pub const Request = struct {
    uri: Uri,
    client: *Client,
    /// This is null when the connection is released.
    connection: ?*Connection,
    keep_alive: bool,

    method: http.Method,
    version: http.Version = .@"HTTP/1.1",
    transfer_encoding: RequestTransfer,
    redirect_behavior: RedirectBehavior,

    /// Whether the request should handle a 100-continue response before sending the request body.
    handle_continue: bool,

    /// The response associated with this request.
    ///
    /// This field is undefined until `wait` is called.
    response: Response,

    /// Standard headers that have default, but overridable, behavior.
    headers: Headers,

    /// These headers are kept including when following a redirect to a
    /// different domain.
    /// Externally-owned; must outlive the Request.
    extra_headers: []const http.Header,

    /// These headers are stripped when following a redirect to a different
    /// domain.
    /// Externally-owned; must outlive the Request.
    privileged_headers: []const http.Header,

    pub const Headers = struct {
        host: Value = .default,
        authorization: Value = .default,
        user_agent: Value = .default,
        connection: Value = .default,
        accept_encoding: Value = .default,
        content_type: Value = .default,

        pub const Value = union(enum) {
            default,
            omit,
            override: []const u8,
        };
    };

    /// Any value other than `not_allowed` or `unhandled` means that integer represents
    /// how many remaining redirects are allowed.
    pub const RedirectBehavior = enum(u16) {
        /// The next redirect will cause an error.
        not_allowed = 0,
        /// Redirects are passed to the client to analyze the redirect response
        /// directly.
        unhandled = std.math.maxInt(u16),
        _,

        pub fn subtractOne(rb: *RedirectBehavior) void {
            switch (rb.*) {
                .not_allowed => unreachable,
                .unhandled => unreachable,
                _ => rb.* = @enumFromInt(@intFromEnum(rb.*) - 1),
            }
        }

        pub fn remaining(rb: RedirectBehavior) u16 {
            assert(rb != .unhandled);
            return @intFromEnum(rb);
        }
    };

    /// Frees all resources associated with the request.
    pub fn deinit(req: *Request) void {
        if (req.connection) |connection| {
            if (!req.response.parser.done) {
                // If the response wasn't fully read, then we need to close the connection.
                connection.closing = true;
            }
            req.client.connection_pool.release(req.client.allocator, connection);
        }
        req.* = undefined;
    }

    // This function must deallocate all resources associated with the request,
    // or keep those which will be used.
    // This needs to be kept in sync with deinit and request.
    fn redirect(req: *Request, uri: Uri) !void {
        assert(req.response.parser.done);

        req.client.connection_pool.release(req.client.allocator, req.connection.?);
        req.connection = null;

        var server_header: std.heap.FixedBufferAllocator = .init(req.response.parser.header_bytes_buffer);
        defer req.response.parser.header_bytes_buffer = server_header.buffer[server_header.end_index..];
        const protocol, const valid_uri = try validateUri(uri, server_header.allocator());

        const new_host = valid_uri.host.?.raw;
        const prev_host = req.uri.host.?.raw;
        const keep_privileged_headers =
            std.ascii.eqlIgnoreCase(valid_uri.scheme, req.uri.scheme) and
            std.ascii.endsWithIgnoreCase(new_host, prev_host) and
            (new_host.len == prev_host.len or new_host[new_host.len - prev_host.len - 1] == '.');
        if (!keep_privileged_headers) {
            // When redirecting to a different domain, strip privileged headers.
            req.privileged_headers = &.{};
        }

        if (switch (req.response.status) {
            .see_other => true,
            .moved_permanently, .found => req.method == .POST,
            else => false,
        }) {
            // A redirect to a GET must change the method and remove the body.
            req.method = .GET;
            req.transfer_encoding = .none;
            req.headers.content_type = .omit;
        }

        if (req.transfer_encoding != .none) {
            // The request body has already been sent. The request is
            // still in a valid state, but the redirect must be handled
            // manually.
            return error.RedirectRequiresResend;
        }

        req.uri = valid_uri;
        req.connection = try req.client.connect(new_host, uriPort(valid_uri, protocol), protocol);
        req.redirect_behavior.subtractOne();
        req.response.parser.reset();

        req.response = .{
            .version = undefined,
            .status = undefined,
            .reason = undefined,
            .keep_alive = undefined,
            .parser = req.response.parser,
        };
    }

    pub const SendError = Connection.WriteError || error{ InvalidContentLength, UnsupportedTransferEncoding };

