//SPDX-License-Identifier: LGPL-3.0-only
//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

// HTTP/1.1 wire format — RFC 9112 syntax on top of the RFC 9110 semantics
// already modelled by :HTTP (HTTPRequest / HTTPResponse are shared with the
// HTTP/3 stack, so a route handler written for one works unchanged on the
// other).
//
// Scope:
//   - request / response serialisation, with the framing headers
//     (host, content-length, transfer-encoding, connection) owned by the
//     serialiser rather than the caller
//   - an incremental parser that consumes arbitrary socket chunks and
//     surfaces one message at a time, so a single connection can carry a
//     keep-alive series (and pipelined requests)
//   - both body framings that matter in practice: content-length and
//     `Transfer-Encoding: chunked` (plus read-to-EOF for responses that
//     use neither, RFC 9112 §6.3 item 8)
//
// Deliberately rejected rather than guessed at, because getting these
// wrong is how request smuggling happens (RFC 9112 §11.2):
//   - Content-Length together with Transfer-Encoding
//   - conflicting duplicate Content-Length values
//   - whitespace between a field name and its colon
//   - CR/LF inside a field value we are asked to serialise
//
// This partition has no transport dependency; ClientHTTP1 and ListenerHTTP1
// pair it with :ClientTCP / :ListenerTCP.

export module Crafter.Network:HTTP1;
import std;
import :HTTP;

namespace Crafter::HTTP1 {
    // ---------------- Versions ----------------
    export inline constexpr std::string_view kVersion10 = "HTTP/1.0";
    export inline constexpr std::string_view kVersion11 = "HTTP/1.1";

    // ---------------- Errors ----------------
    // Thrown for any input we refuse to interpret. Callers turn this into a
    // 400 (server side) or propagate it out of Send() (client side).
    export class HTTP1ProtocolError : public std::runtime_error {
    public:
        using std::runtime_error::runtime_error;
    };

    // ---------------- Lexical helpers (RFC 9110 §5.6) ----------------
    inline bool IsTChar(unsigned char c) {
        constexpr std::string_view extra = "!#$%&'*+-.^_`|~";
        return (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')
            || extra.find(static_cast<char>(c)) != std::string_view::npos;
    }

    inline bool IsToken(std::string_view s) {
        return !s.empty() && std::ranges::all_of(s, [](char c) {
            return IsTChar(static_cast<unsigned char>(c));
        });
    }

    inline std::string ToLowerAscii(std::string_view s) {
        std::string out(s);
        std::ranges::transform(out, out.begin(), [](unsigned char c) {
            return static_cast<char>(c >= 'A' && c <= 'Z' ? c + ('a' - 'A') : c);
        });
        return out;
    }

    inline bool EqualsIgnoreCase(std::string_view a, std::string_view b) {
        return std::ranges::equal(a, b, [](unsigned char x, unsigned char y) {
            auto lower = [](unsigned char c) { return c >= 'A' && c <= 'Z' ? c + ('a' - 'A') : c; };
            return lower(x) == lower(y);
        });
    }

    // Optional whitespace — SP / HTAB only (RFC 9110 §5.6.3).
    inline std::string_view TrimOWS(std::string_view s) {
        while (!s.empty() && (s.front() == ' ' || s.front() == '\t')) s.remove_prefix(1);
        while (!s.empty() && (s.back() == ' ' || s.back() == '\t')) s.remove_suffix(1);
        return s;
    }

    // Split a comma-separated field value into trimmed tokens (#rule).
    export inline std::vector<std::string_view> SplitCommaList(std::string_view value) {
        std::vector<std::string_view> parts;
        while (!value.empty()) {
            auto comma = value.find(',');
            parts.push_back(TrimOWS(value.substr(0, comma)));
            if (comma == std::string_view::npos) break;
            value.remove_prefix(comma + 1);
        }
        std::erase_if(parts, [](std::string_view p) { return p.empty(); });
        return parts;
    }

    // Whether a comma-separated field value contains `token`, compared
    // case-insensitively — the shape of `Connection`, `Expect`,
    // `Transfer-Encoding` and friends.
    export inline bool HasToken(std::string_view fieldValue, std::string_view token) {
        return std::ranges::any_of(SplitCommaList(fieldValue), [token](std::string_view candidate) {
            return EqualsIgnoreCase(candidate, token);
        });
    }

