//! Tiny pure-Rust SHA-256 implementation (private).
//!
//! Kept inline so the crate does not pull a `sha2` runtime dependency for a
//! single hash use case. This implementation follows FIPS 180-4. It does not
//! aim for speed; it aims for correctness and zero deps.

/// SHA-256 hasher.
pub struct SimpleSha256 {
    h: [u32; 8],
    buf: [u8; 64],
    buf_len: usize,
    bit_len: u64,
}

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,
];

const H0: [u32; 8] = [
    0x6a09e667, 0xbb67ae85, 0x3c6ef372, 0xa54ff53a, 0x510e527f, 0x9b05688c, 0x1f83d9ab, 0x5be0cd19,
];

impl SimpleSha256 {
    /// Initialize a fresh hasher.
    pub fn new() -> Self {
        Self {
            h: H0,
            buf: [0u8; 64],
            buf_len: 0,
            bit_len: 0,
        }
    }

    /// Feed `data` into the hasher.
    pub fn update(&mut self, data: &[u8]) {
        self.bit_len = self.bit_len.wrapping_add((data.len() as u64) * 8);
        let mut i = 0;
        // Fill the partial buffer first.
        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;
            i += take;
            if self.buf_len == 64 {
                let block = self.buf;
                self.compress(&block);
                self.buf_len = 0;
            }
        }
        // Then process full 64-byte blocks directly from the input.
        while data.len() - i >= 64 {
            let mut block = [0u8; 64];
            block.copy_from_slice(&data[i..i + 64]);
            self.compress(&block);
            i += 64;
        }
        // Stash the trailing tail in the buffer.
        if i < data.len() {
            let tail_len = data.len() - i;
            self.buf[..tail_len].copy_from_slice(&data[i..]);
            self.buf_len = tail_len;
        }
    }

    /// Consume the hasher and produce the 32-byte digest.
    pub fn finalize(mut self) -> [u8; 32] {
        // FIPS 180-4 padding: append 0x80, then zero bytes until length mod
        // 64 == 56, then 8-byte big-endian length in bits.
        let bit_len = self.bit_len;
        self.buf[self.buf_len] = 0x80;
        self.buf_len += 1;
        if self.buf_len > 56 {
            // Not enough room for the length field; flush this block first.
            for b in &mut self.buf[self.buf_len..] {
                *b = 0;
            }
            let block = self.buf;
            self.compress(&block);
            self.buf_len = 0;
        }
        for b in &mut self.buf[self.buf_len..56] {
            *b = 0;
        }
        self.buf[56..64].copy_from_slice(&bit_len.to_be_bytes());
        let block = self.buf;
        self.compress(&block);

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

    fn compress(&mut self, block: &[u8; 64]) {
        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 = self.h[0];
        let mut b = self.h[1];
        let mut c = self.h[2];
        let mut d = self.h[3];
        let mut e = self.h[4];
        let mut f = self.h[5];
        let mut g = self.h[6];
        let mut hh = self.h[7];

        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 t1 = hh
                .wrapping_add(s1)
                .wrapping_add(ch)
                .wrapping_add(K[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 t2 = s0.wrapping_add(maj);
            hh = g;
            g = f;
            f = e;
            e = d.wrapping_add(t1);
            d = c;
            c = b;
            b = a;
            a = t1.wrapping_add(t2);
        }

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

#[cfg(test)]
mod tests {
    use super::SimpleSha256;

    fn hex(bytes: &[u8]) -> String {
        const HEX: &[u8; 16] = b"0123456789abcdef";
        let mut s = String::with_capacity(bytes.len() * 2);
        for &b in bytes {
            s.push(HEX[(b >> 4) as usize] as char);
            s.push(HEX[(b & 0x0f) as usize] as char);
        }
        s
    }

    #[test]
    fn known_answer_empty() {
        let h = SimpleSha256::new().finalize();
        assert_eq!(
            hex(&h),
            "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
        );
    }

    #[test]
    fn known_answer_abc() {
        let mut h = SimpleSha256::new();
        h.update(b"abc");
        let out = h.finalize();
        assert_eq!(
            hex(&out),
            "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad"
        );
    }

    #[test]
    fn known_answer_two_block() {
        // The classic FIPS 180-2 multi-block test vector (448 bits in,
        // exercises both compression blocks).
        let mut h = SimpleSha256::new();
        h.update(b"abcdbcdecdefdefgefghfghighijhijkijkljklmklmnlmnomnopnopq");
        let out = h.finalize();
        assert_eq!(
            hex(&out),
            "248d6a61d20638b8e5c026930c3e6039a33ce45964ff2167f6ecedd419db06c1"
        );
    }

    #[test]
    fn streaming_matches_one_shot() {
        let input: Vec<u8> = (0..1000).map(|i| (i % 251) as u8).collect();
        let mut one_shot = SimpleSha256::new();
        one_shot.update(&input);
        let a = one_shot.finalize();
        let mut streamed = SimpleSha256::new();
        for chunk in input.chunks(7) {
            streamed.update(chunk);
        }
        let b = streamed.finalize();
        assert_eq!(a, b);
    }
}
