# Mint debt token
## Overview
* [x] cache
* [x] updateState
* [x] getIsolationMode
* [x] validateBorrow
* [ ] mint debt token
* [ ] setBorrowing
* [ ] update IsolationMode debt
* [ ] updateInterestRates
* [ ] transfer underlying to user
* If stable interest was selected -> mint stable debt tokens
* Else -> mint variable debt tokens
## mint stableDebtTokens
{% hint style="info" %}
rate: reserve.currentStableBorrowRate
{% endhint %}
### \_calculateBalanceIncrease
* For a user, calculates the increase in stable debt, due to interest accrued from timestamp of their last update, to current time.
* Serves to update user's prior stable debt
{% hint style="info" %}
`balanceIncrease` reflects interest accrued from lastUpdate till current block.timestamp.
Does not relate to incoming borrow action.
{% endhint %}
### totalSupply
```solidity
vars.previousSupply = totalSupply();
vars.currentAvgStableRate = _avgStableRate;
vars.nextSupply = _totalSupply = vars.previousSupply + amount;
```
#### vars.previousSupply = totalSupply();
`totalSupply` returns the updated supply value, by accounting for recently accrued interest, from `_totalSupplyTimestamp` till now, based on the `_avgStableRate`.
* `_avgStableRate`: internal storage variable on StableDebtToken.sol
* `previousSupply`: updated supply value
* `principalSupply` => `super.totalSupply` calls `totalSupply` on `IncentivizedERC20` which returns `_totalSupply`
{% hint style="info" %}
`previousSupply` is set to `totalSupply()` instead of `_totalSupply` directly, so that the updated supply value can be obtained.
{% endhint %}
With recent interest accounted for, the storage variable `_totalSupply` is updated with the incoming stable debt, `amount`.
```solidity
vars.nextSupply = _totalSupply =
vars.previousSupply + amount;
```
### Calculate new stable rate
* `nextStableRate` is new stable rate, specific to the user
{% code overflow="wrap" %}
```solidity
nextStableRate =
(_avgStableRate * currentBalance) + (amount * rate / currentBalance + amount)
```
{% endcode %}
* `nextStableRate` stored in the user's `UserState.additionalData` struct. `userState` mapping collects all users' `UserState` structs.
* Store update time in `_timestamps`.`_timestamps` mapping collects the last update timestamp for all users.
* Update the timestamp of the last update of `_totalSupply.`
{% hint style="info" %}
rate: `reserve.currentStableBorrowRate`
{% endhint %}
### Update average Stable rate
* `_avgStableRate`: internal uint128 on StableDebtToken.sol
* `vars.currentAvgStableRate` was set to `_avgStableRate` at the start of `mint`
{% code overflow="wrap" %}
```solidity
newAvgStableRate =
(_avgStableRate * previousSupply) + [(rate * amountInRay) / nextSupply]
```
{% endcode %}
{% hint style="info" %}
* previousSupply: updated`totalSupply`
{% endhint %}
### mint tokens
Notice that we do not just mint as per the incoming stable debt amount; but also mint for `balanceIncrease`. This serves to update user's prior stable debt based on interest accrued over time elapsed since last update.
* Since we going to mint, mint to update the user's entire position
`mint` calls the internal function `_mint`:
## mint variableDebtTokens
* `amountScaled`: scale the borrow amount against variableBorrowIndex
* See variableDebtToken [mint](https://calnix.gitbook.io/aave-book/contracts/variabledebttoken#mint)
* If stable interest was selected -> mint stable debt tokens
* Else -> mint variable debt tokens
## mint stableDebtTokens
