# StableDebtToken

The `StableDebtToken` is a specialized token in the Fathom protocol used to track borrowing positions at a stable interest rate. This token does not inherit from `IERC20` to save on code size and avoids standard ERC20 functionalities such as transfers and approvals, given its non-transferable nature.

#### Events

* **Initialized**: Emitted when the token is initialized.
  * `underlyingAsset`: Address of the underlying asset.
  * `pool`: Address of the associated pool.
  * `incentivesController`: Address of the incentives controller.
  * `debtTokenDecimals`: Decimals of the debt token.
  * `debtTokenName`: Name of the debt token.
  * `debtTokenSymbol`: Symbol of the debt token.
  * `params`: Encoded parameters for additional initialization.
* **Mint**: Emitted when new stable debt is minted.
  * `user`: Address of the user who triggered the minting.
  * `onBehalfOf`: Address receiving the stable debt tokens.
  * `amount`: Amount minted (user-entered amount + balance increase from interest).
  * `currentBalance`: Balance of the user based on the previous balance and balance increase from interest.
  * `balanceIncrease`: Increase in balance since the last action of the user `onBehalfOf`.
  * `newRate`: Rate of the debt after the minting.
  * `avgStableRate`: Next average stable rate after the minting.
  * `newTotalSupply`: Next total supply of the stable debt token after the action.
* **Burn**: Emitted when stable debt is burned.
  * `from`: Address from which the debt will be burned.
  * `amount`: Amount being burned (user-entered amount - balance increase from interest).
  * `currentBalance`: Balance of the user based on the previous balance and balance increase from interest.
  * `balanceIncrease`: Increase in balance since the last action of `from`.
  * `avgStableRate`: Next average stable rate after the burning.
  * `newTotalSupply`: Next total supply of the stable debt token after the action.

#### Functions

**`initialize`**

Initializes the debt token.

* **Parameters**:
  * `pool`: The pool contract that is initializing this contract.
  * `underlyingAsset`: The address of the underlying asset of this debt token.
  * `incentivesController`: The smart contract managing potential incentives distribution.
  * `debtTokenDecimals`: The decimals of the debt token, same as the underlying asset's.
  * `debtTokenName`: The name of the token.
  * `debtTokenSymbol`: The symbol of the token.
  * `params`: Encoded parameters for additional initialization.

**`getAverageStableRate`**

Returns the average rate of all the stable rate loans.

* **Returns**:
  * The average stable rate.

**`getUserLastUpdated`**

Returns the timestamp of the last update of the user.

* **Parameters**:
  * `user`: The address of the user.
* **Returns**:
  * The timestamp.

**`getUserStableRate`**

Returns the stable rate of the user's debt.

* **Parameters**:
  * `user`: The address of the user.
* **Returns**:
  * The stable rate of the user.

**`balanceOf`**

Returns the balance of a user.

* **Parameters**:
  * `account`: The address of the user.
* **Returns**:
  * The balance of the user.

**`mint`**

Mints debt tokens to the `onBehalfOf` address.

* **Parameters**:
  * `user`: The address receiving the borrowed underlying.
  * `onBehalfOf`: The address receiving the debt tokens.
  * `amount`: The amount of debt tokens to mint.
  * `rate`: The rate of the debt being minted.
* **Returns**:
  * `bool`: True if it is the first borrow, false otherwise.
  * `uint256`: The total stable debt.
  * `uint256`: The average stable borrow rate.

**`burn`**

Burns debt of `user`.

* **Parameters**:
  * `from`: The address from which the debt will be burned.
  * `amount`: The amount of debt tokens getting burned.
* **Returns**:
  * `uint256`: The total stable debt.
  * `uint256`: The average stable borrow rate.

**`getSupplyData`**

Returns the principal, the total supply, the average stable rate, and the timestamp for the last update.

* **Returns**:
  * `uint256`: The principal.
  * `uint256`: The total supply.
  * `uint256`: The average stable rate.
  * `uint40`: The timestamp of the last update.

**`getTotalSupplyAndAvgRate`**

Returns the total supply and the average stable rate.

* **Returns**:
  * `uint256`: The total supply.
  * `uint256`: The average rate.

**`totalSupply`**

Returns the total supply of the stable debt tokens.

* **Returns**:
  * The total supply.

**`getTotalSupplyLastUpdated`**

Returns the timestamp of the last update of the total supply.

* **Returns**:
  * The timestamp.

**`principalBalanceOf`**

Returns the principal debt balance of the user.

* **Parameters**:
  * `user`: The address of the user.
* **Returns**:
  * The debt balance of the user since the last burn/mint action.

**`UNDERLYING_ASSET_ADDRESS`**

Returns the address of the underlying asset of this stable debt token.

* **Returns**:
  * The address of the underlying asset.

#### Non-Implemented ERC20 Functions

The following ERC20 functions are not implemented because `StableDebtToken` is non-transferable:

* `transfer`
* `allowance`
* `approve`
* `transferFrom`
* `increaseAllowance`
* `decreaseAllowance`


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