// SPDX-License-Identifier: MIT
pragma solidity ^0.8.30;

import {Math} from "@openzeppelin-contracts-5.6.1/utils/math/Math.sol";

import {Constants} from "./Constants.sol";

/// @title InterestMath
/// @notice Simple (non-compounding) interest accrual. Interest is computed on-the-fly from the loan's
///         stored `outstandingPrincipal`, `rate`, and `lastAccrualTimestamp` — there is no stored
///         `accruedInterest`. The debt at any second is `outstandingPrincipal + accrued(...)`.
/// @dev Rounds up: a borderline position reads as marginally more indebted, which is conservative for
///      the lender both when servicing (interest is paid before principal) and when the same figure
///      feeds the health-factor check at margin-call / liquidation time.
library InterestMath {
    /// @notice Interest accrued on `outstandingPrincipal` at `rate` (annual, bps) over `elapsed`
    ///         seconds, in principal base units.
    ///         `accrued = outstandingPrincipal * rate * elapsed / (BPS * SECONDS_PER_YEAR)`.
    /// @dev `rate` (uint16 bps) and `elapsed` are small enough that `rate * elapsed` cannot overflow;
    ///      `Math.mulDiv` carries the `outstandingPrincipal * (rate * elapsed)` product at full width.
    function accrued(uint256 outstandingPrincipal, uint16 rate, uint256 elapsed) internal pure returns (uint256) {
        return Math.mulDiv(
            outstandingPrincipal,
            uint256(rate) * elapsed,
            Constants.BPS * Constants.SECONDS_PER_YEAR,
            Math.Rounding.Ceil
        );
    }
}