//----------------------------------------------------------------------------- 
//     ___                      __ _       _
//    / _ \__ _ _ __ ___  ___  / /(_)_ __ | | __
//   / /_)/ _` | '__/ __|/ _ \/ / | | '_ \| |/ /
//  / ___/ (_| | |  \__ \  __/ /__| | | | |   <
//  \/    \__,_|_|  |___/\___\____/_|_| |_|_|\_\ .
//
//----------------------------------------------------------------------------- 
// Author: Kurt Sassenrath
// Module: msgpack
//
// Token-based reader for unpacking MessagePack data. Ensure lifetime of the
// MessagePack buffer exceeds the lifetime of parsed tokens.
//
// TBD: How best to handle arrays and maps.
//
// Copyright (c) 2023 Kurt Sassenrath.
//
// License TBD.
//----------------------------------------------------------------------------- 
#ifndef msgpack_token_reader_8daff350a0b1a519
#define msgpack_token_reader_8daff350a0b1a519

#include "type.h"
#include "../core/error.h"
#include "../util/endianness.h"
#include "../core/format.h"

#include <tl/expected.hpp>

namespace msgpack {

namespace detail {

constexpr inline decltype(auto) read(std::size_t size, auto& inp)
        noexcept {
    std::array<std::byte, token::word_size> value;
    be_to_host(inp, inp + size, std::begin(value));
    inp += size;
    return value;
}

template <format::type FormatType>
constexpr inline decltype(auto) read_value(std::size_t size, auto& inp) {
    using value_type = typename token_traits<FormatType>::storage_type;
    return token{std::bit_cast<value_type>(read(size, inp))};
}

} // namespace detail

class token_reader {
public:

    constexpr token_reader(std::span<std::byte const> src) noexcept :
        data_(src), curr_{src.begin()}, end_(src.end()) {}

    constexpr auto remaining() noexcept {
        using dist_type = decltype(std::distance(curr_, end_));
        return std::max(dist_type(0), std::distance(curr_, end_));
    }

    // Read the next token. If the reader currently points to the end of the
    // byte buffer, then end_of_message is returned, and if there is still
    // some data present in the buffer, then incomplete_message is returned,
    // potentially hinting that further buffering is required.
    constexpr tl::expected<token, error> read_one() noexcept {
        if (curr_ >= end_) {
            return tl::make_unexpected(error::end_of_message);
        }

        // Enumerate the current byte first by switch statement, then by
        // fix types.
        long int size = 0;
        auto id = *curr_;
        ++curr_;
        switch (id) {
            case format::uint8::marker:
                size = 1;
            case format::uint16::marker:
                size = 2;
            case format::uint32::marker:
                size = 3;
            case format::uint64::marker:
                size = 4;
                if (remaining() < size) {
                    return tl::make_unexpected(error::incomplete_message);
                }
                return detail::read_value<format::type::unsigned_int>(size, curr_);

            case format::int8::marker:
                size = 1;
            case format::int16::marker:
                size = 2;
            case format::int32::marker:
                size = 3;
            case format::int64::marker:
                size = 4;
                if (remaining() < size) {
                    return tl::make_unexpected(error::incomplete_message);
                }
                return detail::read_value<format::type::signed_int>(size, curr_);
        }
        return tl::make_unexpected(error::not_implemented);
    }


    // Read multiple tokens from the byte buffer. The number of tokens parsed
    // can be surmised from the returned span of tokens. If the reader
    // currently points to the end of the byte buffer, then
    // error::end_of_message is returned, and if there is not enough data to
    // fully parse a token, then incomplete_message is returned.
    constexpr tl::expected<std::span<token>, error> read_some(
            std::span<token> token_buffer) noexcept;
private:
    std::span<std::byte const> data_;
    decltype(data_)::iterator curr_;
    decltype(data_)::iterator end_;
};

} // namespace msgpack

#endif // msgpack_token_reader_8daff350a0b1a519