/*

 * Copyright (c) 2023, kleines Filmröllchen <filmroellchen@serenityos.org>.

 *

 * SPDX-License-Identifier: BSD-2-Clause

 */

#pragma once

#include <AK/Endian.h>

#include <AK/Format.h>

#include <AK/Types.h>

#include <LibCrypto/Checksum/ChecksumFunction.h>

namespace Crypto::Checksum {

// A generic 16-bit Cyclic Redundancy Check.

// Just like CRC32, this class receives its polynomial little-endian.

// For example, the polynomial x¹⁶ + x¹² + x⁵ + 1 is represented as 0x8408.

template<u16 polynomial>

class CRC16 : public ChecksumFunction<u16> {

public:

    static constexpr u16 be_polynomial = bitswap(polynomial);

    // This is a big endian table, while CRC-32 uses a little endian table.

    static constexpr auto generate_table()

    {

        Array<u16, 256> data {};

        data[0] = 0;

        u16 value = 0x8000;

        auto i = 1u;

        do {

            if ((value & 0x8000) != 0) {

                value = be_polynomial ^ (value << 1);

            } else {

                value = value << 1;

            }

            for (auto j = 0u; j < i; ++j) {

                data[i + j] = value ^ data[j];

            }

            i <<= 1;

        } while (i < 256);

        return data;

    }

    static constexpr auto table = generate_table();

    virtual ~CRC16() = default;

    CRC16() = default;

    CRC16(ReadonlyBytes data)

    {

        update(data);

    }

    CRC16(u16 initial_state, ReadonlyBytes data)

        : m_state(initial_state)

    {

        update(data);

    }

    // FIXME: This implementation is naive and slow.

    //        Figure out how to adopt the slicing-by-8 algorithm (see CRC32) for 16-bit polynomials.

    virtual void update(ReadonlyBytes data) override

    {

        for (size_t i = 0; i < data.size(); i++) {

            size_t table_index = ((m_state >> 8) ^ data.at(i)) & 0xFF;

            m_state = (table[table_index] ^ (static_cast<u32>(m_state) << 8)) & 0xFFFF;

        }

    }

    virtual u16 digest() override

    {

        return m_state;

    }

private:

    u16 m_state { 0 };

};

}