    /// Send the HTTP request headers to the server.
    pub fn send(req: *Request) SendError!void {
        if (!req.method.requestHasBody() and req.transfer_encoding != .none)
            return error.UnsupportedTransferEncoding;

        const connection = req.connection.?;
        const w = connection.writer();

        try req.method.write(w);
        try w.writeByte(' ');

        if (req.method == .CONNECT) {
            try req.uri.writeToStream(.{ .authority = true }, w);
        } else {
            try req.uri.writeToStream(.{
                .scheme = connection.proxied,
                .authentication = connection.proxied,
                .authority = connection.proxied,
                .path = true,
                .query = true,
            }, w);
        }
        try w.writeByte(' ');
        try w.writeAll(@tagName(req.version));
        try w.writeAll("\r\n");

        if (try emitOverridableHeader("host: ", req.headers.host, w)) {
            try w.writeAll("host: ");
            try req.uri.writeToStream(.{ .authority = true }, w);
            try w.writeAll("\r\n");
        }

        if (try emitOverridableHeader("authorization: ", req.headers.authorization, w)) {
            if (req.uri.user != null or req.uri.password != null) {
                try w.writeAll("authorization: ");
                const authorization = try connection.allocWriteBuffer(
                    @intCast(basic_authorization.valueLengthFromUri(req.uri)),
                );
                assert(basic_authorization.value(req.uri, authorization).len == authorization.len);
                try w.writeAll("\r\n");
            }
        }

        if (try emitOverridableHeader("user-agent: ", req.headers.user_agent, w)) {
            try w.writeAll("user-agent: zig/");
            try w.writeAll(builtin.zig_version_string);
            try w.writeAll(" (std.http)\r\n");
        }

        if (try emitOverridableHeader("connection: ", req.headers.connection, w)) {
            if (req.keep_alive) {
                try w.writeAll("connection: keep-alive\r\n");
            } else {
                try w.writeAll("connection: close\r\n");
            }
        }

        if (try emitOverridableHeader("accept-encoding: ", req.headers.accept_encoding, w)) {
            // https://github.com/ziglang/zig/issues/18937
            //try w.writeAll("accept-encoding: gzip, deflate, zstd\r\n");
            try w.writeAll("accept-encoding: gzip, deflate\r\n");
        }

        switch (req.transfer_encoding) {
            .chunked => try w.writeAll("transfer-encoding: chunked\r\n"),
            .content_length => |len| try w.print("content-length: {d}\r\n", .{len}),
            .none => {},
        }

        if (try emitOverridableHeader("content-type: ", req.headers.content_type, w)) {
            // The default is to omit content-type if not provided because
            // "application/octet-stream" is redundant.
        }

        for (req.extra_headers) |header| {
            assert(header.name.len != 0);

            try w.writeAll(header.name);
            try w.writeAll(": ");
            try w.writeAll(header.value);
            try w.writeAll("\r\n");
        }

        if (connection.proxied) proxy: {
            const proxy = switch (connection.protocol) {
                .plain => req.client.http_proxy,
                .tls => req.client.https_proxy,
            } orelse break :proxy;

            const authorization = proxy.authorization orelse break :proxy;
            try w.writeAll("proxy-authorization: ");
            try w.writeAll(authorization);
            try w.writeAll("\r\n");
        }

        try w.writeAll("\r\n");

        try connection.flush();
    }

    /// Returns true if the default behavior is required, otherwise handles
    /// writing (or not writing) the header.
    fn emitOverridableHeader(prefix: []const u8, v: Headers.Value, w: anytype) !bool {
        switch (v) {
            .default => return true,
            .omit => return false,
            .override => |x| {
                try w.writeAll(prefix);
                try w.writeAll(x);
                try w.writeAll("\r\n");
                return false;
            },
        }
    }

    const TransferReadError = Connection.ReadError || proto.HeadersParser.ReadError;

    const TransferReader = std.io.Reader(*Request, TransferReadError, transferRead);

