```python
def wave_count(readings: list[int]) -> int:
    """Return how many times a run of readings changes direction."""
    if len(readings) < 2:
        return 0
    
    count = 0
    prev_diff = readings[1] - readings[0]
    
    for i in range(2,        current_diff = readings[i] - readings[i-1]
        if prev_diff * current_diff < 0:
            count +=        prev_diff = current_diff
    
    return count
```