From b71077b2f6c107d9c7551257319317f4bbd7dbc7 Mon Sep 17 00:00:00 2001
From: Your Name <philip.tuomey@tribal.credit>
Date: Thu, 2 May 2024 15:13:51 +0100
Subject: [PATCH] fix: threshold check in SD59x18 exp2 function

---
 src/sd59x18/Constants.sol              |  4 ++++
 src/sd59x18/Math.sol                   |  6 +++++-
 test/unit/sd59x18/math/exp2/exp2.t.sol | 10 +++++-----
 3 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/sd59x18/Constants.sol b/src/sd59x18/Constants.sol
index 94d4603b..6d599ff9 100644
--- a/src/sd59x18/Constants.sol
+++ b/src/sd59x18/Constants.sol
@@ -16,6 +16,10 @@ SD59x18 constant EXP_MAX_INPUT = SD59x18.wrap(uEXP_MAX_INPUT);
 int256 constant uEXP2_MAX_INPUT = 192e18 - 1;
 SD59x18 constant EXP2_MAX_INPUT = SD59x18.wrap(uEXP2_MAX_INPUT);
 
+/// @dev Any value less than this will return 0 in {exp2}.
+int256 constant uEXP2_MIN_THRESHOLD = -59_794705707972522261;
+SD59x18 constant EXP2_MIN_THRESHOLD = SD59x18.wrap(uEXP2_MIN_THRESHOLD);
+
 /// @dev Half the UNIT number.
 int256 constant uHALF_UNIT = 0.5e18;
 SD59x18 constant HALF_UNIT = SD59x18.wrap(uHALF_UNIT);
diff --git a/src/sd59x18/Math.sol b/src/sd59x18/Math.sol
index 3fd3fa7b..978934eb 100644
--- a/src/sd59x18/Math.sol
+++ b/src/sd59x18/Math.sol
@@ -6,6 +6,7 @@ import "./Errors.sol" as Errors;
 import {
     uEXP_MAX_INPUT,
     uEXP2_MAX_INPUT,
+    uEXP2_MIN_THRESHOLD,
     uHALF_UNIT,
     uLOG2_10,
     uLOG2_E,
@@ -172,6 +173,9 @@ function exp(SD59x18 x) pure returns (SD59x18 result) {
     if (xInt > uEXP_MAX_INPUT) {
         revert Errors.PRBMath_SD59x18_Exp_InputTooBig(x);
     }
+    if (xInt < uEXP2_MIN_THRESHOLD) {
+        return ZERO;
+    }
 
     unchecked {
         // Inline the fixed-point multiplication to save gas.
@@ -202,7 +206,7 @@ function exp2(SD59x18 x) pure returns (SD59x18 result) {
     int256 xInt = x.unwrap();
     if (xInt < 0) {
         // The inverse of any number less than this is truncated to zero.
-        if (xInt < -59_794705707972522261) {
+        if (xInt < uEXP2_MIN_THRESHOLD) {
             return ZERO;
         }
 
diff --git a/test/unit/sd59x18/math/exp2/exp2.t.sol b/test/unit/sd59x18/math/exp2/exp2.t.sol
index a79450c0..afaa2ab4 100644
--- a/test/unit/sd59x18/math/exp2/exp2.t.sol
+++ b/test/unit/sd59x18/math/exp2/exp2.t.sol
@@ -2,7 +2,7 @@
 pragma solidity >=0.8.19 <0.9.0;
 
 import { sd } from "src/sd59x18/Casting.sol";
-import { E, EXP2_MAX_INPUT, MIN_SD59x18, MIN_WHOLE_SD59x18, PI, UNIT, ZERO } from "src/sd59x18/Constants.sol";
+import { E, EXP2_MAX_INPUT, MIN_SD59x18, MIN_WHOLE_SD59x18, PI, UNIT, ZERO, uEXP2_MIN_THRESHOLD, EXP2_MIN_THRESHOLD } from "src/sd59x18/Constants.sol";
 import { PRBMath_SD59x18_Exp2_InputTooBig } from "src/sd59x18/Errors.sol";
 import { exp2 } from "src/sd59x18/Math.sol";
 import { SD59x18 } from "src/sd59x18/ValueType.sol";
@@ -10,8 +10,6 @@ import { SD59x18 } from "src/sd59x18/ValueType.sol";
 import { SD59x18_Unit_Test } from "../../SD59x18.t.sol";
 
 contract Exp2_Unit_Test is SD59x18_Unit_Test {
-    /// @dev Any input smaller than this makes the result zero.
-    SD59x18 internal constant THRESHOLD = SD59x18.wrap(-59_794705707972522261);
 
     function test_Exp2_Zero() external pure {
         SD59x18 x = ZERO;
@@ -28,7 +26,7 @@ contract Exp2_Unit_Test is SD59x18_Unit_Test {
         delete sets;
         sets.push(set({ x: MIN_SD59x18, expected: 0 }));
         sets.push(set({ x: MIN_WHOLE_SD59x18, expected: 0 }));
-        sets.push(set({ x: THRESHOLD - sd(1), expected: 0 }));
+        sets.push(set({ x: EXP2_MIN_THRESHOLD - sd(1), expected: 0 }));
         return sets;
     }
 
@@ -39,7 +37,9 @@ contract Exp2_Unit_Test is SD59x18_Unit_Test {
 
     function negativeAndGteMinPermitted_Sets() internal returns (Set[] memory) {
         delete sets;
-        sets.push(set({ x: THRESHOLD, expected: 0.000000000000000001e18 }));
+        sets.push(set({ x: EXP2_MIN_THRESHOLD, expected: 0.000000000000000001e18 }));
+        sets.push(set({ x: EXP2_MIN_THRESHOLD - sd(1), expected: 0 }));
+        sets.push(set({ x: -sd(2**255-1), expected: 0 }));
         sets.push(set({ x: -33.333333e18, expected: 0.000000000092398923e18 }));
         sets.push(set({ x: -20.82e18, expected: 0.000000540201132438e18 }));
         sets.push(set({ x: -16e18, expected: 0.0000152587890625e18 }));