    // ---------------- Status reason phrases ----------------
    // HTTP/1.1 keeps a reason phrase in the status line. It carries no
    // meaning (RFC 9112 §4) but some ancient clients log it, and an empty
    // one trips up a few proxies, so emit the registered text where we know
    // it and a generic class name otherwise.
    export inline std::string_view ReasonPhrase(std::string_view status) {
        static const std::unordered_map<std::string_view, std::string_view> known = {
            {"100", "Continue"}, {"101", "Switching Protocols"}, {"103", "Early Hints"},
            {"200", "OK"}, {"201", "Created"}, {"202", "Accepted"}, {"204", "No Content"},
            {"206", "Partial Content"},
            {"301", "Moved Permanently"}, {"302", "Found"}, {"303", "See Other"},
            {"304", "Not Modified"}, {"307", "Temporary Redirect"}, {"308", "Permanent Redirect"},
            {"400", "Bad Request"}, {"401", "Unauthorized"}, {"403", "Forbidden"},
            {"404", "Not Found"}, {"405", "Method Not Allowed"}, {"408", "Request Timeout"},
            {"409", "Conflict"}, {"411", "Length Required"}, {"413", "Content Too Large"},
            {"414", "URI Too Long"}, {"415", "Unsupported Media Type"},
            {"426", "Upgrade Required"}, {"429", "Too Many Requests"},
            {"431", "Request Header Fields Too Large"},
            {"500", "Internal Server Error"}, {"501", "Not Implemented"},
            {"502", "Bad Gateway"}, {"503", "Service Unavailable"}, {"504", "Gateway Timeout"},
            {"505", "HTTP Version Not Supported"},
        };
        if (auto it = known.find(status); it != known.end()) return it->second;
        if (status.size() == 3) {
            switch (status[0]) {
                case '1': return "Informational";
                case '2': return "Success";
                case '3': return "Redirection";
                case '4': return "Client Error";
                case '5': return "Server Error";
                default: break;
            }
        }
        return "Unknown";
    }

    // IMF-fixdate, the only date format an HTTP/1.1 sender may generate
    // (RFC 9110 §5.6.7). Always UTC.
    export inline std::string FormatHTTPDate(std::chrono::system_clock::time_point when) {
        return std::format("{:%a, %d %b %Y %H:%M:%S} GMT",
                           std::chrono::floor<std::chrono::seconds>(when));
    }

    // ---------------- Serialisation ----------------
    export struct RequestOptions {
        // false emits `connection: close`, telling the peer we will not
        // reuse the connection after this exchange.
        bool keepAlive = true;
    };

    export struct ResponseOptions {
        bool keepAlive = true;
        // HTTP/1.0 peers have no keep-alive by default, so reuse has to be
        // stated explicitly for them (RFC 9112 §9.3).
        bool explicitKeepAlive = false;
        // Response to HEAD: send the header section, including the
        // content-length the body *would* have had, but no body.
        bool omitBody = false;
    };

    // Headers whose value is derived from the message rather than copied
    // from the caller's map — emitting the caller's copy as well would
    // produce a duplicate, and for content-length a smuggling vector.
    inline bool IsFramingHeader(std::string_view lowerName) {
        return lowerName == "host" || lowerName == "content-length"
            || lowerName == "transfer-encoding" || lowerName == "connection";
    }

    inline void ValidateFieldName(std::string_view name) {
        if (!IsToken(name)) {
            throw HTTP1ProtocolError("invalid header field name: '" + std::string(name) + "'");
        }
    }

    // A CR or LF smuggled through a field value would split the message.
    inline void ValidateFieldValue(std::string_view name, std::string_view value) {
        if (value.find_first_of("\r\n") != std::string_view::npos) {
            throw HTTP1ProtocolError("header '" + std::string(name) + "' contains CR/LF");
        }
    }

    inline void AppendHeaders(std::string& out,
                              const std::unordered_map<std::string, std::string>& headers) {
        for (const auto& [name, value] : headers) {
            // HTTP/3 pseudo-headers have no HTTP/1.1 equivalent — they are
            // already represented by the start line.
            if (name.empty() || name.front() == ':') continue;
            std::string lower = ToLowerAscii(name);
            if (IsFramingHeader(lower)) continue;
            ValidateFieldName(lower);
            ValidateFieldValue(lower, value);
            out += lower;
            out += ": ";
            out += value;
            out += "\r\n";
        }
    }

