//! SHA-2 hash functions: SHA-256, SHA-384, SHA-512 (FIPS 180-4).

// ---------------------------------------------------------------------------
// SHA-256
// ---------------------------------------------------------------------------

/// SHA-256 round constants (first 32 bits of the fractional parts of the
/// cube roots of the first 64 primes).
const K256: [u32; 64] = [
    0x428a2f98, 0x71374491, 0xb5c0fbcf, 0xe9b5dba5, 0x3956c25b, 0x59f111f1, 0x923f82a4, 0xab1c5ed5,
    0xd807aa98, 0x12835b01, 0x243185be, 0x550c7dc3, 0x72be5d74, 0x80deb1fe, 0x9bdc06a7, 0xc19bf174,
    0xe49b69c1, 0xefbe4786, 0x0fc19dc6, 0x240ca1cc, 0x2de92c6f, 0x4a7484aa, 0x5cb0a9dc, 0x76f988da,
    0x983e5152, 0xa831c66d, 0xb00327c8, 0xbf597fc7, 0xc6e00bf3, 0xd5a79147, 0x06ca6351, 0x14292967,
    0x27b70a85, 0x2e1b2138, 0x4d2c6dfc, 0x53380d13, 0x650a7354, 0x766a0abb, 0x81c2c92e, 0x92722c85,
    0xa2bfe8a1, 0xa81a664b, 0xc24b8b70, 0xc76c51a3, 0xd192e819, 0xd6990624, 0xf40e3585, 0x106aa070,
    0x19a4c116, 0x1e376c08, 0x2748774c, 0x34b0bcb5, 0x391c0cb3, 0x4ed8aa4a, 0x5b9cca4f, 0x682e6ff3,
    0x748f82ee, 0x78a5636f, 0x84c87814, 0x8cc70208, 0x90befffa, 0xa4506ceb, 0xbef9a3f7, 0xc67178f2,
];

/// Initial hash values for SHA-256 (first 32 bits of the fractional parts
/// of the square roots of the first 8 primes).
const H256_INIT: [u32; 8] = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
];

/// SHA-256 hasher with streaming API.
#[derive(Clone)]
pub struct Sha256 {
    state: [u32; 8],
    buf: [u8; 64],
    buf_len: usize,
    total_len: u64,
}

impl Sha256 {
    pub fn new() -> Self {
        Self {
            state: H256_INIT,
            buf: [0u8; 64],
            buf_len: 0,
            total_len: 0,
        }
    }

    pub fn update(&mut self, data: &[u8]) {
        self.total_len += data.len() as u64;
        let mut offset = 0;

        // Fill the buffer if partially filled.
        if self.buf_len > 0 {
            let space = 64 - self.buf_len;
            let copy_len = space.min(data.len());
            self.buf[self.buf_len..self.buf_len + copy_len].copy_from_slice(&data[..copy_len]);
            self.buf_len += copy_len;
            offset += copy_len;

            if self.buf_len == 64 {
                let block = self.buf;
                sha256_compress(&mut self.state, &block);
                self.buf_len = 0;
            }
        }

        // Process full blocks directly.
        while offset + 64 <= data.len() {
            let block: [u8; 64] = data[offset..offset + 64].try_into().unwrap();
            sha256_compress(&mut self.state, &block);
            offset += 64;
        }

        // Buffer remaining bytes.
        let remaining = data.len() - offset;
        if remaining > 0 {
            self.buf[..remaining].copy_from_slice(&data[offset..]);
            self.buf_len = remaining;
        }
    }

    pub fn finalize(mut self) -> [u8; 32] {
        // Pad per FIPS 180-4 §5.1.1.
        let bit_len = self.total_len * 8;

        // Append 0x80 byte.
        self.buf[self.buf_len] = 0x80;
        self.buf_len += 1;

        // If not enough room for the 8-byte length, pad and compress.
        if self.buf_len > 56 {
            for i in self.buf_len..64 {
                self.buf[i] = 0;
            }
            let block = self.buf;
            sha256_compress(&mut self.state, &block);
            self.buf_len = 0;
        }

