From bc22fc862ad309be83aa5c86802e48e6623404f8 Mon Sep 17 00:00:00 2001
From: Trevor Richard <trmidpoint@gmail.com>
Date: Wed, 4 Oct 2023 19:05:45 +0000
Subject: [PATCH] delete old observations to save gas

---
 src/libraries/DrawAccumulatorLib.sol    | 15 +++++++++------
 test/libraries/DrawAccumulatorLib.t.sol | 12 ++++++++++++
 2 files changed, 21 insertions(+), 6 deletions(-)

diff --git a/src/libraries/DrawAccumulatorLib.sol b/src/libraries/DrawAccumulatorLib.sol
index 66152db..c4a9e78 100644
--- a/src/libraries/DrawAccumulatorLib.sol
+++ b/src/libraries/DrawAccumulatorLib.sol
@@ -88,6 +88,15 @@ library DrawAccumulatorLib {
       uint256 remainingAmount = integrateInf(_alpha, relativeDraw, newestObservation_.available);
       uint256 disbursedAmount = integrate(_alpha, 0, relativeDraw, newestObservation_.available);
 
+      uint16 cardinality = ringBufferInfo.cardinality;
+      if (ringBufferInfo.cardinality < MAX_CARDINALITY) {
+        cardinality += 1;
+      } else {
+        // Delete the old observation to save gas (older than 1 year)
+        uint24 overwritingDrawId = accumulator.drawRingBuffer[ringBufferInfo.nextIndex];
+        delete accumulatorObservations[overwritingDrawId];
+      }
+
       accumulator.drawRingBuffer[ringBufferInfo.nextIndex] = _drawId;
       accumulatorObservations[_drawId] = Observation({
         available: SafeCast.toUint96(_amount + remainingAmount),
@@ -99,12 +108,6 @@ library DrawAccumulatorLib {
         )
       });
 
-      uint16 cardinality = ringBufferInfo.cardinality;
-
-      if (ringBufferInfo.cardinality < MAX_CARDINALITY) {
-        cardinality += 1;
-      }
-
       accumulator.ringBufferInfo = RingBufferInfo({
         nextIndex: uint16(RingBufferLib.nextIndex(ringBufferInfo.nextIndex, MAX_CARDINALITY)),
         cardinality: cardinality
diff --git a/test/libraries/DrawAccumulatorLib.t.sol b/test/libraries/DrawAccumulatorLib.t.sol
index ae448e3..1f1a4f5 100644
--- a/test/libraries/DrawAccumulatorLib.t.sol
+++ b/test/libraries/DrawAccumulatorLib.t.sol
@@ -75,6 +75,18 @@ contract DrawAccumulatorLibTest is Test {
     assertEq(accumulator.observations[3].available, 281);
   }
 
+  function testAddOne_deleteExpired() public {
+    // set up accumulator as if we had just completed a buffer loop:
+    accumulator.drawRingBuffer[0] = 101; // set to some draw ID that isn't 0 or the one we'll be writing
+    accumulator.observations[101] = Observation(2, 3);
+
+    DrawAccumulatorLib.add(accumulator, 100, 1, alpha);
+    assertEq(accumulator.ringBufferInfo.cardinality, 1);
+    assertEq(accumulator.ringBufferInfo.nextIndex, 1);
+    assertEq(accumulator.drawRingBuffer[0], 1);
+    assertEq(accumulator.observations[1].available, 100);
+  }
+
   function testGetTotalRemaining() public {
     add(1);