    // Serialise a request in origin-form. `authority` becomes the mandatory
    // Host header (RFC 9112 §3.2); ClientHTTP1 fills it in from the
    // connection when the caller left it empty.
    export inline std::string SerializeRequest(const HTTPRequest& request, RequestOptions options = {}) {
        const std::string method = request.method.empty() ? std::string("GET") : request.method;
        const std::string target = request.path.empty() ? std::string("/") : request.path;
        if (!IsToken(method)) throw HTTP1ProtocolError("invalid request method: '" + method + "'");
        if (target.find_first_of(" \r\n") != std::string::npos) {
            throw HTTP1ProtocolError("invalid request target: '" + target + "'");
        }
        ValidateFieldValue("host", request.authority);

        std::string out;
        out.reserve(256 + request.headers.size() * 32 + request.body.size());
        out += method;
        out += ' ';
        out += target;
        out += " HTTP/1.1\r\n";
        out += "host: ";
        out += request.authority;
        out += "\r\n";
        AppendHeaders(out, request.headers);
        // A body always gets an explicit length; so does a method that is
        // normally expected to carry one, since some servers answer 411 for
        // a bodyless POST that omits it.
        if (!request.body.empty() || method == "POST" || method == "PUT" || method == "PATCH") {
            out += "content-length: ";
            out += std::to_string(request.body.size());
            out += "\r\n";
        }
        if (!options.keepAlive) out += "connection: close\r\n";
        out += "\r\n";
        out += request.body;
        return out;
    }

    export inline std::string SerializeResponse(const HTTPResponse& response, ResponseOptions options = {}) {
        std::string status = response.status.empty() ? std::string("200") : response.status;
        if (status.size() != 3 || !std::ranges::all_of(status, [](char c) { return c >= '0' && c <= '9'; })) {
            throw HTTP1ProtocolError("status must be three digits, got '" + status + "'");
        }

        std::string out;
        out.reserve(256 + response.headers.size() * 32 + response.body.size());
        out += "HTTP/1.1 ";
        out += status;
        out += ' ';
        out += ReasonPhrase(status);
        out += "\r\n";
        AppendHeaders(out, response.headers);
        // 1xx / 204 / 304 must not carry a content-length at all — sending
        // one makes some clients wait for a body that never arrives.
        const bool bodyless = status[0] == '1' || status == "204" || status == "304";
        if (!bodyless) {
            out += "content-length: ";
            out += std::to_string(response.body.size());
            out += "\r\n";
        }
        if (!options.keepAlive) out += "connection: close\r\n";
        else if (options.explicitKeepAlive) out += "connection: keep-alive\r\n";
        out += "\r\n";
        if (!options.omitBody && !bodyless) out += response.body;
        return out;
    }

    // The interim response a server owes a client that sent
    // `Expect: 100-continue` before it will send its body (RFC 9110 §10.1.1).
    export inline std::string SerializeContinue() {
        return "HTTP/1.1 100 Continue\r\n\r\n";
    }

    // ---------------- Incremental parser ----------------
    export enum class MessageKind { Request, Response };

    // Bounds on what we are willing to buffer before declaring the peer
    // hostile. maxBody applies to the decoded body, so it also caps a
    // chunked stream that never ends.
    export struct MessageLimits {
        std::size_t maxStartLine = 8 * 1024;
        std::size_t maxHeaderSection = 64 * 1024;
        std::uint64_t maxBody = 512ull * 1024 * 1024;
    };

    // Feed socket bytes in with Feed(); call Finish() when the peer closes
    // its send side. Complete() then reports whether a whole message is
    // available, and Take*() moves it out. Reset() rearms for the next
    // message on the same connection, keeping any bytes that already
    // belonged to it (HTTP/1.1 pipelining).
    export class MessageParser {
    public:
        explicit MessageParser(MessageKind kind, MessageLimits limits = {})
            : kind(kind), limits(limits) {}