        // Zero-pad up to byte 56, then append 64-bit big-endian length.
        for i in self.buf_len..56 {
            self.buf[i] = 0;
        }
        self.buf[56..64].copy_from_slice(&bit_len.to_be_bytes());
        let block = self.buf;
        sha256_compress(&mut self.state, &block);

        // Produce the digest.
        let mut out = [0u8; 32];
        for (i, word) in self.state.iter().enumerate() {
            out[i * 4..(i + 1) * 4].copy_from_slice(&word.to_be_bytes());
        }
        out
    }
}

impl Default for Sha256 {
    fn default() -> Self {
        Self::new()
    }
}

/// One-shot SHA-256.
pub fn sha256(data: &[u8]) -> [u8; 32] {
    let mut h = Sha256::new();
    h.update(data);
    h.finalize()
}

/// SHA-256 compression function: process one 64-byte block.
fn sha256_compress(state: &mut [u32; 8], block: &[u8; 64]) {
    // Prepare the message schedule.
    let mut w = [0u32; 64];
    for i in 0..16 {
        w[i] = u32::from_be_bytes([
            block[i * 4],
            block[i * 4 + 1],
            block[i * 4 + 2],
            block[i * 4 + 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 i in 0..64 {
        let s1 = e.rotate_right(6) ^ e.rotate_right(11) ^ e.rotate_right(25);
        let ch = (e & f) ^ ((!e) & g);
        let temp1 = h
            .wrapping_add(s1)
            .wrapping_add(ch)
            .wrapping_add(K256[i])
            .wrapping_add(w[i]);
        let s0 = a.rotate_right(2) ^ a.rotate_right(13) ^ a.rotate_right(22);
        let maj = (a & b) ^ (a & c) ^ (b & c);
        let temp2 = s0.wrapping_add(maj);

        h = g;
        g = f;
        f = e;
        e = d.wrapping_add(temp1);
        d = c;
        c = b;
        b = a;
        a = temp1.wrapping_add(temp2);
    }

    state[0] = state[0].wrapping_add(a);
    state[1] = state[1].wrapping_add(b);
    state[2] = state[2].wrapping_add(c);
    state[3] = state[3].wrapping_add(d);
    state[4] = state[4].wrapping_add(e);
    state[5] = state[5].wrapping_add(f);
    state[6] = state[6].wrapping_add(g);
    state[7] = state[7].wrapping_add(h);
}

// ---------------------------------------------------------------------------
// SHA-512
// ---------------------------------------------------------------------------

