Homepage Display: Rank and Filter Content Items for a Personalized Feed

Question

## Problem

You are building the homepage feed for a streaming platform. Given a list of content items and a user's watch history, return the top `k` items to display.

Ranking rules (in priority order):
1. Items the user has NOT watched rank above watched items.
2. Among unwatched, sort by `popularity_score` descending.
3. Among watched, sort by `last_watched_timestamp` descending (most recently watched first).

```python
from typing import List, Dict

def get_homepage(items: List[Dict], watch_history: List[str], k: int) -> List[Dict]:
    # items: [{"id": str, "title": str, "popularity_score": float}]
    # watch_history: list of watched item ids
    # return: top k items as list of dicts
    pass
```

**Example:**
```
items = [
  {"id": "A", "title": "Alpha", "popularity_score": 9.1},
  {"id": "B", "title": "Beta",  "popularity_score": 8.5},
  {"id": "C", "title": "Gamma", "popularity_score": 7.0},
]
watch_history = ["B"]
k = 2

Output: [{"id": "A", ...}, {"id": "C", ...}]
# A and C are unwatched; B is excluded from top-2 slots.
```

## Follow-ups

1. How would you incorporate a "freshness" decay so older content loses score over time?
2. What data structure best supports real-time score updates as popularity changes?
3. How would you A/B test two ranking algorithms while keeping the experiment statistically valid?
4. If the user has no watch history, how do you cold-start the ranking?

Homepage Display: Rank and Filter Content Items for a Personalized Feed

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