        // Responses are framed partly by the request that provoked them: a
        // HEAD response has no body no matter what its content-length says.
        // Tell the parser before feeding it bytes.
        void SetRequestMethod(std::string_view requestMethod) {
            headRequest = EqualsIgnoreCase(requestMethod, "HEAD");
        }

        void Feed(const char* data, std::size_t size) {
            if (size != 0) buffer.append(data, size);
            Advance();
        }

        void Feed(std::span<const char> bytes) { Feed(bytes.data(), bytes.size()); }

        // The peer closed. Completes a read-to-EOF body; anything else
        // that is mid-message is a truncation error.
        void Finish() {
            eof = true;
            Advance();
        }

        bool HeadersComplete() const noexcept { return headersDone; }
        bool Complete() const noexcept { return state == State::Done; }

        // True while the peer is waiting for `100 Continue` before it sends
        // the body. Clear it with ContinueSent() once the interim response
        // is on the wire.
        bool ExpectsContinue() const noexcept { return expectContinue && !Complete(); }
        void ContinueSent() noexcept { expectContinue = false; }

        // Whether the connection may carry another message after this one.
        // Meaningful once HeadersComplete().
        bool KeepAlive() const noexcept { return keepAlive; }

        // No partial message buffered — a clean point to stop reading.
        bool AtMessageBoundary() const noexcept {
            return state == State::StartLine && pos == buffer.size() && !headersDone;
        }

        std::string_view Method() const noexcept { return method; }
        std::string_view Version() const noexcept { return version; }
        std::string_view Status() const noexcept { return status; }

        HTTPRequest TakeRequest() {
            if (state != State::Done) throw HTTP1ProtocolError("request is not complete");
            HTTPRequest request;
            request.method = std::move(method);
            request.path = std::move(path);
            request.scheme = scheme.empty() ? std::string("http") : std::move(scheme);
            // Absolute-form targets carry their own authority and win over
            // Host (RFC 9112 §3.2.2); otherwise Host supplies it. Either way
            // it lives in the named field, matching the HTTP/3 shape, so it
            // is removed from the header map.
            if (authority.empty()) {
                if (auto it = headers.find("host"); it != headers.end()) authority = it->second;
            }
            headers.erase("host");
            request.authority = std::move(authority);
            request.headers = std::move(headers);
            request.body = std::move(body);
            return request;
        }

        HTTPResponse TakeResponse() {
            if (state != State::Done) throw HTTP1ProtocolError("response is not complete");
            HTTPResponse response;
            response.status = std::move(status);
            response.headers = std::move(headers);
            response.body = std::move(body);
            return response;
        }

        // Rearm for the next message. Any bytes past the one just taken are
        // retained and parsed immediately, so a pipelined request can be
        // Complete() as soon as this returns.
        void Reset() {
            buffer.erase(0, pos);
            pos = 0;
            state = State::StartLine;
            method.clear();
            path.clear();
            scheme.clear();
            authority.clear();
            version.clear();
            status.clear();
            lastHeaderName.clear();
            headers.clear();
            body.clear();
            headerBytes = 0;
            bodyRemaining = 0;
            chunkRemaining = 0;
            bodyMode = BodyMode::None;
            headersDone = false;
            expectContinue = false;
            keepAlive = true;
            headRequest = false;
            Advance();
        }

    private:
        enum class State { StartLine, Headers, Body, ChunkSize, ChunkData, ChunkTrailingCRLF, Trailers, Done };
        enum class BodyMode { None, Length, Chunked, UntilClose };

        // Read one CRLF-terminated line. A bare LF is accepted as well: it
        // is not legal to generate, but recipients may recognise it
        // (RFC 9112 §2.2) and some embedded clients still emit it.
        std::optional<std::string_view> TryReadLine(std::size_t maxLength) {
            const std::size_t nl = buffer.find('\n', pos);
            if (nl == std::string::npos) {
                if (buffer.size() - pos > maxLength) {
                    throw HTTP1ProtocolError("line exceeds " + std::to_string(maxLength) + " bytes");
                }
                if (eof && buffer.size() != pos) {
                    throw HTTP1ProtocolError("connection closed mid-message");
                }
                return std::nullopt;
            }
            std::size_t end = nl;
            if (end > pos && buffer[end - 1] == '\r') --end;
            if (end - pos > maxLength) {
                throw HTTP1ProtocolError("line exceeds " + std::to_string(maxLength) + " bytes");
            }
            std::string_view line(buffer.data() + pos, end - pos);
            pos = nl + 1;
            return line;
        }

