modify interest model

Author: keithsue

x/lending/keeper/loan.go

@@ -1,8 +1,6 @@
 package keeper
 
 import (
-	"time"
-
 	sdkmath "cosmossdk.io/math"
 	storetypes "cosmossdk.io/store/types"
 	sdk "github.com/cosmos/cosmos-sdk/types"
@@ -216,10 +214,11 @@ func (k Keeper) GetCurrentInterest(ctx sdk.Context, loan *types.Loan) sdk.Coin {
 
 	case types.LoanStatus_Liquidated:
 		liquidation := k.liquidationKeeper.GetLiquidation(ctx, loan.LiquidationId)
-		interest = types.GetInterest(loan.Interest, time.Duration(loan.Maturity), loan.CreateAt.Unix(), liquidation.LiquidatedTime.Unix())
+		interest = liquidation.DebtAmount.Amount.Sub(loan.BorrowAmount.Amount)
 
 	default:
-		interest = types.GetInterest(loan.Interest, time.Duration(loan.Maturity), loan.CreateAt.Unix(), ctx.BlockTime().Unix())
+		tranche, _ := types.GetTranche(k.GetPool(ctx, loan.PoolId).Tranches, loan.Maturity)
+		interest = types.GetInterest(loan.BorrowAmount.Amount, loan.StartBorrowIndex, tranche.BorrowIndex)
 	}
 
 	return sdk.NewCoin(loan.BorrowAmount.Denom, interest)

x/lending/keeper/msg_server_loan.go

@@ -105,7 +105,7 @@ func (m msgServer) Apply(goCtx context.Context, msg *types.MsgApply) (*types.Msg
 	repaymentEvent.Outcomes = []string{vault}
 	m.dlcKeeper.SetEvent(ctx, repaymentEvent)
 
-	interest := msg.BorrowAmount.Amount.Mul(sdkmath.NewInt(int64(trancheConfig.BorrowAPR))).Mul(sdkmath.NewInt(trancheConfig.Maturity)).Quo(sdkmath.NewInt(int64(types.OneYear))).Quo(types.Permille)
+	interest := types.GetTotalInterest(msg.BorrowAmount.Amount, tranche.BorrowIndex, trancheConfig.Maturity, trancheConfig.BorrowAPR, m.GetBlocksPerYear(ctx))
 	protocolFee := interest.Mul(sdkmath.NewInt(int64(poolConfig.ReserveFactor))).Quo(types.Permille)
 
 	loan := &types.Loan{

x/lending/types/lending.go

@@ -26,11 +26,21 @@ func GetExchangeRate(totalAvailable sdkmath.Int, totalBorrowed sdkmath.Int, tota
 	return sdkmath.LegacyNewDecFromInt(totalAvailable.Add(totalBorrowed)).Quo(totalSTokens.ToLegacyDec())
 }
 
-// GetInterest calculates the loan interest based on the given params
-func GetInterest(totalInterest sdkmath.Int, maturity time.Duration, startTime int64, destTime int64) sdkmath.Int {
-	elapsed := destTime - startTime
+// GetInterest calculates the loan interest based on the given borrow index
+func GetInterest(borrowAmount sdkmath.Int, startBorrowIndex sdkmath.LegacyDec, borrowIndex sdkmath.LegacyDec) sdkmath.Int {
+	return borrowAmount.ToLegacyDec().Mul(borrowIndex).Quo(startBorrowIndex).TruncateInt().Sub(borrowAmount)
+}
+
+// GetTotalInterest calculates the total loan interest based on the given params
+func GetTotalInterest(borrowAmount sdkmath.Int, startBorrowIndex sdkmath.LegacyDec, maturity int64, borrowAPR uint32, blocksPerYear uint64) sdkmath.Int {
+	totalBlocks := uint64(maturity) * blocksPerYear / OneYear
+
+	borrowRatePerBlock := sdkmath.LegacyNewDec(int64(borrowAPR)).Quo(sdkmath.LegacyNewDec(1000)).Quo(sdkmath.LegacyNewDec(int64(blocksPerYear)))
+	borrowIndexRatio := sdkmath.LegacyOneDec().Add(borrowRatePerBlock)
+
+	endBorrowIndex := startBorrowIndex.Mul(borrowIndexRatio.Power(totalBlocks))
 
-	return totalInterest.Mul(sdkmath.NewInt(elapsed)).Quo(sdkmath.NewInt(int64(maturity)))
+	return GetInterest(borrowAmount, startBorrowIndex, endBorrowIndex)
 }
 
 // GetLiquidationPrice calculates the liquidation price according to the liquidation LTV