// SPDX-License-Identifier: CC0-1.0

#![allow(clippy::unreadable_literal)]
#![allow(clippy::inline_always)]
#![allow(clippy::too_many_lines)]
#![allow(clippy::many_single_char_names)]
#![allow(clippy::cast_ptr_alignment)]

//! SHA3-256 from the family of hashes based on the Keccak permutation function.

// The Keccak permutation function is defined by five functions and a state array of N-bits,
// commonly 1600 bits. These 1600 bits are often divided into a 5x5 matrix of `u64` little endian
// numbers, commonly called a "lane." A single Keccak round comprises of a theta, rho, pi, chi, and iota step. Each of these
// steps are easily performed on a lane. Keccakf1600 is a function that performs a number of Keccak rounds, each defined with
// a different round-constant.
//
// SHA3-256 is a hash function that accepts arbitrary data and alters a Keccak state via the
// Keccakf1600 function. Data is chunked into fixed-sizes slices, called padded messages, each with
// a size of "bitrate" or "rate" for short. Each padded message block is XOR'd into parts of the
// state array, followed by a call of Keccakf1600. To pad the final message block, a
// domain-specific identifier is appended to the message (`0x06`), followed by an XOR of the last
// byte with `0x80`.
//
// To read this file, follow the example code: https://keccak.team/keccak_specs_summary.html
// For a detailed specification: https://keccak.team/files/Keccak-reference-3.0.pdf

use core::fmt;

crate::internal_macros::general_hash_type! {
    /// Output of the SHA3-256 hash function.
    pub struct Hash([u8; 32]);

    const DISPLAY_BACKWARD: bool = false;
}
// The number of rows or columns.
const B: usize = 5;
// 1600 bits are divided into 25, 64-bit "lanes."
const NUM_LANES: usize = B * B;
// Let the word size be 64. Let 2^l = 64, then l is 6. In Keccak, the number of rounds is 12 + 2l.
const NUM_ROUNDS: usize = 24;
// The number of bytes "absorbed" into the state-array per message block.
const RATE: usize = 136;
// The number of lanes a message block may be divided into.
const RATE_LANES: usize = RATE / 8;

// These create non-linear relations between rounds to avoid timing analysis.
const ROUND_CONSTANTS: [u64; NUM_ROUNDS] = [
    0x0000000000000001,
    0x0000000000008082,
    0x800000000000808A,
    0x8000000080008000,
    0x000000000000808B,
    0x0000000080000001,
    0x8000000080008081,
    0x8000000000008009,
    0x000000000000008A,
    0x0000000000000088,
    0x0000000080008009,
    0x000000008000000A,
    0x000000008000808B,
    0x800000000000008B,
    0x8000000000008089,
    0x8000000000008003,
    0x8000000000008002,
    0x8000000000000080,
    0x000000000000800A,
    0x800000008000000A,
    0x8000000080008081,
    0x8000000000008080,
    0x0000000080000001,
    0x8000000080008008,
];

const ROTATION_OFFSETS: [[u32; 5]; 5] = [
    [0, 36, 3, 41, 18],
    [1, 44, 10, 45, 2],
    [62, 6, 43, 15, 61],
    [28, 55, 25, 21, 56],
    [27, 20, 39, 8, 14],
];

// A `x` and `y` index into a flattened matrix.
#[inline(always)]
const fn ind(x: usize, y: usize) -> usize { x + B * y }

// A flattened 5x5 matrix of little-endian `u64`.
#[derive(Clone, Default)]
struct KeccakState([u64; NUM_LANES]);

// A row-column labeled output of the current state.
impl fmt::Debug for KeccakState {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        for y in 0..B {
            for x in 0..B {
                writeln!(f, "[{},{}]: {:016x}", x, y, self.lane(x, y).to_le())?;
            }
        }
        Ok(())
    }
}

impl KeccakState {
    const fn new() -> Self { Self([0u64; NUM_LANES]) }

    #[inline(always)]
    fn assign(&mut self, x: usize, y: usize, val: u64) { self.0[ind(x, y)] = val; }

    #[inline(always)]
    const fn lane(&self, x: usize, y: usize) -> u64 { self.0[ind(x, y)] }

