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test: fix uint128 overflow in IndexingMath round-trip fuzz #57

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test: fix uint128 overflow in IndexingMath round-trip fuzz #57

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11 changes: 10 additions & 1 deletion test/IndexingMath.t.sol
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Original file line number Diff line number Diff line change
Expand Up @@ -311,7 +311,9 @@ contract IndexingMathTests is Test {

// NOTE: Rounding up twice can inflate the principal by up to `ceil(EXP_SCALED_ONE / index)`, so reserve the
// full worst-case headroom for the current `index` before the round trip.
uint112 maxRoundTripInflation_ = uint112((_EXP_SCALED_ONE + index - 1) / index);
// NOTE: Widen to uint256 before adding, since `_EXP_SCALED_ONE + index` overflows a uint128 for indexes
// within 1e12 of `type(uint128).max`.
uint112 maxRoundTripInflation_ = uint112((uint256(_EXP_SCALED_ONE) + index - 1) / index);
uint112 boundedPrincipal_ = uint112(bound(principal, 0, type(uint112).max - maxRoundTripInflation_));

// Rounding the present amount up and back up can never deflate the principal.
Expand All @@ -323,6 +325,13 @@ contract IndexingMathTests is Test {
);
}

/// @dev Pins the round trip at the top of the index range, where the worst-case inflation headroom is computed.
/// Regression: the headroom used to be derived in uint128 arithmetic, which overflowed here.
function test_roundTrip_atMaxIndex() external view {
this.testFuzz_roundTrip(1, type(uint128).max);
this.testFuzz_roundTrip(type(uint112).max, type(uint128).max);
}

function testFuzz_getSafePrincipalAmountRoundedUp(uint112 principal, uint128 index, uint112 maxPrincipalAmount)
external
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