StabilityPool: Deposits and Compounded Balance

Key takeaway: The Stability Pool is the protocol's first line of defense against undercollateralized Troves. Depositors lock LUSD here; in exchange, they receive a share of every liquidated Trove's collateral. The accounting innovation: deposits are tracked with a multiplicative running product P and a per-scale sum scaleToSum. Each depositor's storage is a single snapshot taken at deposit time. Their current compounded balance is derived in O(1) as initialValue × P_current / P_snapshot. Every liquidation updates P (multiplied by 1 - lossFraction) in a single write, and every depositor's balance shrinks proportionally without ever being touched. This is the cleanest example of scalable per-share reward distribution in DeFi.

What You Are Building

In this section you build the deposit side of the Stability Pool: provideToSP, withdrawFromSP, getCompoundedLUSDDeposit, and the storage that supports them. The ETH gain accounting (getDepositorETHGain, _updateRewardSumAndProduct, and the liquidation offset callback) lives in section 10.

For section 9, the ETH gain helper is stubbed to return 0, and P stays at its initial value 1e18 because no liquidations have happened yet. Even so, the deposit math is correct — compounded = initial × P / snapshot.P = initial × 1e18 / 1e18 = initial. Section 10 turns on the P decay and proves the formula works under liquidations.