/// SHA-512 round constants (first 64 bits of the fractional parts of the
/// cube roots of the first 80 primes).
const K512: [u64; 80] = [
    0x428a2f98d728ae22,
    0x7137449123ef65cd,
    0xb5c0fbcfec4d3b2f,
    0xe9b5dba58189dbbc,
    0x3956c25bf348b538,
    0x59f111f1b605d019,
    0x923f82a4af194f9b,
    0xab1c5ed5da6d8118,
    0xd807aa98a3030242,
    0x12835b0145706fbe,
    0x243185be4ee4b28c,
    0x550c7dc3d5ffb4e2,
    0x72be5d74f27b896f,
    0x80deb1fe3b1696b1,
    0x9bdc06a725c71235,
    0xc19bf174cf692694,
    0xe49b69c19ef14ad2,
    0xefbe4786384f25e3,
    0x0fc19dc68b8cd5b5,
    0x240ca1cc77ac9c65,
    0x2de92c6f592b0275,
    0x4a7484aa6ea6e483,
    0x5cb0a9dcbd41fbd4,
    0x76f988da831153b5,
    0x983e5152ee66dfab,
    0xa831c66d2db43210,
    0xb00327c898fb213f,
    0xbf597fc7beef0ee4,
    0xc6e00bf33da88fc2,
    0xd5a79147930aa725,
    0x06ca6351e003826f,
    0x142929670a0e6e70,
    0x27b70a8546d22ffc,
    0x2e1b21385c26c926,
    0x4d2c6dfc5ac42aed,
    0x53380d139d95b3df,
    0x650a73548baf63de,
    0x766a0abb3c77b2a8,
    0x81c2c92e47edaee6,
    0x92722c851482353b,
    0xa2bfe8a14cf10364,
    0xa81a664bbc423001,
    0xc24b8b70d0f89791,
    0xc76c51a30654be30,
    0xd192e819d6ef5218,
    0xd69906245565a910,
    0xf40e35855771202a,
    0x106aa07032bbd1b8,
    0x19a4c116b8d2d0c8,
    0x1e376c085141ab53,
    0x2748774cdf8eeb99,
    0x34b0bcb5e19b48a8,
    0x391c0cb3c5c95a63,
    0x4ed8aa4ae3418acb,
    0x5b9cca4f7763e373,
    0x682e6ff3d6b2b8a3,
    0x748f82ee5defb2fc,
    0x78a5636f43172f60,
    0x84c87814a1f0ab72,
    0x8cc702081a6439ec,
    0x90befffa23631e28,
    0xa4506cebde82bde9,
    0xbef9a3f7b2c67915,
    0xc67178f2e372532b,
    0xca273eceea26619c,
    0xd186b8c721c0c207,
    0xeada7dd6cde0eb1e,
    0xf57d4f7fee6ed178,
    0x06f067aa72176fba,
    0x0a637dc5a2c898a6,
    0x113f9804bef90dae,
    0x1b710b35131c471b,
    0x28db77f523047d84,
    0x32caab7b40c72493,
    0x3c9ebe0a15c9bebc,
    0x431d67c49c100d4c,
    0x4cc5d4becb3e42b6,
    0x597f299cfc657e2a,
    0x5fcb6fab3ad6faec,
    0x6c44198c4a475817,
];

/// Initial hash values for SHA-512.
const H512_INIT: [u64; 8] = [
    0x6a09e667f3bcc908,
    0xbb67ae8584caa73b,
    0x3c6ef372fe94f82b,
    0xa54ff53a5f1d36f1,
    0x510e527fade682d1,
    0x9b05688c2b3e6c1f,
    0x1f83d9abfb41bd6b,
    0x5be0cd19137e2179,
];

/// Initial hash values for SHA-384 (different from SHA-512).
const H384_INIT: [u64; 8] = [
    0xcbbb9d5dc1059ed8,
    0x629a292a367cd507,
    0x9159015a3070dd17,
    0x152fecd8f70e5939,
    0x67332667ffc00b31,
    0x8eb44a8768581511,
    0xdb0c2e0d64f98fa7,
    0x47b5481dbefa4fa4,
];

/// SHA-512 hasher with streaming API.
#[derive(Clone)]
pub struct Sha512 {
    state: [u64; 8],
    buf: [u8; 128],
    buf_len: usize,
    total_len: u128,
}

impl Sha512 {
    pub fn new() -> Self {
        Self {
            state: H512_INIT,
            buf: [0u8; 128],
            buf_len: 0,
            total_len: 0,
        }
    }

    fn with_init(init: [u64; 8]) -> Self {
        Self {
            state: init,
            buf: [0u8; 128],
            buf_len: 0,
            total_len: 0,
        }
    }

    pub fn update(&mut self, data: &[u8]) {
        self.total_len += data.len() as u128;
        let mut offset = 0;

        if self.buf_len > 0 {
            let space = 128 - self.buf_len;
            let copy_len = space.min(data.len());
            self.buf[self.buf_len..self.buf_len + copy_len].copy_from_slice(&data[..copy_len]);
            self.buf_len += copy_len;
            offset += copy_len;

            if self.buf_len == 128 {
                let block = self.buf;
                sha512_compress(&mut self.state, &block);
                self.buf_len = 0;
            }
        }

