Geographic Advertiser Access Control - Region-Based Permission Filtering

Question

## Round 1 - Coding

## Problem

Advertisers are restricted to serve ads only in certain geographic regions. Each region is defined as a rectangle on a 2D coordinate grid. Given a user's location and a list of advertisers with their allowed regions, return the IDs of advertisers eligible to serve an ad to that user.

```python
def eligible_advertisers(
    user_location: tuple[float, float],
    advertisers: list[dict]
) -> list[int]:
    # advertisers[i] = {
    #   "id": int,
    #   "regions": list of (x1, y1, x2, y2) rectangles (inclusive bounds)
    # }
    # user_location = (x, y)
    # Return sorted list of advertiser IDs whose regions contain the user
    ...
```

```
Example:
user_location = (3.0, 4.0)
advertisers = [
    {"id": 1, "regions": [(0,0,5,5), (10,10,20,20)]},
    {"id": 2, "regions": [(4,4,8,8)]},
    {"id": 3, "regions": [(1,1,3,3)]},
]
# User (3,4): inside advertiser 1's first region; not in 2 or 3
eligible_advertisers(user_location, advertisers) -> [1]
```

## Follow-ups

1. How would you speed up queries if there are 1M advertisers and 10K user location queries per second?
2. What spatial indexing structures would you consider (R-tree, grid bucketing)?
3. How do you handle non-rectangular regions (e.g. polygons or radius-based circles)?
4. How would you make the system respect geo-blocking at country/state level?

Geographic Advertiser Access Control - Region-Based Permission Filtering

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