File Parts Validator: Validate Chunked File Uploads and Detect Missing or Corrupt Parts

Question

## Problem

A large file is split into numbered parts and uploaded in arbitrary order. Each part has a part number (1-indexed), a checksum, and a byte payload. Implement a validator that (1) accepts parts as they arrive, (2) identifies missing part numbers once an `end` signal is received, and (3) verifies checksums, reporting corrupt parts.

```python
class FilePartsValidator:
    def __init__(self, expected_parts: int): ...

def receive_part(self, part_num: int, checksum: str, data: bytes) -> None: ...

def finalize(self) -> dict:
        """
        Returns {
          "missing": [part_nums],
          "corrupt": [part_nums],
          "valid": bool
        }
        """
```

**Example:**
```
v = FilePartsValidator(3)
v.receive_part(1, "abc", b"hello")
v.receive_part(3, "bad", b"world")  # checksum mismatch
v.finalize()
# -> {"missing": [2], "corrupt": [3], "valid": False}
```

## Follow-ups

1. How do you compute and verify the checksum — MD5, SHA-256, or CRC32? What are the tradeoffs?
2. If parts can be retransmitted (duplicates), how does your model handle re-receipt of a valid part?
3. How would you reassemble the file in order from received parts with minimal memory overhead?
4. What streaming approach avoids holding all parts in memory simultaneously?

File Parts Validator: Validate Chunked File Uploads and Detect Missing or Corrupt Parts

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