From f83d02b394d7cc40e5ce5647c286d061bb2f9599 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Sat, 16 Feb 2019 15:09:43 +1100
Subject: [PATCH] Update previous epoch function

---
 eth2/types/src/beacon_state.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/eth2/types/src/beacon_state.rs b/eth2/types/src/beacon_state.rs
index 6a4033d5c..2e1df45f2 100644
--- a/eth2/types/src/beacon_state.rs
+++ b/eth2/types/src/beacon_state.rs
@@ -203,7 +203,12 @@ impl BeaconState {
     ///
     /// Spec v0.2.0
     pub fn previous_epoch(&self, spec: &ChainSpec) -> Epoch {
-        self.current_epoch(spec).saturating_sub(1_u64)
+        let current_epoch = self.current_epoch(&spec);
+        if current_epoch == spec.genesis_epoch {
+            current_epoch
+        } else {
+            current_epoch - 1
+        }
     }
 
     /// The epoch following `self.current_epoch()`.