15 @@ impl CommitteeCache {
         &self.shuffling
     }
 
+    /// Returns the shuffled list of active validator indices for the initialized epoch.
+    ///
+    /// Always returns `&[]` for a non-initialized epoch.
+    ///
+    /// Spec v0.6.1
+    pub fn shuffling(&self) -> &[usize] {
+        &self.shuffling
+    }
+
     /// Return `Some(CrosslinkCommittee)` if the given shard has a committee during the given
     /// `epoch`.
     ///