From b0e965a5f770b19003c26595a1cf6d85c94ed762 Mon Sep 17 00:00:00 2001
From: Daniel Beal <git@dbeal.dev>
Date: Thu, 7 Nov 2024 12:04:41 +0900
Subject: [PATCH] ability for cachedDebt to become negative

to avoid subtraction overflow
---
 contracts/BaseDebtCache.sol         |  6 +++---
 contracts/DebtCache.sol             | 15 +++++++--------
 contracts/interfaces/IDebtCache.sol |  4 ++--
 3 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/contracts/BaseDebtCache.sol b/contracts/BaseDebtCache.sol
index 7a5396083a..c1216b1def 100644
--- a/contracts/BaseDebtCache.sol
+++ b/contracts/BaseDebtCache.sol
@@ -26,7 +26,7 @@ contract BaseDebtCache is Owned, MixinSystemSettings, IDebtCache {
     using SafeMath for uint;
     using SafeDecimalMath for uint;
 
-    uint internal _cachedDebt;
+    int internal _cachedDebt;
     mapping(bytes32 => uint) internal _cachedSynthDebt;
     mapping(bytes32 => uint) internal _excludedIssuedDebt;
     uint internal _cacheTimestamp;
@@ -111,7 +111,7 @@ contract BaseDebtCache is Owned, MixinSystemSettings, IDebtCache {
         return getDebtSnapshotStaleTime();
     }
 
-    function cachedDebt() external view returns (uint) {
+    function cachedDebt() external view returns (int) {
         return _cachedDebt;
     }
 
@@ -316,7 +316,7 @@ contract BaseDebtCache is Owned, MixinSystemSettings, IDebtCache {
         external
         view
         returns (
-            uint debt,
+            int debt,
             uint timestamp,
             bool isInvalid,
             bool isStale
diff --git a/contracts/DebtCache.sol b/contracts/DebtCache.sol
index be465a6985..fbcc179bbd 100644
--- a/contracts/DebtCache.sol
+++ b/contracts/DebtCache.sol
@@ -41,10 +41,10 @@ contract DebtCache is BaseDebtCache {
 
         // Subtract out the excluded non-SNX backed debt from our total
         _cachedSynthDebt[EXCLUDED_DEBT_KEY] = excludedDebt;
-        uint newDebt = snxCollateralDebt.floorsub(excludedDebt);
-        _cachedDebt = newDebt;
+        uint newDebt = snxCollateralDebt - excludedDebt;
+        _cachedDebt = int(newDebt);
         _cacheTimestamp = block.timestamp;
-        emit DebtCacheUpdated(newDebt);
+        emit DebtCacheUpdated(int(newDebt));
         emit DebtCacheSnapshotTaken(block.timestamp);
 
         // (in)validate the cache if necessary
@@ -87,11 +87,10 @@ contract DebtCache is BaseDebtCache {
         uint delta = SafeDecimalMath.abs(amount);
         if (amount > 0) {
             _cachedSynthDebt[sUSD] = _cachedSynthDebt[sUSD].add(delta);
-            _cachedDebt = _cachedDebt.add(delta);
         } else {
             _cachedSynthDebt[sUSD] = _cachedSynthDebt[sUSD].sub(delta);
-            _cachedDebt = _cachedDebt.sub(delta);
         }
+        _cachedDebt = _cachedDebt + amount;
 
         emit DebtCacheUpdated(_cachedDebt);
     }
@@ -131,10 +130,10 @@ contract DebtCache is BaseDebtCache {
 
         // Apply the debt update.
         if (cachedSum != currentSum) {
-            uint debt = _cachedDebt;
+            int debt = _cachedDebt;
             // apply the delta between the cachedSum and currentSum
             // add currentSum before sub cachedSum to prevent overflow as cachedSum > debt for large amount of excluded debt
-            debt = debt.add(currentSum).sub(cachedSum);
+            debt = debt + int(currentSum) - int(cachedSum);
             _cachedDebt = debt;
             emit DebtCacheUpdated(debt);
         }
@@ -147,7 +146,7 @@ contract DebtCache is BaseDebtCache {
 
     /* ========== EVENTS ========== */
 
-    event DebtCacheUpdated(uint cachedDebt);
+    event DebtCacheUpdated(int cachedDebt);
     event DebtCacheSnapshotTaken(uint timestamp);
     event DebtCacheValidityChanged(bool indexed isInvalid);
 }
diff --git a/contracts/interfaces/IDebtCache.sol b/contracts/interfaces/IDebtCache.sol
index 1c18056c88..faf0e5cc6f 100644
--- a/contracts/interfaces/IDebtCache.sol
+++ b/contracts/interfaces/IDebtCache.sol
@@ -5,7 +5,7 @@ import "./IIssuer.sol";
 interface IDebtCache {
     // Views
 
-    function cachedDebt() external view returns (uint);
+    function cachedDebt() external view returns (int);
 
     function cachedSynthDebt(bytes32 currencyKey) external view returns (uint);
 
@@ -37,7 +37,7 @@ interface IDebtCache {
         external
         view
         returns (
-            uint debt,
+            int debt,
             uint timestamp,
             bool isInvalid,
             bool isStale