varint.h

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// BSD 3-Clause License
// Copyright (c) 2024, 🍀☀🌕🌥 🌊
// See the LICENSE file in the project root for full license information.
 
#pragma once
 
#include "kcenon/network/detail/utils/result_types.h"
 
#include <cstdint>
#include <span>
#include <utility>
#include <vector>
 
namespace kcenon::network::protocols::quic
{
    constexpr uint64_t varint_max = 4611686018427387903ULL;
 
    class varint
    {
    public:
        [[nodiscard]] static auto encode(uint64_t value) -> std::vector<uint8_t>;
 
        [[nodiscard]] static auto encode_with_length(uint64_t value, size_t min_length)
            -> Result<std::vector<uint8_t>>;
 
        [[nodiscard]] static auto decode(std::span<const uint8_t> data)
            -> Result<std::pair<uint64_t, size_t>>;
 
        [[nodiscard]] static constexpr auto encoded_length(uint64_t value) noexcept -> size_t
        {
            if (value <= 63)
            {
                return 1;
            }
            if (value <= 16383)
            {
                return 2;
            }
            if (value <= 1073741823)
            {
                return 4;
            }
            return 8;
        }
 
        [[nodiscard]] static constexpr auto length_from_prefix(uint8_t first_byte) noexcept -> size_t
        {
            return size_t{1} << (first_byte >> 6);
        }
 
        [[nodiscard]] static constexpr auto is_valid(uint64_t value) noexcept -> bool
        {
            return value <= varint_max;
        }
 
        static constexpr uint64_t max_1byte = 63;
        static constexpr uint64_t max_2byte = 16383;
        static constexpr uint64_t max_4byte = 1073741823;
        static constexpr uint64_t max_8byte = varint_max;
 
    private:
        static constexpr uint8_t prefix_1byte = 0x00;
        static constexpr uint8_t prefix_2byte = 0x40;
        static constexpr uint8_t prefix_4byte = 0x80;
        static constexpr uint8_t prefix_8byte = 0xC0;
        static constexpr uint8_t prefix_mask = 0xC0;
        static constexpr uint8_t value_mask = 0x3F;
    };
 
} // namespace kcenon::network::protocols::quic