Fix kink curve utilization accounting

[liobrasil]

Summary

• remove the remaining FlowALPMath dependency from FlowALPInterestRates
• fix KinkCurve utilization to use total supplied semantics instead of double-counting debt in the denominator
• update direct KinkCurve tests and add a regression test covering the live TokenState path

Testing

• flow test cadence/tests/interest_curve_test.cdc
• flow test cadence/tests/kink_curve_utilization_regression_test.cdc
• flow test cadence/tests/update_interest_rate_test.cdc
• flow test cadence/tests/interest_accrual_integration_test.cdc

Credit

• Credit to @UlyanaAndrukhiv for finding the utilization bug and surfacing the mismatch in the current KinkCurve behavior.

[UlyanaAndrukhiv]

Looks good to me 👍 The only thing I wanted to mention is that in Aave formula, when the denominator is 0, utilization is defined as 0, whereas in our implementation it becomes 100%. Just want to pointing this out.

[liobrasil]

Looks good to me 👍 The only thing I wanted to mention is that in Aave formula, when the denominator is 0, utilization is defined as 0, whereas in our implementation it becomes 100%. Just want to pointing this out.

I think the key distinction is what L_t = 0 means in the Aave notation.

In Aave:

• totalDebt = total borrowed amount outstanding
• availableLiquidity = idle liquidity still in the reserve
• L_t corresponds to availableLiquidity + totalDebt, i.e. total reserve liquidity

So if L_t = 0, then both terms must be 0:

• availableLiquidity = 0
• totalDebt = 0

That is the empty-reserve case: no supplied liquidity and no outstanding borrows.

That is different from the fully-utilized case, where:

• availableLiquidity = 0
• totalDebt > 0

In that case, L_t = totalDebt > 0, so utilization is 100%, not undefined.

This is also how the Aave implementation behaves: it only computes borrowUsageRatio when totalDebt != 0, so the empty-reserve case stays at 0 utilization:
https://github.com/aave/aave-v3-core/blob/master/contracts/protocol/pool/DefaultReserveInterestRateStrategy.sol#L169-L177

So our defensive branch is making the same distinction:

• creditBalance = 0, debitBalance = 0 -> empty reserve -> base-rate / 0-utilization behavior
• creditBalance = 0, debitBalance > 0 -> positive debt with no credit-side balance -> saturate to 100%, i.e. effectively full utilization

[UlyanaAndrukhiv]

Thanks @liobrasil, got it! I missed that case is already handled by the early if debitBalance == 0.0 { return baseRate } check 😅