# ExpTokenBasedFormula

[Git Source](https://github.com/dustinstacy/boncurs/blob/7928cae257b46ede89b50d06eaae18601fcd0340/contracts/exponential/utils/ExpTokenBasedFormula.sol)

This contract provides functions for calculating the purchase and sale return values on an exponential token-based curve using a scaling factor. The formula converts a desired token amount into a reserve token cost and the sale of a token amount into a reserve token return.

## State Variables

### MAX\_SCALE

*Max reserve ratio in basis points*

```solidity
uint32 private constant MAX_SCALE = 1000000;
```

### WAD

*Wei as decimal precision for calculations*

```solidity
uint256 private constant WAD = 10 ** 18;
```

## Functions

### \_calculateExpTokenBasedPurchaseCost

Returns the purchase cost for a given conversion (in the reserve token) for a desired amount (in the main token).

```solidity
function _calculateExpTokenBasedPurchaseCost(uint256 supply, uint256 initialCost, uint32 scalingFactor, uint256 amount)
    internal
    pure
    returns (uint256 purchaseCost);
```

**Parameters**

| Name            | Type      | Description                                          |
| --------------- | --------- | ---------------------------------------------------- |
| `supply`        | `uint256` | token total supply                                   |
| `initialCost`   | `uint256` | initial cost of the token                            |
| `scalingFactor` | `uint32`  | scaling factor, represented in basis points, 1-10000 |
| `amount`        | `uint256` | amount of tokens to purchase                         |

**Returns**

| Name           | Type      | Description            |
| -------------- | --------- | ---------------------- |
| `purchaseCost` | `uint256` | cost of the conversion |

### \_calculateExpTokenBasedSaleReturn

Returns the sale value for a given amount (in the main token) as a conversion into the reserve token.

```solidity
function _calculateExpTokenBasedSaleReturn(uint256 supply, uint256 initialCost, uint32 scalingFactor, uint256 amount)
    internal
    pure
    returns (uint256 saleReturn);
```

**Parameters**

| Name            | Type      | Description                                          |
| --------------- | --------- | ---------------------------------------------------- |
| `supply`        | `uint256` | token total supply                                   |
| `initialCost`   | `uint256` | initial cost of the token                            |
| `scalingFactor` | `uint32`  | scaling factor, represented in basis points, 1-10000 |
| `amount`        | `uint256` | amount of tokens to sell                             |

**Returns**

| Name         | Type      | Description              |
| ------------ | --------- | ------------------------ |
| `saleReturn` | `uint256` | return of the conversion |

## Errors

### ExpTokenBasedFormula\_\_InvalidInput

```solidity
error ExpTokenBasedFormula__InvalidInput();
```