    fn transferReader(req: *Request) TransferReader {
        return .{ .context = req };
    }

    fn transferRead(req: *Request, buf: []u8) TransferReadError!usize {
        if (req.response.parser.done) return 0;

        var index: usize = 0;
        while (index == 0) {
            const amt = try req.response.parser.read(req.connection.?, buf[index..], req.response.skip);
            if (amt == 0 and req.response.parser.done) break;
            index += amt;
        }

        return index;
    }

    pub const WaitError = RequestError || SendError || TransferReadError ||
        proto.HeadersParser.CheckCompleteHeadError || Response.ParseError ||
        error{
            TooManyHttpRedirects,
            RedirectRequiresResend,
            HttpRedirectLocationMissing,
            HttpRedirectLocationInvalid,
            CompressionInitializationFailed,
            CompressionUnsupported,
        };

    /// Waits for a response from the server and parses any headers that are sent.
    /// This function will block until the final response is received.
    ///
    /// If handling redirects and the request has no payload, then this
    /// function will automatically follow redirects. If a request payload is
    /// present, then this function will error with
    /// error.RedirectRequiresResend.
    ///
    /// Must be called after `send` and, if any data was written to the request
    /// body, then also after `finish`.
    pub fn wait(req: *Request) WaitError!void {
        while (true) {
            // This while loop is for handling redirects, which means the request's
            // connection may be different than the previous iteration. However, it
            // is still guaranteed to be non-null with each iteration of this loop.
            const connection = req.connection.?;

            while (true) { // read headers
                try connection.fill();

                const nchecked = try req.response.parser.checkCompleteHead(connection.peek());
                connection.drop(@intCast(nchecked));

                if (req.response.parser.state.isContent()) break;
            }

            try req.response.parse(req.response.parser.get());

            if (req.response.status == .@"continue") {
                // We're done parsing the continue response; reset to prepare
                // for the real response.
                req.response.parser.done = true;
                req.response.parser.reset();

                if (req.handle_continue)
                    continue;

                return; // we're not handling the 100-continue
            }

            // we're switching protocols, so this connection is no longer doing http
            if (req.method == .CONNECT and req.response.status.class() == .success) {
                connection.closing = false;
                req.response.parser.done = true;
                return; // the connection is not HTTP past this point
            }

            connection.closing = !req.response.keep_alive or !req.keep_alive;

            // Any response to a HEAD request and any response with a 1xx
            // (Informational), 204 (No Content), or 304 (Not Modified) status
            // code is always terminated by the first empty line after the
            // header fields, regardless of the header fields present in the
            // message.
            if (req.method == .HEAD or req.response.status.class() == .informational or
                req.response.status == .no_content or req.response.status == .not_modified)
            {
                req.response.parser.done = true;
                return; // The response is empty; no further setup or redirection is necessary.
            }

            switch (req.response.transfer_encoding) {
                .none => {
                    if (req.response.content_length) |cl| {
                        req.response.parser.next_chunk_length = cl;

                        if (cl == 0) req.response.parser.done = true;
                    } else {
                        // read until the connection is closed
                        req.response.parser.next_chunk_length = std.math.maxInt(u64);
                    }
                },
                .chunked => {
                    req.response.parser.next_chunk_length = 0;
                    req.response.parser.state = .chunk_head_size;
                },
            }

            if (req.response.status.class() == .redirect and req.redirect_behavior != .unhandled) {
                // skip the body of the redirect response, this will at least
                // leave the connection in a known good state.
                req.response.skip = true;
                assert(try req.transferRead(&.{}) == 0); // we're skipping, no buffer is necessary

                if (req.redirect_behavior == .not_allowed) return error.TooManyHttpRedirects;

                const location = req.response.location orelse
                    return error.HttpRedirectLocationMissing;