        void ParseRequestLine(std::string_view line) {
            const auto first = line.find(' ');
            if (first == std::string_view::npos) throw HTTP1ProtocolError("malformed request line");
            const auto second = line.find(' ', first + 1);
            if (second == std::string_view::npos) {
                // HTTP/0.9 simple-request. Not supported, and accepting it
                // would leave us unable to answer with headers.
                throw HTTP1ProtocolError("malformed request line (missing version)");
            }
            method = std::string(line.substr(0, first));
            const std::string_view target = line.substr(first + 1, second - first - 1);
            version = std::string(line.substr(second + 1));
            if (!IsToken(method)) throw HTTP1ProtocolError("invalid method token");
            if (target.empty()) throw HTTP1ProtocolError("empty request target");
            CheckVersion();

            if (target.front() == '/') {
                path = std::string(target);                     // origin-form
            } else if (target == "*") {
                path = "*";                                     // asterisk-form (OPTIONS)
            } else if (const auto sep = target.find("://"); sep != std::string_view::npos) {
                // absolute-form, as sent to a proxy.
                scheme = ToLowerAscii(target.substr(0, sep));
                const std::string_view rest = target.substr(sep + 3);
                const auto slash = rest.find('/');
                authority = std::string(rest.substr(0, slash));
                path = slash == std::string_view::npos ? std::string("/") : std::string(rest.substr(slash));
            } else {
                // authority-form — only legal for CONNECT, which this
                // listener does not implement.
                authority = std::string(target);
                path = std::string(target);
            }
        }

        void ParseStatusLine(std::string_view line) {
            const auto first = line.find(' ');
            if (first == std::string_view::npos) throw HTTP1ProtocolError("malformed status line");
            version = std::string(line.substr(0, first));
            CheckVersion();
            std::string_view rest = TrimOWS(line.substr(first + 1));
            const auto space = rest.find(' ');
            status = std::string(space == std::string_view::npos ? rest : rest.substr(0, space));
            if (status.size() != 3
                || !std::ranges::all_of(status, [](char c) { return c >= '0' && c <= '9'; })) {
                throw HTTP1ProtocolError("malformed status code '" + status + "'");
            }
            // The reason phrase is discarded: it carries no semantics and
            // HTTPResponse has nowhere to put it.
        }

        void CheckVersion() {
            if (version != kVersion11 && version != kVersion10) {
                throw HTTP1ProtocolError("unsupported HTTP version '" + version + "'");
            }
        }

        void AddHeader(std::string name, std::string_view value) {
            auto [it, inserted] = headers.try_emplace(std::move(name), std::string(value));
            if (!inserted) {
                // Repeated field lines are equivalent to one comma-joined
                // value (RFC 9110 §5.3) — except content-length, where
                // disagreeing values are a smuggling attempt.
                if (it->first == "content-length" && it->second != value) {
                    throw HTTP1ProtocolError("conflicting content-length values");
                }
                if (it->first != "content-length") {
                    it->second += ", ";
                    it->second += value;
                }
            }
        }

        void ParseHeaderLine(std::string_view line) {
            headerBytes += line.size() + 2;
            if (headerBytes > limits.maxHeaderSection) {
                throw HTTP1ProtocolError("header section exceeds "
                                         + std::to_string(limits.maxHeaderSection) + " bytes");
            }
            if (line.front() == ' ' || line.front() == '\t') {
                // obs-fold. RFC 9112 §5.2 allows replacing it with a space
                // rather than rejecting the message.
                if (lastHeaderName.empty()) throw HTTP1ProtocolError("obs-fold before any header");
                auto it = headers.find(lastHeaderName);
                if (it != headers.end()) {
                    it->second += ' ';
                    it->second += TrimOWS(line);
                }
                return;
            }
            const auto colon = line.find(':');
            if (colon == std::string_view::npos) throw HTTP1ProtocolError("header line has no colon");
            const std::string_view rawName = line.substr(0, colon);
            // "No whitespace is allowed between the field name and colon" —
            // a recipient MUST reject such a message (RFC 9112 §5.1).
            if (!IsToken(rawName)) throw HTTP1ProtocolError("invalid header field name");
            lastHeaderName = ToLowerAscii(rawName);
            AddHeader(lastHeaderName, TrimOWS(line.substr(colon + 1)));
        }

