# Coupon Token

{% hint style="info" %}
**Source:** `src/fira_bonding/core/YieldContracts/CouponToken.sol`, `src/fira_bonding/core/YieldContracts/InterestManagerCT.sol`
{% endhint %}

## Contract summary

CT extends `InterestManagerCT` and `FiraERC20`. Like BT, the FW, BT, factory, expiry, and `doCacheIndexSameBlock` fields are all immutable. All external mutating functions use `nonReentrant`.

## Minting BT+CT

`mintBC(receiverBT, receiverCT)` takes FW that has been pre-transferred to the CT contract, converts the FW amount to underlying asset units via the BC index, and mints equal BT and CT. The BC index is `max(FW.exchangeRate(), storedIndex)` — monotonically non-decreasing.

`mintBCMulti` does the same for multiple receivers in one call.

## Redeeming BT+CT

`redeemBC(receiver)` burns BT from `address(this)` (must be pre-transferred) and burns CT if not expired. FW is returned based on the current BC index.

**Post-expiry:** The redeemable FW amount is calculated at the *current* BC index, but the excess between the post-expiry first index and the current index accrues as treasury interest. Post-expiry yield goes to the protocol, not to redeemers.

## Interest accounting

`InterestManagerCT` tracks per-user interest via `UserInterest { index, accrued }`. On every CT transfer (`_beforeTokenTransfer`), interest is distributed to both sender and receiver:

```
interest = principal × (currentIndex - prevIndex) / (prevIndex × currentIndex)
```

Claims go through `_doTransferOutInterest`, which deducts `interestFeeRate` and sends the fee to treasury.

### Key design decisions

* Interest is based on CT balance, not FW balance
* Protocol fees are taken at claim time, not at accrual time
* `_distributeInterestForTwo` skips `address(0)` and `address(this)` to avoid phantom accounting

## BC index caching

If `doCacheIndexSameBlock` is true, the BC index is only updated once per block. This prevents sandwich attacks where the FW exchange rate is manipulated within a block to extract value from CT holders.

## Post-expiry state

`_setPostExpiryData()` is called on the first interaction after expiry. It snapshots the BC index and reward indices. After this, the interest index is frozen to `postExpiry.firstBCIndex`, and all subsequent yield flows to treasury.

`redeemInterestAndRewardsPostExpiryForTreasury()` sweeps all post-expiry interest to the treasury.

## Inherited contracts

* `InterestManagerCT` — Per-user interest tracking
* `FiraERC20` — ERC-20 with reentrancy guard


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