//! 32-bit CRC implementations using our polynomial types
//!
//! A cyclic redundancy check (CRC), is a common checksum algorithm
//! that is simple to implement in circuitry, and effective at detecting
//! bit-level errors.
//!
//! Looking at CRCs mathematically, they are nothing more than the remainder
//! after polynomial division by a constant, allowing for efficient
//! implementations that leverage our polynomial types and hardware-accelerated
//! carry-less multiplication.
//!
//! More information on how CRCs work can be found in [`crc`'s module-level
//! documentation][crc-mod].
//!
//! [crc-mod]: https://docs.rs/gf256/latest/gf256/crc

use std::iter;
use std::convert::TryFrom;
use ::gf256::traits::FromLossy;
use ::gf256::*;

/// This is a common polynomial for 32-bit CRCs, normally the highest
/// bit of the polynomial is omitted, so this may often be seen as just
/// 0x04c11db7
///
const POLYNOMIAL: p64 = p64(0x104c11db7);

/// A naive CRC implementation using the textbook definition of polynomial
/// remainder, the input is padded with 32-bits of zeros to represent the
/// correct polynomial.
///
/// The bit-invert of the CRC is a bit strange when mapped to the
/// textbook definition as this appears as xoring the input with
/// 32-bits of ones followed by zeros.
///
/// We also have to bit-reverse the input/output in order to match
/// the common CRC32 behavior.
///
pub fn naive_crc(data: &[u8]) -> u32 {
    let mut crc = p64(0);

    for b in
        data.iter().copied()
            // pad with 32-bits
            .chain(iter::repeat(0x00).take(4))
            // invert the first 32-bits
            .zip(iter::repeat(0xff).take(4).chain(iter::repeat(0x00)))
            .map(|(m, b)| m ^ b)
    {
        crc = (crc << 8) | p64::from(b.reverse_bits());
        crc = crc % POLYNOMIAL;
    }

    u32::try_from(crc).unwrap().reverse_bits() ^ 0xffffffff
}

/// A CRC implementation that uses the first common optimization:
/// delaying the addition of the next byte to when overflow can occur
///
pub fn less_naive_crc(data: &[u8]) -> u32 {
    let mut crc = p32(0xffffffff);

    for b in data {
        crc = crc + (p32::from(b.reverse_bits()) << 24);
        crc = p32::try_from((p64::from(crc) << 8) % POLYNOMIAL).unwrap();
    }

    u32::from(crc).reverse_bits() ^ 0xffffffff
}

/// A CRC implementation using the same technique as less_naive_crc but
/// operating on a 32-bit word at a time
///
pub fn word_less_naive_crc(data: &[u8]) -> u32 {
    let mut crc = p32(0xffffffff);

    // iterate over 4-byte words
    let mut words = data.chunks_exact(4);
    for word in &mut words {
        let word = <[u8; 4]>::try_from(word).unwrap();
        crc = crc + p32::from_le_bytes(word).reverse_bits();
        crc = p32::try_from((p64::from(crc) << 32) % POLYNOMIAL).unwrap();
    }

    for b in words.remainder() {
        crc = crc + (p32::from(b.reverse_bits()) << 24);
        crc = p32::try_from((p64::from(crc) << 8) % POLYNOMIAL).unwrap();
    }

    u32::from(crc).reverse_bits() ^ 0xffffffff
}

/// A table-based CRC implementation using precomputed remainders
/// post-addition
///
/// This requires a 4*256 = 1024 byte table (computed at compile-time thanks
/// to Rust's const evaluation), and is the most common CRC implementation
/// thanks to its portability and speed.
///
pub fn table_crc(data: &[u8]) -> u32 {
    const CRC_TABLE: [u32; 256] = {
        let mut table = [0; 256];
        let mut i = 0;
        while i < table.len() {
            let x = (i as u32).reverse_bits();
            let x = p64((x as u64) << 8).naive_rem(POLYNOMIAL).0 as u32;
            table[i] = x.reverse_bits();
            i += 1;
        }
        table
    };

