//! SHA-256 of a model file, streamed (a model can be hundreds of megabytes).

use std::io::{self, Read};
use std::path::Path;

use sha2::{Digest, Sha256};

/// Lowercase hex SHA-256 of everything `r` yields.
pub fn sha256_reader<R: Read>(mut r: R) -> io::Result<String> {
    let mut h = Sha256::new();
    let mut buf = vec![0u8; 1 << 20];
    loop {
        let n = r.read(&mut buf)?;
        if n == 0 {
            break;
        }
        h.update(buf.get(..n).unwrap_or(&[]));
    }
    Ok(hex(&h.finalize()))
}

/// Lowercase hex SHA-256 of a file.
pub fn sha256_file(path: &Path) -> io::Result<String> {
    sha256_reader(io::BufReader::new(std::fs::File::open(path)?))
}

/// Lowercase hex SHA-256 of `bytes`.
pub fn sha256_hex(bytes: &[u8]) -> String {
    hex(&Sha256::digest(bytes))
}

fn hex(bytes: &[u8]) -> String {
    bytes.iter().map(|b| format!("{b:02x}")).collect()
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn known_digests() {
        assert_eq!(sha256_hex(b""), "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855");
        assert_eq!(sha256_hex(b"abc"), "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
        assert_eq!(sha256_reader(&b"abc"[..]).unwrap(), sha256_hex(b"abc"));
    }
}