#pragma once

#include <cstdint>
#include <optional>
#include <span>
#include <string>
#include <utility>

#include "wireframe/byteio.hpp"
#include "wireframe/l7/dissector.hpp"

// Hand-rolled DNS message parsing: header + the first question record.
// Answer/authority/additional records aren't decoded (not needed for a
// one-line summary), so name-compression pointers there are never
// followed - a pointer in the question section itself is rejected
// rather than chased, keeping this a pure forward scan with no risk of
// a pointer loop.
namespace wireframe::net {

inline constexpr std::uint16_t kDnsPort = 53;

struct DnsHeader {
    std::uint16_t id;
    bool is_response;
    std::uint8_t opcode;
    std::uint8_t rcode;
    std::uint16_t qdcount;
    std::uint16_t ancount;
};

struct DnsQuestion {
    std::string name;
    std::uint16_t qtype;
};

struct DnsMessage {
    DnsHeader header;
    std::optional<DnsQuestion> question;  // first question only
};

// Reads a (possibly multi-label) dotted name starting at offset.
// Returns the name and the offset just past it, or nullopt on
// truncation or a compression pointer (0xC0 prefix - valid in
// answer/authority records, not supported here).
inline std::optional<std::pair<std::string, std::size_t>> read_dns_name(
    std::span<const unsigned char> bytes, std::size_t offset) {
    std::string name;
    while (true) {
        if (offset >= bytes.size()) return std::nullopt;
        std::uint8_t len = bytes[offset];
        if (len == 0) {
            ++offset;
            break;
        }
        if ((len & 0xC0) == 0xC0) return std::nullopt;  // compression pointer: unsupported
        ++offset;
        if (offset + len > bytes.size()) return std::nullopt;
        if (!name.empty()) name += '.';
        for (std::uint8_t i = 0; i < len; ++i) name += static_cast<char>(bytes[offset + i]);
        offset += len;
    }
    return std::make_pair(std::move(name), offset);
}

inline std::optional<DnsMessage> parse_dns(std::span<const unsigned char> bytes) {
    if (bytes.size() < 12) return std::nullopt;

    DnsHeader header{};
    header.id = read_be16(bytes, 0);
    std::uint16_t flags = read_be16(bytes, 2);
    header.is_response = (flags & 0x8000) != 0;
    header.opcode = static_cast<std::uint8_t>((flags >> 11) & 0x0F);
    header.rcode = static_cast<std::uint8_t>(flags & 0x0F);
    header.qdcount = read_be16(bytes, 4);
    header.ancount = read_be16(bytes, 6);

    DnsMessage msg{header, std::nullopt};
    if (header.qdcount >= 1) {
        if (auto result = read_dns_name(bytes, 12)) {
            auto& [name, next_offset] = *result;
            if (next_offset + 4 <= bytes.size()) {
                msg.question = DnsQuestion{std::move(name), read_be16(bytes, next_offset)};
            }
        }
    }
    return msg;
}

class DnsDissector : public L7Dissector {
public:
    std::uint16_t port() const override { return kDnsPort; }

    std::optional<std::string> summarize(std::span<const unsigned char> payload) const override {
        auto msg = parse_dns(payload);
        if (!msg) return std::nullopt;

        std::string out = "DNS ";
        out += msg->header.is_response ? "response" : "query";
        out += " id=" + std::to_string(msg->header.id);
        if (msg->header.is_response) out += " ancount=" + std::to_string(msg->header.ancount);
        if (msg->question) {
            out += " " + msg->question->name + " type=" + std::to_string(msg->question->qtype);
        }
        return out;
    }
};

}  // namespace wireframe::net