### Inputs:
1. **Distance to destination** (`dist`): The total distance you need to travel.
2. **Fuel tank capacity** (`miles`): The maximum number of miles your car can travel on a full tank.
3. **Number of gas stations** (`n`): The number of gas stations along the route.
4. **Gas station locations** (`gas_stations`): A list of distances where gas stations are located from your starting point.
### Objective:
Determine the minimum number of refills required to reach the destination. If it’s not possible to reach the destination, return `-1`.
### Solution
1. **Initial Setup:**
- You start with a full tank of gas, so the car can travel `miles` distance before needing a refill.
- The variable `num_refill` keeps track of the total number of refills.
- `curr_refill` tracks your current position at a gas station, and `limit` keeps track of how far you can travel with the current fuel level.
2. **Loop Until the Destination is Reachable:**
- While the destination is beyond your current limit (i.e., `limit < dist`), you need to check whether you can reach the destination or the next gas station.
- If you can't reach the next gas station (or there are no more stations), return `-1` to indicate that the trip is impossible.
3. **Find the Furthest Reachable Gas Station:**
- The algorithm moves to the furthest reachable gas station that is within the current fuel limit.
- It iterates through the list of gas stations and moves `curr_refill` to the furthest station within range.
4. **Refill at the Furthest Station:**
- After finding the furthest gas station within reach, refill the tank, increase the refill count, and update the `limit` to reflect how far you can now travel after refilling.
5. **Continue Until the Destination is Reachable:**
- The loop continues, finding the next furthest reachable station and refilling until you can reach the destination.
6. **Return the Number of Refills:**
- If you reach the destination, return the number of refills made during the journey.
### Step-by-Step Breakdown
1. **Check Feasibility**:
- Start with a full tank, and as long as the destination cannot be reached with the current fuel (`limit < dist`), look for gas stations within range.
2. **Handle Edge Cases**:
- If the gas station is too far away from the current limit or there are no more gas stations within reach, return `-1` to indicate that the destination is unreachable.
3. **Optimize Refills**:
- Always stop at the furthest reachable gas station to minimize the number of refills. This prevents unnecessary stops at closer stations.
### Example Walkthrough:
#### Test Case 1:
- **Input:** `car_fueling(950, 400, 4, [200, 375, 550, 750])`
- **Explanation:**
- Start with a full tank (can travel 400 miles).
- Reach the first station at 200 miles, but continue to the furthest station at 375 miles.
- Refill at 375 miles and continue.
- Reach the next station at 550 miles but continue to the furthest one at 750 miles.
- Refill at 750 miles and then reach the destination at 950 miles.
- **Result:** 2 refills are needed.
#### Test Case 2:
- **Input:** `car_fueling(10, 3, 4, [1, 2, 5, 9])`
- **Explanation:**
- Start with a full tank (can travel 3 miles).
- The first station is at 1 mile, but you cannot reach the next station at 5 miles with the current fuel capacity.
- **Result:** The trip is impossible, so return `-1`.
