//! SHA-1 (FIPS 180-4), used only to check the `Sec-WebSocket-Accept` header of a WebSocket
//! handshake (RFC 6455, section 4.2.2). Never use it for anything that needs to resist attack.

/// The digest of `data`.
pub fn sha1(data: &[u8]) -> [u8; 20] {
    let mut h = Sha1::new();
    h.update(data);
    h.finish()
}

/// SHA-1 fed in pieces.
#[derive(Debug, Clone)]
pub struct Sha1 {
    state: [u32; 5],
    block: [u8; 64],
    filled: usize,
    length: u64,
}

impl Default for Sha1 {
    fn default() -> Self {
        Self::new()
    }
}

impl Sha1 {
    pub fn new() -> Sha1 {
        Sha1 {
            state: [
                0x6745_2301,
                0xEFCD_AB89,
                0x98BA_DCFE,
                0x1032_5476,
                0xC3D2_E1F0,
            ],
            block: [0; 64],
            filled: 0,
            length: 0,
        }
    }

    pub fn update(&mut self, mut data: &[u8]) {
        // Add 8 * data.len() to `length` (wrapping; `u64::try_from`, never `as`). Copy bytes into
        // `block` from `filled` on; each time it is full (64), `compress` it and set `filled` to 0.
        // Use `split_at` and `get_mut(..)`, no indexing that can go out of bounds.
        self.length = self
            .length
            .wrapping_add(8u64.wrapping_mul(u64::try_from(data.len()).unwrap_or(u64::MAX)));
        while !data.is_empty() {
            let space = 64 - self.filled;
            let take = data.len().min(space);
            let (left, right) = data.split_at(take);
            if let Some(dst) = self.block.get_mut(self.filled..self.filled + take) {
                dst.copy_from_slice(left);
            }
            self.filled += take;
            if self.filled == 64 {
                let block = self.block;
                self.compress(&block);
                self.filled = 0;
            }
            data = right;
        }
    }

    pub fn finish(mut self) -> [u8; 20] {
        // Save `length`. Feed 0x80 then zeros so that 56 bytes of the block are filled (56 -
        // filled, or 120 - filled when filled >= 56), then the saved length as 8 big-endian bytes,
        // through `update`. `update` adds to `length`: put the saved value back after. Then the
        // five state words, big-endian.
        let saved = self.length;
        let zeros = if self.filled < 56 {
            56 - self.filled
        } else {
            120 - self.filled
        };
        let total = zeros + 8;
        let mut pad = [0u8; 128];
        pad[0] = 0x80;
        pad[zeros..total].copy_from_slice(&saved.to_be_bytes());
        self.update(&pad[..total]);
        self.length = saved;

        let mut out = [0u8; 20];
        for (i, word) in self.state.iter().enumerate() {
            out[i * 4..i * 4 + 4].copy_from_slice(&word.to_be_bytes());
        }
        out
    }

    fn compress(&mut self, block: &[u8; 64]) {
        // FIPS 180-4, section 6.1.2: w[0..16] are the block as big-endian u32s; w[i] = (w[i-3] ^
        // w[i-8] ^ w[i-14] ^ w[i-16]).rotate_left(1) for 16..80. Eighty rounds with f and k by
        // range: 0..=19 (b & c) | (!b & d), 0x5A827999; 20..=39 b ^ c ^ d, 0x6ED9EBA1; 40..=59 (b &
        // c) | (b & d) | (c & d), 0x8F1BBCDC; 60..=79 b ^ c ^ d, 0xCA62C1D6. All additions
        // wrapping. Add a..e into state.
        let mut w: [u32; 80] = [0; 80];
        let (chunks, _) = block.as_chunks::<4>();
        for (i, chunk) in chunks.iter().enumerate() {
            w[i] = u32::from_be_bytes(*chunk);
        }
        for i in 16..80 {
            w[i] = (w[i - 3] ^ w[i - 8] ^ w[i - 14] ^ w[i - 16]).rotate_left(1);
        }

        let (mut a, mut b, mut c, mut d, mut e) = (
            self.state[0],
            self.state[1],
            self.state[2],
            self.state[3],
            self.state[4],
        );
        for (round, &ww) in w.iter().enumerate() {
            let (f, k) = if round <= 19 {
                ((b & c) | (!b & d), 0x5A82_7999)
            } else if round <= 39 {
                (b ^ c ^ d, 0x6ED9_EBA1)
            } else if round <= 59 {
                ((b & c) | (b & d) | (c & d), 0x8F1B_BCDC)
            } else {
                (b ^ c ^ d, 0xCA62_C1D6)
            };
            let temp = a
                .rotate_left(5)
                .wrapping_add(f)
                .wrapping_add(e)
                .wrapping_add(k);
            e = d;
            d = c;
            c = b.rotate_left(30);
            b = a;
            a = temp.wrapping_add(ww);
        }

        self.state[0] = self.state[0].wrapping_add(a);
        self.state[1] = self.state[1].wrapping_add(b);
        self.state[2] = self.state[2].wrapping_add(c);
        self.state[3] = self.state[3].wrapping_add(d);
        self.state[4] = self.state[4].wrapping_add(e);
    }
}