typedlm 0.1.0

//! SHA-256, written out rather than pulled in as a dependency: small and fixed by its
//! standard. Fingerprints datasets so a baseline is only compared with a run on the same
//! examples. Taken from chartlet (MIT, CASOON).

/// The SHA-256 round constants from FIPS 180-4.
#[rustfmt::skip]
const K: [u32; 64] = [
    0x428a_2f98, 0x7137_4491, 0xb5c0_fbcf, 0xe9b5_dba5, 0x3956_c25b, 0x59f1_11f1, 0x923f_82a4,
    0xab1c_5ed5, 0xd807_aa98, 0x1283_5b01, 0x2431_85be, 0x550c_7dc3, 0x72be_5d74, 0x80de_b1fe,
    0x9bdc_06a7, 0xc19b_f174, 0xe49b_69c1, 0xefbe_4786, 0x0fc1_9dc6, 0x240c_a1cc, 0x2de9_2c6f,
    0x4a74_84aa, 0x5cb0_a9dc, 0x76f9_88da, 0x983e_5152, 0xa831_c66d, 0xb003_27c8, 0xbf59_7fc7,
    0xc6e0_0bf3, 0xd5a7_9147, 0x06ca_6351, 0x1429_2967, 0x27b7_0a85, 0x2e1b_2138, 0x4d2c_6dfc,
    0x5338_0d13, 0x650a_7354, 0x766a_0abb, 0x81c2_c92e, 0x9272_2c85, 0xa2bf_e8a1, 0xa81a_664b,
    0xc24b_8b70, 0xc76c_51a3, 0xd192_e819, 0xd699_0624, 0xf40e_3585, 0x106a_a070, 0x19a4_c116,
    0x1e37_6c08, 0x2748_774c, 0x34b0_bcb5, 0x391c_0cb3, 0x4ed8_aa4a, 0x5b9c_ca4f, 0x682e_6ff3,
    0x748f_82ee, 0x78a5_636f, 0x84c8_7814, 0x8cc7_0208, 0x90be_fffa, 0xa450_6ceb, 0xbef9_a3f7,
    0xc671_78f2,
];

/// The SHA-256 digest of `message`, as FIPS 180-4 defines it. The working variables keep the standard's one-letter names
/// so the round function can be read against the specification.
#[allow(clippy::many_single_char_names)]
#[must_use]
pub(crate) fn sha256(message: &[u8]) -> [u8; 32] {
    let mut state: [u32; 8] = [
        0x6a09_e667,
        0xbb67_ae85,
        0x3c6e_f372,
        0xa54f_f53a,
        0x510e_527f,
        0x9b05_688c,
        0x1f83_d9ab,
        0x5be0_cd19,
    ];
    let bits = (message.len() as u64).wrapping_mul(8);
    let mut padded = message.to_vec();
    padded.push(0x80);
    while padded.len() % 64 != 56 {
        padded.push(0);
    }
    padded.extend_from_slice(&bits.to_be_bytes());

    // `chunks_exact` rather than `as_chunks`, which needs Rust 1.88 (MSRV is 1.85).
    for block in padded.chunks_exact(64) {
        let mut w = [0u32; 64];
        for (word, bytes) in w.iter_mut().zip(block.chunks_exact(4)) {
            *word = u32::from_be_bytes([bytes[0], bytes[1], bytes[2], bytes[3]]);
        }
        for i in 16..64 {
            let s0 = w[i - 15].rotate_right(7) ^ w[i - 15].rotate_right(18) ^ (w[i - 15] >> 3);
            let s1 = w[i - 2].rotate_right(17) ^ w[i - 2].rotate_right(19) ^ (w[i - 2] >> 10);
            w[i] = w[i - 16]
                .wrapping_add(s0)
                .wrapping_add(w[i - 7])
                .wrapping_add(s1);
        }
        let [mut a, mut b, mut c, mut d, mut e, mut f, mut g, mut h] = state;
        for (k, w) in K.iter().zip(w) {
            let s1 = e.rotate_right(6) ^ e.rotate_right(11) ^ e.rotate_right(25);
            let choice = (e & f) ^ (!e & g);
            let t1 = h
                .wrapping_add(s1)
                .wrapping_add(choice)
                .wrapping_add(*k)
                .wrapping_add(w);
            let s0 = a.rotate_right(2) ^ a.rotate_right(13) ^ a.rotate_right(22);
            let majority = (a & b) ^ (a & c) ^ (b & c);
            let t2 = s0.wrapping_add(majority);
            h = g;
            g = f;
            f = e;
            e = d.wrapping_add(t1);
            d = c;
            c = b;
            b = a;
            a = t1.wrapping_add(t2);
        }
        for (word, added) in state.iter_mut().zip([a, b, c, d, e, f, g, h]) {
            *word = word.wrapping_add(added);
        }
    }

    let mut digest = [0u8; 32];
    for (bytes, word) in digest.chunks_exact_mut(4).zip(state) {
        bytes.copy_from_slice(&word.to_be_bytes());
    }
    digest
}

/// Lowercase hexadecimal, as the manifest writes a digest.
pub(crate) fn hex(digest: &[u8]) -> String {
    use std::fmt::Write as _;
    digest.iter().fold(String::new(), |mut text, byte| {
        write!(text, "{byte:02x}").expect("writing to String cannot fail");
        text
    })
}

#[cfg(test)]
mod tests {
    use super::{hex, sha256};

    #[test]
    fn sha256_matches_the_standard_test_vectors() {
        let cases: [(&[u8], &str); 4] = [
            (
                b"",
                "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
            ),
            (
                b"abc",
                "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad",
            ),
            (
                b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq",
                "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1",
            ),
            (
                b"abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu",
                "cf5b16a778af8380036ce59e7b0492370b249b11e8f07a51afac45037afee9d1",
            ),
        ];
        for (message, expected) in cases {
            assert_eq!(hex(&sha256(message)), expected);
        }
        // A million times `a`: many blocks, and a length that needs every byte of the counter.
        assert_eq!(
            hex(&sha256(&vec![b'a'; 1_000_000])),
            "cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0"
        );
        // 55 and 56 bytes: the last length that pads within the block, and the first that cannot.
        assert_eq!(
            hex(&sha256(&[b'a'; 55])),
            "9f4390f8d30c2dd92ec9f095b65e2b9ae9b0a925a5258e241c9f1e910f734318"
        );
        assert_eq!(
            hex(&sha256(&[b'a'; 56])),
            "b35439a4ac6f0948b6d6f9e3c6af0f5f590ce20f1bde7090ef7970686ec6738a"
        );
    }
}