        while offset + 128 <= data.len() {
            let block: [u8; 128] = data[offset..offset + 128].try_into().unwrap();
            sha512_compress(&mut self.state, &block);
            offset += 128;
        }

        let remaining = data.len() - offset;
        if remaining > 0 {
            self.buf[..remaining].copy_from_slice(&data[offset..]);
            self.buf_len = remaining;
        }
    }

    pub fn finalize(mut self) -> [u8; 64] {
        // Pad per FIPS 180-4 §5.1.2.
        let bit_len = self.total_len * 8;

        self.buf[self.buf_len] = 0x80;
        self.buf_len += 1;

        // Need 16 bytes for the 128-bit length at the end.
        if self.buf_len > 112 {
            for i in self.buf_len..128 {
                self.buf[i] = 0;
            }
            let block = self.buf;
            sha512_compress(&mut self.state, &block);
            self.buf_len = 0;
        }

        for i in self.buf_len..112 {
            self.buf[i] = 0;
        }
        self.buf[112..128].copy_from_slice(&bit_len.to_be_bytes());
        let block = self.buf;
        sha512_compress(&mut self.state, &block);

        let mut out = [0u8; 64];
        for (i, word) in self.state.iter().enumerate() {
            out[i * 8..(i + 1) * 8].copy_from_slice(&word.to_be_bytes());
        }
        out
    }
}

impl Default for Sha512 {
    fn default() -> Self {
        Self::new()
    }
}

/// One-shot SHA-512.
pub fn sha512(data: &[u8]) -> [u8; 64] {
    let mut h = Sha512::new();
    h.update(data);
    h.finalize()
}

/// SHA-512 compression function: process one 128-byte block.
fn sha512_compress(state: &mut [u64; 8], block: &[u8; 128]) {
    let mut w = [0u64; 80];
    for i in 0..16 {
        w[i] = u64::from_be_bytes([
            block[i * 8],
            block[i * 8 + 1],
            block[i * 8 + 2],
            block[i * 8 + 3],
            block[i * 8 + 4],
            block[i * 8 + 5],
            block[i * 8 + 6],
            block[i * 8 + 7],
        ]);
    }
    for i in 16..80 {
        let s0 = w[i - 15].rotate_right(1) ^ w[i - 15].rotate_right(8) ^ (w[i - 15] >> 7);
        let s1 = w[i - 2].rotate_right(19) ^ w[i - 2].rotate_right(61) ^ (w[i - 2] >> 6);
        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 i in 0..80 {
        let s1 = e.rotate_right(14) ^ e.rotate_right(18) ^ e.rotate_right(41);
        let ch = (e & f) ^ ((!e) & g);
        let temp1 = h
            .wrapping_add(s1)
            .wrapping_add(ch)
            .wrapping_add(K512[i])
            .wrapping_add(w[i]);
        let s0 = a.rotate_right(28) ^ a.rotate_right(34) ^ a.rotate_right(39);
        let maj = (a & b) ^ (a & c) ^ (b & c);
        let temp2 = s0.wrapping_add(maj);

        h = g;
        g = f;
        f = e;
        e = d.wrapping_add(temp1);
        d = c;
        c = b;
        b = a;
        a = temp1.wrapping_add(temp2);
    }

    state[0] = state[0].wrapping_add(a);
    state[1] = state[1].wrapping_add(b);
    state[2] = state[2].wrapping_add(c);
    state[3] = state[3].wrapping_add(d);
    state[4] = state[4].wrapping_add(e);
    state[5] = state[5].wrapping_add(f);
    state[6] = state[6].wrapping_add(g);
    state[7] = state[7].wrapping_add(h);
}

// ---------------------------------------------------------------------------
// SHA-384
// ---------------------------------------------------------------------------

