#pragma once

#include <cstdint>
#include <optional>
#include <span>

#include "wireframe/byteio.hpp"

namespace wireframe::net {

// Lower 6 bits of the flags byte: URG ACK PSH RST SYN FIN. CWR/ECE (the
// top 2 bits) are masked off - not needed for now.
inline constexpr std::uint8_t kTcpFin = 0x01;
inline constexpr std::uint8_t kTcpSyn = 0x02;
inline constexpr std::uint8_t kTcpRst = 0x04;
inline constexpr std::uint8_t kTcpPsh = 0x08;
inline constexpr std::uint8_t kTcpAck = 0x10;
inline constexpr std::uint8_t kTcpUrg = 0x20;

struct TcpHeader {
    std::uint16_t src_port;
    std::uint16_t dst_port;
    std::uint32_t seq;
    std::uint32_t ack;
    std::uint8_t data_offset;  // header length in 32-bit words
    std::uint8_t flags;
    std::uint16_t window;
};

struct TcpSegment {
    TcpHeader header;
    std::span<const unsigned char> payload;
};

inline std::optional<TcpSegment> parse_tcp(std::span<const unsigned char> bytes) {
    if (bytes.size() < 20) return std::nullopt;

    std::uint8_t data_offset = static_cast<std::uint8_t>(bytes[12] >> 4);
    std::size_t header_len = static_cast<std::size_t>(data_offset) * 4;
    if (header_len < 20 || bytes.size() < header_len) return std::nullopt;

    TcpHeader header{};
    header.src_port = read_be16(bytes, 0);
    header.dst_port = read_be16(bytes, 2);
    header.seq = read_be32(bytes, 4);
    header.ack = read_be32(bytes, 8);
    header.data_offset = data_offset;
    header.flags = bytes[13] & 0x3F;
    header.window = read_be16(bytes, 14);

    return TcpSegment{header, bytes.subspan(header_len)};
}

}  // namespace wireframe::net