Interest Rate Boundary Conditions Testing

[UlyanaAndrukhiv]

Context

Tests cover normal utilization ranges, need extreme scenarios.

Missing Test Coverage:

• Extreme Utilization

  • Utilization = 99.9% (nearly all liquidity borrowed)
  • Interest rate at maximum cap (400%)
  • KinkCurve steep slope behavior
  • Borrowing cost discourages further borrowing

• Zero Balance Edge Cases

  • totalCreditBalance = 0 (no liquidity)
  • Attempting to borrow should fail
  • Interest rate calculation with zero denominator
  • New deposit enables borrowing

• Empty Pool

  • totalDebitBalance = 0 (no borrows)
  • Credit interest rate = 0
  • First borrow triggers rate update
  • Utilization jumps from 0% to X%

• Kink Point Transitions

  • Utilization exactly at kink (e.g., 80.0%)
  • Rate calculation uses slope1 or slope2?
  • Smooth transition validation
  • No discontinuities or jumps

• Maximum Rate Enforcement

  • Interest curve returns 450% (exceeds cap)
  • Protocol enforces 400% maximum
  • Validation in postcondition
  • No overflow or panic

• Long Time Period Accrual

  • Interest accrues for 1 year (31,557,600 seconds)
  • Interest accrues for 10 years
  • Compound interest doesn't overflow
  • UFix128 precision maintained

• Time Jump Scenarios

  • Blockchain halts for 1 day
  • First transaction after restart
  • Interest accrual for large time delta
  • No overflow in dt calculation

Recommended Tests:

Test: Utilization 99.9%, validate interest rate
Test: totalCreditBalance = 0, borrow attempt fails
Test: totalDebitBalance = 0, credit rate = 0
Test: Utilization exactly 80.0% (kink point)
Test: Interest curve returns 450%, enforced to 400%
Test: Accrue interest for 10 years, no overflow
Test: Time jump 1 day, interest accrual correct