                // This mutates the beginning of header_bytes_buffer and uses that
                // for the backing memory of the returned Uri.
                try req.redirect(req.uri.resolve_inplace(
                    location,
                    &req.response.parser.header_bytes_buffer,
                ) catch |err| switch (err) {
                    error.UnexpectedCharacter,
                    error.InvalidFormat,
                    error.InvalidPort,
                    => return error.HttpRedirectLocationInvalid,
                    error.NoSpaceLeft => return error.HttpHeadersOversize,
                });
                try req.send();
            } else {
                req.response.skip = false;
                if (!req.response.parser.done) {
                    switch (req.response.transfer_compression) {
                        .identity => req.response.compression = .none,
                        .compress, .@"x-compress" => return error.CompressionUnsupported,
                        .deflate => req.response.compression = .{
                            .deflate = std.compress.zlib.decompressor(req.transferReader()),
                        },
                        .gzip, .@"x-gzip" => req.response.compression = .{
                            .gzip = std.compress.gzip.decompressor(req.transferReader()),
                        },
                        // https://github.com/ziglang/zig/issues/18937
                        //.zstd => req.response.compression = .{
                        //    .zstd = std.compress.zstd.decompressStream(req.client.allocator, req.transferReader()),
                        //},
                        .zstd => return error.CompressionUnsupported,
                    }
                }

                break;
            }
        }
    }

    pub const ReadError = TransferReadError || proto.HeadersParser.CheckCompleteHeadError ||
        error{ DecompressionFailure, InvalidTrailers };

    pub const Reader = std.io.Reader(*Request, ReadError, read);

    pub fn reader(req: *Request) Reader {
        return .{ .context = req };
    }

    /// Reads data from the response body. Must be called after `wait`.
    pub fn read(req: *Request, buffer: []u8) ReadError!usize {
        const out_index = switch (req.response.compression) {
            .deflate => |*deflate| deflate.read(buffer) catch return error.DecompressionFailure,
            .gzip => |*gzip| gzip.read(buffer) catch return error.DecompressionFailure,
            // https://github.com/ziglang/zig/issues/18937
            //.zstd => |*zstd| zstd.read(buffer) catch return error.DecompressionFailure,
            else => try req.transferRead(buffer),
        };
        if (out_index > 0) return out_index;

        while (!req.response.parser.state.isContent()) { // read trailing headers
            try req.connection.?.fill();

            const nchecked = try req.response.parser.checkCompleteHead(req.connection.?.peek());
            req.connection.?.drop(@intCast(nchecked));
        }

        return 0;
    }

    /// Reads data from the response body. Must be called after `wait`.
    pub fn readAll(req: *Request, buffer: []u8) !usize {
        var index: usize = 0;
        while (index < buffer.len) {
            const amt = try read(req, buffer[index..]);
            if (amt == 0) break;
            index += amt;
        }
        return index;
    }

    pub const WriteError = Connection.WriteError || error{ NotWriteable, MessageTooLong };

    pub const Writer = std.io.Writer(*Request, WriteError, write);

    pub fn writer(req: *Request) Writer {
        return .{ .context = req };
    }

    /// Write `bytes` to the server. The `transfer_encoding` field determines how data will be sent.
    /// Must be called after `send` and before `finish`.
    pub fn write(req: *Request, bytes: []const u8) WriteError!usize {
        switch (req.transfer_encoding) {
            .chunked => {
                if (bytes.len > 0) {
                    try req.connection.?.writer().print("{x}\r\n", .{bytes.len});
                    try req.connection.?.writer().writeAll(bytes);
                    try req.connection.?.writer().writeAll("\r\n");
                }

                return bytes.len;
            },
            .content_length => |*len| {
                if (len.* < bytes.len) return error.MessageTooLong;

                const amt = try req.connection.?.write(bytes);
                len.* -= amt;
                return amt;
            },
            .none => return error.NotWriteable,
        }
    }

    /// Write `bytes` to the server. The `transfer_encoding` field determines how data will be sent.
    /// Must be called after `send` and before `finish`.
    pub fn writeAll(req: *Request, bytes: []const u8) WriteError!void {
        var index: usize = 0;
        while (index < bytes.len) {
            index += try write(req, bytes[index..]);
        }
    }

    pub const FinishError = WriteError || error{MessageNotCompleted};

    /// Finish the body of a request. This notifies the server that you have no more data to send.
    /// Must be called after `send`.
    pub fn finish(req: *Request) FinishError!void {
        switch (req.transfer_encoding) {
            .chunked => try req.connection.?.writer().writeAll("0\r\n\r\n"),
            .content_length => |len| if (len != 0) return error.MessageNotCompleted,
            .none => {},
        }

        try req.connection.?.flush();
    }
}