    let mut crc = 0xffffffff;

    for b in data {
        crc = (crc >> 8) ^ CRC_TABLE[usize::from((crc as u8) ^ b)];
    }

    crc ^ 0xffffffff
}

/// A smaller table-based CRC implementation using 4-bit precomputed
/// remainders post-addition
///
/// This requires a 4*16 = 64 byte table (computed at compile-time thanks
/// to Rust's const evaluation), significantly reducing the code-size
/// at the cost of 2x the number of operations. This CRC implementation
/// is common on embedded systems.
///
pub fn small_table_crc(data: &[u8]) -> u32 {
    const CRC_SMALL_TABLE: [u32; 16] = {
        let mut table = [0; 16];
        let mut i = 0;
        while i < table.len() {
            let x = (i as u32).reverse_bits();
            let x = p64((x as u64) << 4).naive_rem(POLYNOMIAL).0 as u32;
            table[i] = x.reverse_bits();
            i += 1;
        }
        table
    };

    let mut crc = 0xffffffff;

    for b in data {
        crc = (crc >> 4) ^ CRC_SMALL_TABLE[usize::from(((crc as u8) ^ (b >> 0)) & 0xf)];
        crc = (crc >> 4) ^ CRC_SMALL_TABLE[usize::from(((crc as u8) ^ (b >> 4)) & 0xf)];
    }

    crc ^ 0xffffffff
}

/// A hardware-accelerated CRC implementation using Barret reduction
///
/// This leverages polynomial multiplication instructions (pclmulqdq,
/// pmull, etc) to provide an efficient CRC implementation without the need
/// of a lookup table.
///
/// You may notice that polynomial multiplication is not the polynomial
/// remainder operation needed for CRC, and that is where Barret reduction
/// comes in. Barret reduction allows you to turn division/remainder
/// by a constant into a cheaper multiply by a different constant.
///
/// Fortunately Rust makes it easy to precompute this constant at
/// compile-time.
///
pub fn barret_crc(data: &[u8]) -> u32 {
    // Normally this would be 0x10000000000000000 / __polynomial, but
    // we eagerly do one step of division so we avoid needing a 4x wide
    // type. We can also drop the highest bit if we add the high bits
    // manually we use use this constant.
    //
    // = x % p
    // = 0xffffffff & (x + p*(((x >> 32) * [0x10000000000000000/p]) >> 32))
    // = 0xffffffff & (x + p*(((x >> 32) * [(p << 32)/p + 0x100000000]) >> 32))
    // = 0xffffffff & (x + p*((((x >> 32) * [(p << 32)/p]) >> 32) + (x >> 32)))
    //                                      \-----+-----/
    //                                            '-- Barret constant
    //
    // Note that the shifts and masks can go away if we operate on u32s,
    // leaving 2 xmuls and 2 xors.
    //
    const BARRET_CONSTANT: p32 = {
        p32(p64(POLYNOMIAL.0 << 32).naive_div(POLYNOMIAL).0 as u32)
    };

    let mut crc = p32(0xffffffff);

    for b in data {
        crc = crc ^ (p32::from(b.reverse_bits()) << 24);
        crc = (crc << 8)
            + ((crc >> 24u32).widening_mul(BARRET_CONSTANT).1 + (crc >> 24u32))
                .wrapping_mul(p32::from_lossy(POLYNOMIAL));
    }