        std::optional<std::string_view> Header(std::string_view name) const {
            auto it = headers.find(std::string(name));
            if (it == headers.end()) return std::nullopt;
            return std::string_view(it->second);
        }

        void ResolveKeepAlive() {
            const auto connection = Header("connection");
            const bool close = connection && HasToken(*connection, "close");
            const bool explicitKeep = connection && HasToken(*connection, "keep-alive");
            keepAlive = version == kVersion11 ? !close : (explicitKeep && !close);
        }

        void ResolveFraming() {
            const auto transferEncoding = Header("transfer-encoding");
            const auto contentLength = Header("content-length");
            if (transferEncoding && contentLength) {
                // Ambiguous framing — the classic smuggling primitive.
                throw HTTP1ProtocolError("both content-length and transfer-encoding present");
            }

            if (transferEncoding) {
                auto codings = SplitCommaList(*transferEncoding);
                if (codings.empty() || !EqualsIgnoreCase(codings.back(), "chunked")) {
                    throw HTTP1ProtocolError("unsupported transfer-encoding '"
                                             + std::string(*transferEncoding) + "'");
                }
                if (codings.size() > 1) {
                    throw HTTP1ProtocolError("stacked transfer-codings are not supported");
                }
                bodyMode = BodyMode::Chunked;
                return;
            }

            if (contentLength) {
                if (contentLength->empty()
                    || !std::ranges::all_of(*contentLength, [](char c) { return c >= '0' && c <= '9'; })) {
                    throw HTTP1ProtocolError("malformed content-length '"
                                             + std::string(*contentLength) + "'");
                }
                std::uint64_t length = 0;
                const auto result = std::from_chars(contentLength->data(),
                                                    contentLength->data() + contentLength->size(), length);
                if (result.ec != std::errc{}) throw HTTP1ProtocolError("content-length out of range");
                if (length > limits.maxBody) {
                    throw HTTP1ProtocolError("body exceeds " + std::to_string(limits.maxBody) + " bytes");
                }
                bodyRemaining = length;
                bodyMode = length == 0 ? BodyMode::None : BodyMode::Length;
                return;
            }

            // No explicit framing. A request has no body; a response runs
            // to connection close (RFC 9112 §6.3).
            if (kind == MessageKind::Request) {
                bodyMode = BodyMode::None;
            } else {
                bodyMode = BodyMode::UntilClose;
                keepAlive = false;
            }
        }

        // Returns false when this was an interim (1xx) response and parsing
        // must restart on the next status line.
        bool FinishHeaders() {
            ResolveKeepAlive();

            if (kind == MessageKind::Response) {
                if (status[0] == '1') {
                    // Interim response (100 Continue, 103 Early Hints, …).
                    // It has no body and is not the answer to the request,
                    // so drop it and keep reading.
                    headers.clear();
                    lastHeaderName.clear();
                    status.clear();
                    version.clear();
                    headerBytes = 0;
                    state = State::StartLine;
                    return false;
                }
                if (status == "204" || status == "304" || headRequest) {
                    bodyMode = BodyMode::None;
                    headersDone = true;
                    state = State::Done;
                    return true;
                }
            } else if (auto expect = Header("expect")) {
                expectContinue = HasToken(*expect, "100-continue");
            }

            ResolveFraming();
            headersDone = true;
            switch (bodyMode) {
                case BodyMode::None:       state = State::Done;      break;
                case BodyMode::Length:     state = State::Body;      break;
                case BodyMode::Chunked:    state = State::ChunkSize; break;
                case BodyMode::UntilClose: state = State::Body;      break;
            }
            return true;
        }

        void AppendBody(const char* data, std::size_t size) {
            if (body.size() + size > limits.maxBody) {
                throw HTTP1ProtocolError("body exceeds " + std::to_string(limits.maxBody) + " bytes");
            }
            body.append(data, size);
        }