/// SHA-384 hasher (SHA-512 with different initial values, truncated to 384 bits).
#[derive(Clone)]
pub struct Sha384 {
    inner: Sha512,
}

impl Sha384 {
    pub fn new() -> Self {
        Self {
            inner: Sha512::with_init(H384_INIT),
        }
    }

    pub fn update(&mut self, data: &[u8]) {
        self.inner.update(data);
    }

    pub fn finalize(self) -> [u8; 48] {
        let full = self.inner.finalize();
        let mut out = [0u8; 48];
        out.copy_from_slice(&full[..48]);
        out
    }
}

impl Default for Sha384 {
    fn default() -> Self {
        Self::new()
    }
}

/// One-shot SHA-384.
pub fn sha384(data: &[u8]) -> [u8; 48] {
    let mut h = Sha384::new();
    h.update(data);
    h.finalize()
}

// ---------------------------------------------------------------------------
// Tests
// ---------------------------------------------------------------------------

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

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

    // -----------------------------------------------------------------------
    // SHA-256 NIST test vectors (FIPS 180-4 / NIST CSRC examples)
    // -----------------------------------------------------------------------

    #[test]
    fn sha256_empty() {
        assert_eq!(
            hex(&sha256(b"")),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
    }

    #[test]
    fn sha256_abc() {
        assert_eq!(
            hex(&sha256(b"abc")),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
    }

    #[test]
    fn sha256_448bit() {
        // "abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq" (448 bits)
        assert_eq!(
            hex(&sha256(
                b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq"
            )),
            "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1"
        );
    }

    #[test]
    fn sha256_896bit() {
        // "abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu"
        assert_eq!(
            hex(&sha256(b"abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu")),
            "cf5b16a778af8380036ce59e7b0492370b249b11e8f07a51afac45037afee9d1"
        );
    }

    #[test]
    fn sha256_million_a() {
        // 1,000,000 repetitions of 'a'
        let mut h = Sha256::new();
        // Feed in chunks to test streaming.
        let chunk = [b'a'; 1000];
        for _ in 0..1000 {
            h.update(&chunk);
        }
        assert_eq!(
            hex(&h.finalize()),
            "cdc76e5c9914fb9281a1c7e284d73e67f1809a48a497200e046d39ccc7112cd0"
        );
    }

    #[test]
    fn sha256_streaming() {
        // Verify streaming matches one-shot.
        let data = b"The quick brown fox jumps over the lazy dog";
        let expected = sha256(data);

        let mut h = Sha256::new();
        h.update(&data[..10]);
        h.update(&data[10..20]);
        h.update(&data[20..]);
        assert_eq!(h.finalize(), expected);
    }

    // -----------------------------------------------------------------------
    // SHA-512 NIST test vectors
    // -----------------------------------------------------------------------

    #[test]
    fn sha512_empty() {
        assert_eq!(
            hex(&sha512(b"")),
            "cf83e1357eefb8bdf1542850d66d8007d620e4050b5715dc83f4a921d36ce9ce47d0d13c5d85f2b0ff8318d2877eec2f63b931bd47417a81a538327af927da3e"
        );
    }

    #[test]
    fn sha512_abc() {
        assert_eq!(
            hex(&sha512(b"abc")),
            "ddaf35a193617abacc417349ae20413112e6fa4e89a97ea20a9eeee64b55d39a2192992a274fc1a836ba3c23a3feebbd454d4423643ce80e2a9ac94fa54ca49f"
        );
    }

    #[test]
    fn sha512_896bit() {
        assert_eq!(
            hex(&sha512(b"abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu")),
            "8e959b75dae313da8cf4f72814fc143f8f7779c6eb9f7fa17299aeadb6889018501d289e4900f7e4331b99dec4b5433ac7d329eeb6dd26545e96e55b874be909"
        );
    }

    #[test]
    fn sha512_million_a() {
        let mut h = Sha512::new();
        let chunk = [b'a'; 1000];
        for _ in 0..1000 {
            h.update(&chunk);
        }
        assert_eq!(
            hex(&h.finalize()),
            "e718483d0ce769644e2e42c7bc15b4638e1f98b13b2044285632a803afa973ebde0ff244877ea60a4cb0432ce577c31beb009c5c2c49aa2e4eadb217ad8cc09b"
        );
    }