    u32::from(crc).reverse_bits() ^ 0xffffffff
}

/// A hardware-accelerated CRC implementation using the same technique as
/// barret_crc, but operating on a 32-bit word at a time
///
pub fn word_barret_crc(data: &[u8]) -> u32 {
    // Normally this would be 0x10000000000000000 / __polynomial, but
    // we eagerly do one step of division so we avoid needing a 4x wide
    // type. We can also drop the highest bit if we add the high bits
    // manually we use use this constant.
    //
    // = x % p
    // = 0xffffffff & (x + p*(((x >> 32) * [0x10000000000000000/p]) >> 32))
    // = 0xffffffff & (x + p*(((x >> 32) * [(p << 32)/p + 0x100000000]) >> 32))
    // = 0xffffffff & (x + p*((((x >> 32) * [(p << 32)/p]) >> 32) + (x >> 32)))
    //                                      \-----+-----/
    //                                            '-- Barret constant
    //
    // Note that the shifts and masks can go away if we operate on u32s,
    // leaving 2 xmuls and 2 xors.
    //
    const BARRET_CONSTANT: p32 = {
        p32(p64(POLYNOMIAL.0 << 32).naive_div(POLYNOMIAL).0 as u32)
    };

    let mut crc = p32(0xffffffff);

    // iterate over 4-byte words
    let mut words = data.chunks_exact(4);
    for word in &mut words {
        let word = <[u8; 4]>::try_from(word).unwrap();
        crc = crc ^ p32::from_le_bytes(word).reverse_bits();
        crc = (crc.widening_mul(BARRET_CONSTANT).1 + crc)
                .wrapping_mul(p32::from_lossy(POLYNOMIAL));
    }

    for b in words.remainder() {
        crc = crc ^ (p32::from(b.reverse_bits()) << 24);
        crc = (crc << 8)
            + ((crc >> 24u32).widening_mul(BARRET_CONSTANT).1 + (crc >> 24u32))
                .wrapping_mul(p32::from_lossy(POLYNOMIAL));
    }

    u32::from(crc).reverse_bits() ^ 0xffffffff
}

/// A hardware-accelerated CRC implementation using Barret reduction without
/// needing to bit-reverse the internal representation
///
/// CRC32 and polynomial multiplication instructions unfortunately are defined
/// with different bit-endianness. This would normally mean we need to
/// bit-reverse the incoming data before we can use polynomial multiplication.
///
/// However, polynomial multiplication has the odd property that it is
/// symmetric, brev(a) * brev(b) = brev((a * b) << 1)
///
/// This means we can rewrite our Barret reduction CRC to operate entirely
/// on a bit-reversed representation, shaving off several instructions.
///
/// In theory this should be faster, but measurements show this as actually
/// being slightly slower, perhaps the extra 1-bit shift costs more on
/// machines with bit-reverse instructions?
///
pub fn reversed_barret_crc(data: &[u8]) -> u32 {
    // Normally this would be 0x10000000000000000 / __polynomial, but
    // we eagerly do one step of division so we avoid needing a 4x wide
    // type. We can also drop the highest bit if we add the high bits
    // manually we use use this constant.
    //
    // = x % p
    // = 0xffffffff & (x + p*(((x >> 32) * [0x10000000000000000/p]) >> 32))
    // = 0xffffffff & (x + p*(((x >> 32) * [(p << 32)/p + 0x100000000]) >> 32))
    // = 0xffffffff & (x + p*((((x >> 32) * [(p << 32)/p]) >> 32) + (x >> 32)))
    //                                      \-----+-----/
    //                                            '-- Barret constant
    //
    // Note that the shifts and masks can go away if we operate on u32s,
    // leaving 2 xmuls and 2 xors.
    //
    const BARRET_CONSTANT: p32 = {
        p32(p64(POLYNOMIAL.0 << 32).naive_div(POLYNOMIAL).0 as u32)
    };
    const POLYNOMIAL_REV: p32 = p32(POLYNOMIAL.0 as u32).reverse_bits();
    const BARRET_CONSTANT_REV: p32 = BARRET_CONSTANT.reverse_bits();

    let mut crc = p32(0xffffffff);

    for b in data {
        crc = crc ^ p32::from(*b);
        let (lo, _) = (crc << 24u32).widening_mul(BARRET_CONSTANT_REV);
        let (lo, hi) = ((lo << 1u32) + (crc << 24u32)).widening_mul(POLYNOMIAL_REV);
        crc = (crc >> 8u32) + ((hi << 1u32) | (lo >> 31u32));
    }

