182 lines
6.1 KiB
C++
182 lines
6.1 KiB
C++
//-----------------------------------------------------------------------------
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// ___ __ _ _
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// / _ \__ _ _ __ ___ ___ / /(_)_ __ | | __
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// / /_)/ _` | '__/ __|/ _ \/ / | | '_ \| |/ /
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// / ___/ (_| | | \__ \ __/ /__| | | | | <
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// \/ \__,_|_| |___/\___\____/_|_| |_|_|\_\ .
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//
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//-----------------------------------------------------------------------------
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// Author: Kurt Sassenrath
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// Module: msgpack
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//
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// Default packer implementation, which aims to deduce the best format to use
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// for a given value. For example, if a 32-bit unsigned integer type only
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// contains the value 5, a uint32 format would serialize into:
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//
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// 0xce, 0x00, 0x00, 0x00, 0x05
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//
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// Instead, the packer will note that this value could be stored in a positive
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// fixint, which is simply:
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//
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// 0x05
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//
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// The same optimization will be applied to variable-length types, like strings,
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// bytes, arrays, and maps.
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//
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// This flexibility comes at the cost of CPU instructions. For embedded targets,
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// writer (to be renamed verbatim_packer in the future) may be a better choice.
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//
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// Future goals for this particular packer:
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// 1. Support containers/ranges seamlessly.
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// 2. Support packing of trivial POD structures without an explicit
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// pack_adapter.
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//
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// Copyright (c) 2023 Kurt Sassenrath.
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//
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// License TBD.
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//-----------------------------------------------------------------------------
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#ifndef msgpack_core_packer_1d5939e9c1498568
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#define msgpack_core_packer_1d5939e9c1498568
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#include "parselink/msgpack/core/error.h"
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#include "parselink/msgpack/core/format.h"
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#include "parselink/msgpack/util/endianness.h"
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#include <tl/expected.hpp>
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#include <limits>
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#include <type_traits>
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namespace msgpack {
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namespace detail {
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// This is a generic helper function for writing integral bytes.
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template <typename T, typename Itr>
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constexpr auto write_integral(T value, Itr out, std::size_t sz) noexcept {
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auto bytes = ::detail::as_bytes(host_to_be(value));
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for (std::size_t i = 0; i < sz; ++i) {
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*out++ = *(bytes.end() - sz + i);
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}
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return out;
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}
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// Depending on the format, a number of bytes will be necessary to represent
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// either the value (integer formats) or the length (variable length formats).
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template <format::type T>
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struct pack_helper {};
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template <>
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struct pack_helper<format::type::unsigned_int> {
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static constexpr std::size_t num_bytes(std::uint64_t value) noexcept {
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if (value <= std::uint64_t(format::positive_fixint::mask)) return 0;
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return std::bit_ceil(std::uint64_t((std::bit_width(value) + 7) >> 3));
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}
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static constexpr std::byte marker(std::uint64_t value) noexcept {
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switch (num_bytes(value)) {
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case 0: return static_cast<std::byte>(value);
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case 1: return format::uint8::marker;
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case 2: return format::uint16::marker;
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case 4: return format::uint32::marker;
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default: return format::uint64::marker;
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}
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}
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};
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template <>
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struct pack_helper<format::type::signed_int> {
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static constexpr std::size_t num_bytes(std::int64_t value) noexcept {
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// Probably a better way to do this.
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if (value < 0 && value >= -32) return 0;
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auto underlying = static_cast<std::uint64_t>(value);
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// save a branch; these should be cheap on modern hardware.
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std::uint64_t counts[2] = {
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static_cast<std::uint64_t>(std::countl_zero(underlying)),
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static_cast<std::uint64_t>(std::countl_one(underlying))};
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std::uint64_t width = 1 + std::numeric_limits<std::uint64_t>::digits
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- counts[underlying >> 63];
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return std::bit_ceil((width + 7) >> 3);
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}
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static constexpr std::byte marker(std::int64_t value) noexcept {
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switch (num_bytes(value)) {
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case 0: return static_cast<std::byte>(value);
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case 1: return format::int8::marker;
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case 2: return format::int16::marker;
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case 4: return format::int32::marker;
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default: return format::int64::marker;
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}
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}
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};
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template <>
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struct pack_helper<format::type::string> {
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static constexpr std::size_t num_bytes(std::uint64_t value) noexcept {
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if (value <= std::uint32_t(format::fixstr::mask)) return 0;
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return std::bit_ceil(std::uint32_t((std::bit_width(value) + 7) >> 3));
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}
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static constexpr auto marker(std::string_view value) noexcept {
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switch (num_bytes(value.size())) {
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case 0: return format::fixstr::marker | std::byte(value.size());
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case 1: return format::str8::marker;
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case 2: return format::str16::marker;
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case 4:
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default: return format::str32::marker;
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}
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}
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};
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} // namespace detail
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// Pack adapter is the basis for packing native values into MessagePack format.
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template <typename T>
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struct pack_adapter {};
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template <typename T>
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concept builtin_packable_type = requires(T const& t, std::byte* b) {
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{ pack_adapter<T>::size(t) } -> std::same_as<std::size_t>;
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{ pack_adapter<T>::write(t, b) } -> std::same_as<decltype(b)>;
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{ pack_adapter<T>::marker(t) } -> std::same_as<std::byte>;
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};
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template <typename T>
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concept packable_type = builtin_packable_type<T>;
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template <format::type F>
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struct builtin_pack_adapter {
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static constexpr auto format_type = F;
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static constexpr auto size(auto value) noexcept {
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return detail::pack_helper<F>::num_bytes(value);
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}
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static constexpr auto marker(auto value) noexcept {
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return detail::pack_helper<F>::marker(value);
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}
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};
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template <std::signed_integral T>
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struct pack_adapter<T> : builtin_pack_adapter<format::type::signed_int> {
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template <typename Itr>
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static constexpr Itr write(T value, Itr out) noexcept {
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return detail::write_integral(value, out, size(value));
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}
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};
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template <std::unsigned_integral T>
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struct pack_adapter<T> : builtin_pack_adapter<format::type::unsigned_int> {
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template <typename Itr>
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static constexpr Itr write(T value, Itr out) noexcept {
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return detail::write_integral(value, out, size(value));
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}
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};
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} // namespace msgpack
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#endif // msgpack_core_packer_1d5939e9c1498568
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