# Sha256 Utility Overview

`Sha256.cpp` provides a small helper to compute the SHA-256 digest of a file.
The function is used during import to assign a stable, unique identifier for
books; this id becomes the primary key in SQLite and the filename for cached
covers.

## Responsibilities

- Stream a file from disk.
- Feed the bytes through OpenSSL’s SHA-256 implementation.
- Return the digest as a lowercase hexadecimal string.

## Expected usage

```cpp
std::string digest = sha256_file(path);
```

The caller is responsible for handling I/O errors (the helper throws if the file
cannot be read). Since the digest is deterministic, re-importing the same file
produces the same id, allowing updates to merge cleanly.