```python
"""Count the hits a key stream scores on a direct-mapped cache."""

def slot_hits(keys: list[int], slots: int) -> int:
    if not isinstance(slots, int) or slots <= 0:
        raise ValueError("Slot count must be a positive integer.")
    
    held = [None] * slots
    hits = 0
    
    for key in keys:
        index = abs(key) % slots
        if held[index] == key:
            hits += 1
        else:
            held[index] = key
    
    return hits
```