How are funding rates calculated in GRVT?

The following steps are followed to determine funding rate:

Fair Impact Bid/Ask
- Calculate average fill prices for a set notional amount on both the bid and ask sides.
Fair Price
- Take the median of:
  1. Average of Fair Impact Bid/Ask
  2. Average of best bid/ask
  3. Last traded price
Index Price
- A weighted average from several external exchanges.
Premium
- Computed every second:
```
Premium = Fair_Price – Index_Price
```
Premium Rate
- The median of (Premium / Index_Price) over the past minute.
- Updated every minute.
Capping the Rate
- Limited to ±1% (configurable) to prevent extreme funding swings.
Funding Index
- Tracks cumulative funding changes, starting at 0.
- Updated every minute:
```
Funding_Index += Capped_Premium_Rate * (Index_Price) / (8 * 60)
```
Funding Payment
- Triggered whenever a position changes size (due to trades, transfers, deposits, etc.).
- If your position size was Last_Position_Size and the Funding Index is F, then:
```
Funding_Payment = -1 × (Last_Position_Size) × (F - Funding_Index_Start)
```
- Afterward, Funding_Index_Start is updated to the current Funding Index if you still have a position, or reset to 0 if you fully closed it.