// SHA-256 (FIPS 180-4) — minimal block-oriented implementation for HKDF/HMAC.
// No external crypto dependencies.

const K: [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,
];

#[inline(always)]
fn rotr(x: u32, n: u32) -> u32 {
    x.rotate_right(n)
}

fn compress(state: &mut [u32; 8], block: &[u8; 64]) {
    let mut w = [0u32; 64];
    for i in 0..16 {
        w[i] = u32::from_be_bytes(block[i * 4..(i + 1) * 4].try_into().unwrap());
    }
    for i in 16..64 {
        let s0 = rotr(w[i - 15], 7) ^ rotr(w[i - 15], 18) ^ (w[i - 15] >> 3);
        let s1 = rotr(w[i - 2], 17) ^ rotr(w[i - 2], 19) ^ (w[i - 2] >> 10);
        w[i] = w[i - 16]
            .wrapping_add(s0)
            .wrapping_add(w[i - 7])
            .wrapping_add(s1);
    }

    let mut a = state[0];
    let mut b = state[1];
    let mut c = state[2];
    let mut d = state[3];
    let mut e = state[4];
    let mut f = state[5];
    let mut g = state[6];
    let mut h = state[7];

    for i in 0..64 {
        let s1 = rotr(e, 6) ^ rotr(e, 11) ^ rotr(e, 25);
        let ch = (e & f) ^ (!e & g);
        let t1 = h
            .wrapping_add(s1)
            .wrapping_add(ch)
            .wrapping_add(K[i])
            .wrapping_add(w[i]);
        let s0 = rotr(a, 2) ^ rotr(a, 13) ^ rotr(a, 22);
        let maj = (a & b) ^ (a & c) ^ (b & c);
        let t0 = s0.wrapping_add(maj);

        h = g;
        g = f;
        f = e;
        e = d.wrapping_add(t1);
        d = c;
        c = b;
        b = a;
        a = t0.wrapping_add(t1);
    }

    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);
}

pub(crate) struct Sha256 {
    state: [u32; 8],
    buf: [u8; 64],
    buf_len: usize,
    /// Total input bytes ingested (for final 64-bit bit length).
    byte_len: u64,
}

impl Sha256 {
    pub(crate) fn new() -> Self {
        Self {
            state: [
                0x6a09e667,
                0xbb67ae85,
                0x3c6ef372,
                0xa54ff53a,
                0x510e527f,
                0x9b05688c,
                0x1f83d9ab,
                0x5be0cd19,
            ],
            buf: [0u8; 64],
            buf_len: 0,
            byte_len: 0,
        }
    }

    pub(crate) fn update(&mut self, mut data: &[u8]) {
        self.byte_len += data.len() as u64;

        if self.buf_len > 0 {
            let take = (64 - self.buf_len).min(data.len());
            self.buf[self.buf_len..self.buf_len + take].copy_from_slice(&data[..take]);
            self.buf_len += take;
            data = &data[take..];
            if self.buf_len == 64 {
                compress(&mut self.state, &self.buf);
                self.buf_len = 0;
            }
        }

        while data.len() >= 64 {
            compress(&mut self.state, data[..64].try_into().unwrap());
            data = &data[64..];
        }

        if !data.is_empty() {
            self.buf[..data.len()].copy_from_slice(data);
            self.buf_len = data.len();
        }
    }

    pub(crate) fn finalize(mut self) -> [u8; 32] {
        let mut block = [0u8; 64];
        let l = self.buf_len;
        block[..l].copy_from_slice(&self.buf[..l]);
        block[l] = 0x80;
        let bits = self.byte_len.saturating_mul(8);

        if l < 56 {
            block[l + 1..56].fill(0);
            block[56..64].copy_from_slice(&bits.to_be_bytes());
            compress(&mut self.state, &block);
        } else {
            block[l + 1..64].fill(0);
            compress(&mut self.state, &block);
            let mut block2 = [0u8; 64];
            block2[56..64].copy_from_slice(&bits.to_be_bytes());
            compress(&mut self.state, &block2);
        }

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

pub(crate) fn sha256_digest(data: &[u8]) -> [u8; 32] {
    let mut h = Sha256::new();
    h.update(data);
    h.finalize()
}

#[cfg(test)]
mod tests {
    use super::sha256_digest;

    #[test]
    fn nist_empty() {
        let got = sha256_digest(b"");
        const EXP: [u8; 32] = hex_literal::hex!(
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
        assert_eq!(got, EXP);
    }
}