    #[inline(always)]
    const fn column_xor(&self, x: usize) -> u64 {
        self.0[ind(x, 0)]
            ^ self.0[ind(x, 1)]
            ^ self.0[ind(x, 2)]
            ^ self.0[ind(x, 3)]
            ^ self.0[ind(x, 4)]
    }

    #[inline(always)]
    fn xor_assign(&mut self, x: usize, y: usize, val: u64) { self.0[ind(x, y)] ^= val }

    #[inline(always)]
    const fn chi(&self, x: usize, y: usize) -> u64 {
        self.lane(x, y) ^ (!self.lane((x + 1) % B, y) & self.lane((x + 2) % B, y))
    }
}

fn keccak_round(state: &mut KeccakState, round_constant: u64) {
    // Theta
    let mut c: [u64; B] = [0; B];
    (0..B).for_each(|x| {
        c[x] = state.column_xor(x);
    });
    let mut d: [u64; B] = [0; B];
    (0..B).for_each(|x| {
        // Avoid an underflow here with a mod trick
        d[x] = c[(x + B - 1) % B] ^ (c[(x + 1) % B].rotate_left(1));
    });
    (0..B).for_each(|x| {
        (0..B).for_each(|y| {
            state.xor_assign(x, y, d[x]);
        });
    });

    // Rho and Pi combined
    let mut b = KeccakState::default();
    (0..B).for_each(|x| {
        (0..B).for_each(|y| {
            let offset = ROTATION_OFFSETS[x][y];
            let val = state.lane(x, y).rotate_left(offset);
            let b_y = ((2 * x) + (3 * y)) % B;
            b.assign(y, b_y, val);
        });
    });

    // Chi
    (0..B).for_each(|x| {
        (0..B).for_each(|y| {
            state.assign(x, y, b.chi(x, y));
        });
    });

    // Iota
    state.xor_assign(0, 0, round_constant);
}

fn keccakf1600(state: &mut KeccakState) {
    for c in ROUND_CONSTANTS {
        keccak_round(state, c);
    }
}

/// Engine to compute the SHA3-256 hash function.
#[derive(Debug, Clone)]
pub struct HashEngine {
    h: KeccakState,
    bytes_hashed: u64,
    buffer: [u8; RATE],
}

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

impl HashEngine {
    /// Constructs a new SHA3-256 hash engine.
    pub const fn new() -> Self {
        Self { h: KeccakState::new(), bytes_hashed: 0, buffer: [0; RATE] }
    }

    fn absorb(state: &mut KeccakState, block: &[u8]) {
        for lane in 0..RATE_LANES {
            let x = lane % 5;
            let y = lane / 5;
            let mut pad_block = [0u8; 8];
            pad_block.copy_from_slice(&block[8 * lane..8 * lane + 8]);
            let shuffle = u64::from_le_bytes(pad_block);
            state.xor_assign(x, y, shuffle);
        }
    }

    fn process_blocks(state: &mut KeccakState, blocks: &[u8]) {
        debug_assert!(!blocks.is_empty() && blocks.len() % RATE == 0);

        for block in blocks.chunks_exact(RATE) {
            Self::absorb(state, block);
            keccakf1600(state);
        }
    }
}

impl crate::HashEngine for HashEngine {
    type Hash = Hash;
    const BLOCK_SIZE: usize = RATE;

    crate::internal_macros::engine_input_impl!();

    fn n_bytes_hashed(&self) -> u64 { self.bytes_hashed }

    fn finalize(mut self) -> Self::Hash {
        let incomplete_block_len = crate::incomplete_block_len(&self);
        self.buffer[incomplete_block_len + 1..].fill(0);
        self.buffer[incomplete_block_len] = 0x06;
        self.buffer[RATE - 1] ^= 0x80;
        Self::process_blocks(&mut self.h, &self.buffer);

        let mut out = [0u8; 32];
        out[..8].copy_from_slice(&self.h.lane(0, 0).to_le_bytes());
        out[8..16].copy_from_slice(&self.h.lane(1, 0).to_le_bytes());
        out[16..24].copy_from_slice(&self.h.lane(2, 0).to_le_bytes());
        out[24..].copy_from_slice(&self.h.lane(3, 0).to_le_bytes());
        Hash(out)
    }
}
