# DSR Manager - Detailed Documentation

The `DsrManager` provides an easy to use smart contract that allows service providers to deposit/withdraw dai into the DSR contract [pot](https://docs.makerdao.com/smart-contract-modules/rates-module/pot-detailed-documentation), and activate/deactivate the Dai Savings Rate to start earning savings on a pool of dai in a single function call. To understand the DsrManager, it is necessary to have an understanding of the [pot](https://docs.makerdao.com/smart-contract-modules/rates-module/pot-detailed-documentation) first. The DSR is set by Maker Governance, and will typically be less than the base stability fee to remain sustainable. The purpose of DSR is to offer another incentive for holding Dai.

## Deployment Details

* Mainnet: [0x373238337Bfe1146fb49989fc222523f83081dDb](https://etherscan.io/address/0x373238337Bfe1146fb49989fc222523f83081dDb#code)
* Kovan: [0x7f5d60432DE4840a3E7AE7218f7D6b7A2412683a](https://kovan.etherscan.io/address/0x7f5d60432DE4840a3E7AE7218f7D6b7A2412683a#code)
* Ropsten: [0x74ddba71e98d26ceb071a7f3287260eda8daa045](https://ropsten.etherscan.io/address/0x74ddba71e98d26ceb071a7f3287260eda8daa045#code)

## Contract Details

### Math

* `wad` - some quantity of tokens, as a fixed point integer with 18 decimal places.
* `ray` - a fixed point integer, with 27 decimal places.
* `rad` - a fixed point integer, with 45 decimal places.
* `mul(uint, uint)`, `rmul(uint, uint)`, `add(uint, uint)` & `sub(uint, uint)` - will revert on overflow or underflow
* `Rdiv` - Divide two `ray`s and return a new `ray`. Always rounds down. A `ray` is a decimal number with 27 digits of precision that is being represented as an integer.
* `Rdivup` - Divide two `ray`s and return a new `ray`. Always rounds up. A `ray` is a decimal number with 27 digits of precision that is being represented as an integer.

### Storage

* `pot` - stores the contract address of the main Dai Savings Rate contract `pot`.
* `dai` - stores the contract address of dai.
* `daiJoin` - stores the contract address of the Dai token adapter.
* `supply` - the supply of Dai in the DsrManager.
* `pieOf` - `mapping (addresses=>uint256)` mapping of user addresses and normalized Dai balances (`amount of dai / chi`) deposited into `pot`.
* `pie` - stores the address' `pot` balance.
* `chi` - the rate accumulator. This is the always increasing value which decides how much dai is given when `drip()` is called.
* `vat` - an address that conforms to a `VatLike` interface.
* `rho` - the last time that `drip` is called.

## Functions and mechanics

### daiBalance(address usr) returns (uint wad)

* Calculates and returns the Dai balance of the specified address usr in the DsrManager contract. (Existing Dai balance + accrued dsr)

### join(address dst, uint wad)

* `uint wad` this parameter specifies the amount of Dai that you want to join to the pot. The `wad` amount of Dai must be present in the account of `msg.sender`.
* address `dst` specifies a destination address for the deposited dai in the pot. Allows a hot wallet address (`msg.sender`) to deposit dai into the pot and transfer ownership of that dai to a cold wallet (or any other address for that matter)
* The normalized balance `pie` is calculated by dividing wad with the rate acumulator `chi`.
* the `dst`'s `pieOf` amount is updated to include the `pie`.
* The total supply amount is also updated by adding the `pie`.
* `wad` amount of dai is transferred to the DsrManager contract
* The DsrManager contract joins `wad` amount of dai into the MCD system through the dai token adapter `daiJoin`.
* The DsrManager contract `join`s `pie` amount of dai to the `pot`.

### exit(address dst, uint wad)

* `exit()` essentially functions as the exact opposite of `join()`.
* `uint wad` this parameter is based on the amount of dai that you want to `exit` the `pot`.
* address `dst` specifies a destination address for the retrieved dai from the `pot`. Allows a cold wallet address (`msg.sender`) to retrieve dai from the `pot` and transfer ownership of that dai to a hot wallet (or any other address for that matter)
* The normalized balance `pie` is calculated by dividing wad with the rate acumulator `chi`.
* The `msg.sender`’s `pieOf` amount is updated by subtracting the `pie`.
* The total supply amount is also updated by subtracting the `pie`.
* The contract calls exit on the `pot` contract.
* It calculates the amount of dai to retrieve by multiplying `pie` with `chi`.
* Then exits the dai from the dai token adapter `daiJoin` to the destination address `dst`.

### exitAll(address dst)

* `exitAll()` functions like the `exit` function, except it simply looks into the mapping `pieOf`, to determine how much dai the `msg.sender` has, and `exit`s the entire amount of dai, instead of a specified amount.

## Gotchas / Integration Concerns

* In order to use the `join` function, you need to `approve` the contract to transfer Dai from your wallet. You need to call `approve` on the Dai token, specifying the `DsrManager` contract and the amount that the contract should be able to pull (can be set to `-1`, if you want to set an unlimited approval)