Math Reference

WAD                         = 10^18
USD_VALUE_DECIMALS          = 9            // USD values are nano-USD
BASIS_POINTS                = 10_000
SECONDS_PER_YEAR            = 31_556_952
BALANCE_TO_BORROW_THRESHOLD = 1.1 × WAD
MIN_INITIAL_SHARES          = 1_000
CLOSE_FACTOR_BPS            = 5_000
MAX_LIQUIDATION_BONUS_BPS   = 500
FULL_LIQUIDATION_DEBT_VALUE = $100         // in nano-USD
WRITE_OFF_COLLATERAL_VALUE  = $1           // in nano-USD

u = total_borrow / (accounted_liquidity + total_borrow)          // WAD, 0 if empty

polynomial    = linear·u + linear·u^32 + jump·u^64
rate_per_sec  = rate_multiplier × polynomial / SECONDS_PER_YEAR

growth       = rate_per_sec × elapsed
interest     = floor((total_borrow × growth + interest_remainder) / WAD)
fee          = floor((interest × reserve_factor_bps + fee_remainder) / 10_000)

total_borrow' = total_borrow + interest
fees'         = fees + fee
index'        = index × total_borrow' / total_borrow

lender_assets  = accounted_liquidity + total_borrow − protocol_fees
available_cash = accounted_liquidity − protocol_fees

supply_shares  = floor(assets × total_supply_shares / lender_assets)    // 1:1 if empty
redeem_assets  = floor(shares × lender_assets / total_supply_shares)

new_shares  = ceil(assets × total_borrow_shares / total_borrow)         // 1:1 if empty
user_debt   = ceil(user_shares × total_borrow / total_borrow_shares)
repay_share = floor(paid × total_borrow_shares / total_borrow)          // all shares if paid ≥ debt

value = amount × price, rescaled by the price exponent and mint decimals to 9 decimals
HF    = Σ collateral_usd / Σ debt_usd                    // unweighted
borrow / withdraw / margin_execute allowed ⇔ debt = 0 or HF > 1.10
liquidatable                               ⇔ debt > 0 and HF ≤ 1.10

max_bonus = 1 + liquidation_bonus_bps / 10_000            // bonus ≤ 5%
bonus     = max_bonus           if HF < 1
          = min(HF, max_bonus)  otherwise

full      = HF ≤ max_bonus  or  debt_usd ≤ $100
max_repay = debt            if full
          = 50% of debt     otherwise

repaid      = min(repay, max_repay)
seize_value = repaid_value × bonus
seized      = seize_value × held / held_value             // capped at held, repaid shrinks to match