    #[test]
    fn sha512_streaming() {
        let data = b"The quick brown fox jumps over the lazy dog";
        let expected = sha512(data);

        let mut h = Sha512::new();
        h.update(&data[..5]);
        h.update(&data[5..]);
        assert_eq!(h.finalize(), expected);
    }

    // -----------------------------------------------------------------------
    // SHA-384 NIST test vectors
    // -----------------------------------------------------------------------

    #[test]
    fn sha384_empty() {
        assert_eq!(
            hex(&sha384(b"")),
            "38b060a751ac96384cd9327eb1b1e36a21fdb71114be07434c0cc7bf63f6e1da274edebfe76f65fbd51ad2f14898b95b"
        );
    }

    #[test]
    fn sha384_abc() {
        assert_eq!(
            hex(&sha384(b"abc")),
            "cb00753f45a35e8bb5a03d699ac65007272c32ab0eded1631a8b605a43ff5bed8086072ba1e7cc2358baeca134c825a7"
        );
    }

    #[test]
    fn sha384_896bit() {
        assert_eq!(
            hex(&sha384(b"abcdefghbcdefghicdefghijdefghijkefghijklfghijklmghijklmnhijklmnoijklmnopjklmnopqklmnopqrlmnopqrsmnopqrstnopqrstu")),
            "09330c33f71147e83d192fc782cd1b4753111b173b3b05d22fa08086e3b0f712fcc7c71a557e2db966c3e9fa91746039"
        );
    }

    #[test]
    fn sha384_million_a() {
        let mut h = Sha384::new();
        let chunk = [b'a'; 1000];
        for _ in 0..1000 {
            h.update(&chunk);
        }
        assert_eq!(
            hex(&h.finalize()),
            "9d0e1809716474cb086e834e310a4a1ced149e9c00f248527972cec5704c2a5b07b8b3dc38ecc4ebae97ddd87f3d8985"
        );
    }

    #[test]
    fn sha384_streaming() {
        let data = b"The quick brown fox jumps over the lazy dog";
        let expected = sha384(data);

        let mut h = Sha384::new();
        h.update(&data[..15]);
        h.update(&data[15..]);
        assert_eq!(h.finalize(), expected);
    }

    // -----------------------------------------------------------------------
    // Edge cases
    // -----------------------------------------------------------------------

    #[test]
    fn sha256_exactly_one_block() {
        // 55 bytes + 1 byte (0x80) + 8 bytes (length) = 64 = one block after padding
        let data = [0xABu8; 55];
        let result = sha256(&data);
        // Verify streaming matches.
        let mut h = Sha256::new();
        h.update(&data);
        assert_eq!(h.finalize(), result);
    }

    #[test]
    fn sha256_exactly_56_bytes() {
        // 56 bytes requires two blocks (56 + 1 + 8 > 64).
        let data = [0xCDu8; 56];
        let result = sha256(&data);
        let mut h = Sha256::new();
        h.update(&data);
        assert_eq!(h.finalize(), result);
    }

    #[test]
    fn sha512_exactly_one_block() {
        // 111 bytes + 1 + 16 = 128
        let data = [0xABu8; 111];
        let result = sha512(&data);
        let mut h = Sha512::new();
        h.update(&data);
        assert_eq!(h.finalize(), result);
    }

    #[test]
    fn sha512_exactly_112_bytes() {
        // 112 + 1 + 16 > 128 => requires second block
        let data = [0xCDu8; 112];
        let result = sha512(&data);
        let mut h = Sha512::new();
        h.update(&data);
        assert_eq!(h.finalize(), result);
    }

    #[test]
    fn sha256_byte_at_a_time() {
        let data = b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq";
        let expected = sha256(data);
        let mut h = Sha256::new();
        for byte in data.iter() {
            h.update(&[*byte]);
        }
        assert_eq!(h.finalize(), expected);
    }
}