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fix: threshold check in SD59x18 exp2 function #228

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fix: threshold check in SD59x18 exp2 function #228

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4 changes: 4 additions & 0 deletions src/sd59x18/Constants.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -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);
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6 changes: 5 additions & 1 deletion src/sd59x18/Math.sol
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Expand Up @@ -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,
Expand Down Expand Up @@ -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) {
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Unexplained. We need to add an explanatory comment

return ZERO;
}

unchecked {
// Inline the fixed-point multiplication to save gas.
Expand Down Expand Up @@ -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;
}

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10 changes: 5 additions & 5 deletions test/unit/sd59x18/math/exp2/exp2.t.sol
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Expand Up @@ -2,16 +2,14 @@
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";
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has to be ordered alphabetically

import { PRBMath_SD59x18_Exp2_InputTooBig } from "src/sd59x18/Errors.sol";
import { exp2 } from "src/sd59x18/Math.sol";
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;
Expand All @@ -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;
}

Expand All @@ -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 }));
Comment on lines +40 to +42
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smallest value has to be at the top

Comment on lines +40 to +42
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We can use MIN_SD59x18 instead of -sd(2**255-1).

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 }));
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