    u32::from(crc) ^ 0xffffffff
}

/// A hardware-accelerated CRC implementation using the same technique as
/// reversed_barret_crc, but operating on a 32-bit word at a time
///
pub fn word_reversed_barret_crc(data: &[u8]) -> u32 {
    // Normally this would be 0x10000000000000000 / __polynomial, but
    // we eagerly do one step of division so we avoid needing a 4x wide
    // type. We can also drop the highest bit if we add the high bits
    // manually we use use this constant.
    //
    // = x % p
    // = 0xffffffff & (x + p*(((x >> 32) * [0x10000000000000000/p]) >> 32))
    // = 0xffffffff & (x + p*(((x >> 32) * [(p << 32)/p + 0x100000000]) >> 32))
    // = 0xffffffff & (x + p*((((x >> 32) * [(p << 32)/p]) >> 32) + (x >> 32)))
    //                                      \-----+-----/
    //                                            '-- Barret constant
    //
    // Note that the shifts and masks can go away if we operate on u32s,
    // leaving 2 xmuls and 2 xors.
    //
    const BARRET_CONSTANT: p32 = {
        p32(p64(POLYNOMIAL.0 << 32).naive_div(POLYNOMIAL).0 as u32)
    };
    const POLYNOMIAL_REV: p32 = p32(POLYNOMIAL.0 as u32).reverse_bits();
    const BARRET_CONSTANT_REV: p32 = BARRET_CONSTANT.reverse_bits();

    let mut crc = p32(0xffffffff);

    // iterate over 4-byte words
    let mut words = data.chunks_exact(4);
    for word in &mut words {
        let word = <[u8; 4]>::try_from(word).unwrap();
        crc = crc ^ p32::from_le_bytes(word);
        let (lo, _) = crc.widening_mul(BARRET_CONSTANT_REV);
        let (lo, hi) = ((lo << 1u32) + crc).widening_mul(POLYNOMIAL_REV);
        crc = (hi << 1u32) | (lo >> 31u32);
    }

    for b in words.remainder() {
        crc = crc ^ p32::from(*b);
        let (lo, _) = (crc << 24u32).widening_mul(BARRET_CONSTANT_REV);
        let (lo, hi) = ((lo << 1u32) + (crc << 24u32)).widening_mul(POLYNOMIAL_REV);
        crc = (crc >> 8u32) + ((hi << 1u32) | (lo >> 31u32));
    }

    u32::from(crc) ^ 0xffffffff
}


fn main() {
    let input = b"Hello World!";
    let expected = 0x1c291ca3;
    println!();
    println!("testing crc({:?})", String::from_utf8_lossy(input));

    let output = naive_crc(input);
    println!("{:<24} => 0x{:08x}", "naive_crc", output);
    assert_eq!(output, expected);

    let output = less_naive_crc(input);
    println!("{:<24} => 0x{:08x}", "less_naive_crc", output);
    assert_eq!(output, expected);

    let output = word_less_naive_crc(input);
    println!("{:<24} => 0x{:08x}", "word_less_naive_crc", output);
    assert_eq!(output, expected);

    let output = table_crc(input);
    println!("{:<24} => 0x{:08x}", "table_crc", output);
    assert_eq!(output, expected);

    let output = small_table_crc(input);
    println!("{:<24} => 0x{:08x}", "small_table_crc", output);
    assert_eq!(output, expected);

    let output = barret_crc(input);
    println!("{:<24} => 0x{:08x}", "barret_crc", output);
    assert_eq!(output, expected);

    let output = word_barret_crc(input);
    println!("{:<24} => 0x{:08x}", "word_barret_crc", output);
    assert_eq!(output, expected);

    let output = reversed_barret_crc(input);
    println!("{:<24} => 0x{:08x}", "reversed_barret_crc", output);
    assert_eq!(output, expected);

    let output = word_reversed_barret_crc(input);
    println!("{:<24} => 0x{:08x}", "word_reversed_barret_crc", output);
    assert_eq!(output, expected);

    println!();
}