        // Drive the state machine as far as the buffered bytes allow, then
        // drop what has been consumed so a long-lived connection (or a
        // large body arriving in many chunks) doesn't grow the buffer
        // without bound.
        void Advance() {
            AdvanceState();
            if (pos != 0 && pos == buffer.size()) {
                buffer.clear();
                pos = 0;
            }
        }

        void AdvanceState() {
            for (;;) {
                switch (state) {
                    case State::StartLine: {
                        auto line = TryReadLine(limits.maxStartLine);
                        if (!line) return;
                        // A server should tolerate stray empty lines left
                        // over from a previous message (RFC 9112 §2.2).
                        if (line->empty()) continue;
                        if (kind == MessageKind::Request) ParseRequestLine(*line);
                        else ParseStatusLine(*line);
                        state = State::Headers;
                        continue;
                    }
                    case State::Headers: {
                        auto line = TryReadLine(limits.maxHeaderSection);
                        if (!line) return;
                        if (line->empty()) {
                            FinishHeaders();
                            continue;
                        }
                        ParseHeaderLine(*line);
                        continue;
                    }
                    case State::Body: {
                        const std::size_t available = buffer.size() - pos;
                        if (bodyMode == BodyMode::UntilClose) {
                            AppendBody(buffer.data() + pos, available);
                            pos = buffer.size();
                            if (!eof) return;
                            state = State::Done;
                            continue;
                        }
                        const std::size_t take = static_cast<std::size_t>(
                            std::min<std::uint64_t>(available, bodyRemaining));
                        AppendBody(buffer.data() + pos, take);
                        pos += take;
                        bodyRemaining -= take;
                        if (bodyRemaining != 0) {
                            if (eof) throw HTTP1ProtocolError("connection closed mid-body");
                            return;
                        }
                        state = State::Done;
                        continue;
                    }
                    case State::ChunkSize: {
                        auto line = TryReadLine(limits.maxStartLine);
                        if (!line) return;
                        // Chunk extensions after ';' are legal and ignorable.
                        std::string_view digits = TrimOWS(line->substr(0, line->find(';')));
                        if (digits.empty()) throw HTTP1ProtocolError("empty chunk size");
                        std::uint64_t size = 0;
                        const auto result = std::from_chars(digits.data(), digits.data() + digits.size(),
                                                            size, 16);
                        if (result.ec != std::errc{} || result.ptr != digits.data() + digits.size()) {
                            throw HTTP1ProtocolError("malformed chunk size '" + std::string(digits) + "'");
                        }
                        if (size == 0) {
                            state = State::Trailers;
                            continue;
                        }
                        chunkRemaining = size;
                        state = State::ChunkData;
                        continue;
                    }
                    case State::ChunkData: {
                        const std::size_t available = buffer.size() - pos;
                        if (available == 0) {
                            if (eof) throw HTTP1ProtocolError("connection closed mid-chunk");
                            return;
                        }
                        const std::size_t take = static_cast<std::size_t>(
                            std::min<std::uint64_t>(available, chunkRemaining));
                        AppendBody(buffer.data() + pos, take);
                        pos += take;
                        chunkRemaining -= take;
                        if (chunkRemaining != 0) {
                            if (eof) throw HTTP1ProtocolError("connection closed mid-chunk");
                            return;
                        }
                        state = State::ChunkTrailingCRLF;
                        continue;
                    }
                    case State::ChunkTrailingCRLF: {
                        auto line = TryReadLine(2);
                        if (!line) return;
                        if (!line->empty()) throw HTTP1ProtocolError("chunk not terminated by CRLF");
                        state = State::ChunkSize;
                        continue;
                    }
                    case State::Trailers: {
                        auto line = TryReadLine(limits.maxHeaderSection);
                        if (!line) return;
                        if (line->empty()) {
                            state = State::Done;
                            continue;
                        }
                        // Trailer fields are merged into the header map;
                        // callers see one flat view of the message.
                        ParseHeaderLine(*line);
                        continue;
                    }
                    case State::Done:
                        return;
                }
            }
        }

        MessageKind kind;
        MessageLimits limits;
        State state = State::StartLine;
        BodyMode bodyMode = BodyMode::None;

        std::string buffer;
        std::size_t pos = 0;
        bool eof = false;

        std::string method;
        std::string path;
        std::string scheme;
        std::string authority;
        std::string version;
        std::string status;
        std::string lastHeaderName;
        std::unordered_map<std::string, std::string> headers;
        std::string body;

        std::size_t headerBytes = 0;
        std::uint64_t bodyRemaining = 0;
        std::uint64_t chunkRemaining = 0;
        bool headersDone = false;
        bool expectContinue = false;
        bool keepAlive = true;
        bool headRequest = false;
    };
}