From 9c2dfba843c2210df161537f186eb87cb3c18bda Mon Sep 17 00:00:00 2001
From: Michael Sproul <michael@sigmaprime.io>
Date: Wed, 20 Mar 2019 10:47:19 +1100
Subject: [PATCH] Operation pool: prune attestations

---
 eth2/operation_pool/src/lib.rs | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/eth2/operation_pool/src/lib.rs b/eth2/operation_pool/src/lib.rs
index 93ff74652..4932a7e0a 100644
--- a/eth2/operation_pool/src/lib.rs
+++ b/eth2/operation_pool/src/lib.rs
@@ -161,8 +161,18 @@ impl OperationPool {
             .collect()
     }
 
-    pub fn prune_attestations(&self, _finalized_state: &BeaconState, _spec: &ChainSpec) {
-        // TODO
+    /// Remove attestations which are too old to be included in a block.
+    // TODO: we could probably prune other attestations here:
+    // - ones that are completely covered by attestations included in the state
+    // - maybe ones invalidated by the confirmation of one fork over another
+    pub fn prune_attestations(&mut self, finalized_state: &BeaconState, spec: &ChainSpec) {
+        self.attestations.retain(|_, attestations| {
+            // All the attestations in this bucket have the same data, so we only need to
+            // check the first one.
+            attestations.first().map_or(false, |att| {
+                finalized_state.slot < att.data.slot + spec.slots_per_epoch
+            })
+        });
     }
 
     /// Add a deposit to the pool.