# Borrow Fixed Rate

```mermaid
sequenceDiagram
  actor User
  participant Router as Router (ActionBorrow)
  participant FRLM as Fixed Rate LendingMarket (FRLM)
  participant FM as FiraMarket (BT/FW/CT)
  participant FW as FW (StandardizedYield)

  Note over User,Router: User approves collateralToken to Router (ERC20 approve)
  User->>Router: supplyAndBorrowSingleToken(marketParams, FRLM, FM, collateral, btToBorrow, output, limit, receiver)
  Router->>Router: transferFrom(user, collateralToken, collateral)
  Router->>FRLM: approve(collateralToken, collateral)
  Router->>FRLM: supplyCollateral(marketParams, collateral, user, "")
  FRLM-->>Router: position updated (collateral credited to user)
  Router->>FRLM: borrow(marketParams, btToBorrow, 0, user, Router)
  FRLM-->>Router: transfer BT to Router (loanToken = BT, debt on user)
  Router->>FM: transfer BT to FM, swapExactBtForFw(Router, btBorrowed, "")
  FM-->>Router: FW out to Router
  Router->>FW: transfer FW to FW contract
  Router->>FW: redeem(Router, fwOut, tokenRedeemFw, 0, true)
  FW-->>Router: tokenOut (e.g., USDC)
  Router-->>User: transfer tokenOut to receiver
```

**Also available**: `borrowSingleToken` — same flow but skips the collateral supply step (assumes collateral already supplied).

**Key references**: `src/fira_bonding/router/ActionBorrow.sol::supplyAndBorrowSingleToken`, `borrowSingleToken`.

## Details

* **Goal**: Supply collateral and receive a target token in one transaction (via BT→FW→token path).
* **Preconditions**:
  * User approved `collateralToken` to the router.
  * `tokensToBorrow > 0`, `collateralAmount > 0`, `receiver != 0x0`.
  * Market is initialized; `firaMarket` and `lendingMarket` set.
  * User must have authorized the router on the LendingMarket (`setAuthorization(router, true)`).
* **Parameters** (high level):
  * `marketParams` (Fira Lending), `collateralAmount`, `tokensToBorrow` (BT),
  * `output` (TokenOutput: `tokenOut`, `tokenRedeemFw`, `minTokenOut`, `firaSwap`, `swapData`),
  * `limitOrderData` (kept for interface compatibility, orderbook not used in current version).

## Views and events

**Views**

* Lending: `getUserPosition(marketParams, user)` → `supplyAssets`, `supplyShares`, `borrowAssets`, `borrowShares`, `collateralAssets`.
* Lending: `position(id, user)`, `market(id)`.
* Market (fixed-rate): `readTokens()`, `isExpired()`.

**Events**

* Lending: `EventsLib.SupplyCollateral`, `EventsLib.Borrow`.
* Market: `Swap` (BT→FW), `UpdateImpliedRate`.
* FW: `Deposit` (tokenIn→FW), `Redeem` (FW→tokenOut).

## Reward share (fixed-rate borrow)

* Suggested formula: `userBorrowBT / totalBorrowBT` over the epoch (TWAP).
* Functions to call:
  * `LendingMarket.getUserPosition(marketParamsBT, user)` → `borrowAssets` (userBorrowBT)
  * `LendingMarket.market(idBT).totalBorrowAssets` → `totalBorrowBT` (consider `accrueInterest(marketParamsBT)` before reading)
  * `FiraMarket.readTokens()` → BT address; `FiraMarket.isExpired()`
* Events to index: `EventsLib.Borrow`, `FiraMarket.Swap` (BT→FW), `IFiraWrappedStandardized.Redeem`