Head

    pub const Head = struct {
        bytes: []const u8,
        version: http.Version,
        status: http.Status,
        reason: []const u8,
        location: ?[]const u8 = null,
        content_type: ?[]const u8 = null,
        content_disposition: ?[]const u8 = null,

        keep_alive: bool,

        /// If present, the number of bytes in the response body.
        content_length: ?u64 = null,

        transfer_encoding: http.TransferEncoding = .none,
        content_encoding: http.ContentEncoding = .identity,

        pub const ParseError = error{
            HttpConnectionHeaderUnsupported,
            HttpContentEncodingUnsupported,
            HttpHeaderContinuationsUnsupported,
            HttpHeadersInvalid,
            HttpTransferEncodingUnsupported,
            InvalidContentLength,
        };

        pub fn parse(bytes: []const u8) ParseError!Head {
            var res: Head = .{
                .bytes = bytes,
                .status = undefined,
                .reason = undefined,
                .version = undefined,
                .keep_alive = false,
            };
            var it = mem.splitSequence(u8, bytes, "\r\n");

            const first_line = it.first();
            if (first_line.len < 12) return error.HttpHeadersInvalid;

            const version: http.Version = switch (int64(first_line[0..8])) {
                int64("HTTP/1.0") => .@"HTTP/1.0",
                int64("HTTP/1.1") => .@"HTTP/1.1",
                else => return error.HttpHeadersInvalid,
            };
            if (first_line[8] != ' ') return error.HttpHeadersInvalid;
            const status: http.Status = @fromBackingInt(@intCast(parseInt3(first_line[9..12])));
            const reason = mem.trimStart(u8, first_line[12..], " ");

            res.version = version;
            res.status = status;
            res.reason = reason;
            res.keep_alive = switch (version) {
                .@"HTTP/1.0" => false,
                .@"HTTP/1.1" => true,
            };

            while (it.next()) |line| {
                if (line.len == 0) return res;
                switch (line[0]) {
                    ' ', '\t' => return error.HttpHeaderContinuationsUnsupported,
                    else => {},
                }

                var line_it = mem.splitScalar(u8, line, ':');
                const header_name = line_it.next().?;
                const header_value = mem.trim(u8, line_it.rest(), " \t");
                if (header_name.len == 0) return error.HttpHeadersInvalid;

                if (std.ascii.eqlIgnoreCase(header_name, "connection")) {
                    res.keep_alive = !std.ascii.eqlIgnoreCase(header_value, "close");
                } else if (std.ascii.eqlIgnoreCase(header_name, "content-type")) {
                    res.content_type = header_value;
                } else if (std.ascii.eqlIgnoreCase(header_name, "location")) {
                    res.location = header_value;
                } else if (std.ascii.eqlIgnoreCase(header_name, "content-disposition")) {
                    res.content_disposition = header_value;
                } else if (std.ascii.eqlIgnoreCase(header_name, "transfer-encoding")) {
                    // Transfer-Encoding: second, first
                    // Transfer-Encoding: deflate, chunked
                    var iter = mem.splitBackwardsScalar(u8, header_value, ',');

                    const first = iter.first();
                    const trimmed_first = mem.trim(u8, first, " ");

                    var next: ?[]const u8 = first;
                    if (std.meta.stringToEnum(http.TransferEncoding, trimmed_first)) |transfer| {
                        if (res.transfer_encoding != .none) return error.HttpHeadersInvalid; // we already have a transfer encoding
                        res.transfer_encoding = transfer;

                        next = iter.next();
                    }

                    if (next) |second| {
                        const trimmed_second = mem.trim(u8, second, " ");

                        if (http.ContentEncoding.fromString(trimmed_second)) |transfer| {
                            if (res.content_encoding != .identity) return error.HttpHeadersInvalid; // double compression is not supported
                            res.content_encoding = transfer;
                        } else {
                            return error.HttpTransferEncodingUnsupported;
                        }
                    }

                    if (iter.next()) |_| return error.HttpTransferEncodingUnsupported;
                } else if (std.ascii.eqlIgnoreCase(header_name, "content-length")) {
                    const content_length = std.fmt.parseInt(u64, header_value, 10) catch return error.InvalidContentLength;

                    if (res.content_length != null and res.content_length != content_length) return error.HttpHeadersInvalid;

                    res.content_length = content_length;
                } else if (std.ascii.eqlIgnoreCase(header_name, "content-encoding")) {
                    if (res.content_encoding != .identity) return error.HttpHeadersInvalid;

                    const trimmed = mem.trim(u8, header_value, " ");

                    if (http.ContentEncoding.fromString(trimmed)) |ce| {
                        res.content_encoding = ce;
                    } else {
                        return error.HttpContentEncodingUnsupported;
                    }
                }
            }
            return error.HttpHeadersInvalid; // missing empty line
        }

        test parse {
            const response_bytes = "HTTP/1.1 200 OK\r\n" ++
                "LOcation:url\r\n" ++
                "content-tYpe: text/plain\r\n" ++
                "content-disposition:attachment; filename=example.txt \r\n" ++
                "content-Length:10\r\n" ++
                "TRansfer-encoding:\tdeflate, chunked \r\n" ++
                "connectioN:\t keep-alive \r\n\r\n";

            const head = try Head.parse(response_bytes);

            try testing.expectEqual(.@"HTTP/1.1", head.version);
            try testing.expectEqualStrings("OK", head.reason);
            try testing.expectEqual(.ok, head.status);

            try testing.expectEqualStrings("url", head.location.?);
            try testing.expectEqualStrings("text/plain", head.content_type.?);
            try testing.expectEqualStrings("attachment; filename=example.txt", head.content_disposition.?);

            try testing.expectEqual(true, head.keep_alive);
            try testing.expectEqual(10, head.content_length.?);
            try testing.expectEqual(.chunked, head.transfer_encoding);
            try testing.expectEqual(.deflate, head.content_encoding);
        }

        pub fn iterateHeaders(h: Head) http.HeaderIterator {
            return .init(h.bytes);
        }

        test iterateHeaders {
            const response_bytes = "HTTP/1.1 200 OK\r\n" ++
                "LOcation:url\r\n" ++
                "content-tYpe: text/plain\r\n" ++
                "content-disposition:attachment; filename=example.txt \r\n" ++
                "content-Length:10\r\n" ++
                "TRansfer-encoding:\tdeflate, chunked \r\n" ++
                "connectioN:\t keep-alive \r\n\r\n";

            const head = try Head.parse(response_bytes);
            var it = head.iterateHeaders();
            {
                const header = it.next().?;
                try testing.expectEqualStrings("LOcation", header.name);
                try testing.expectEqualStrings("url", header.value);
                try testing.expect(!it.is_trailer);
            }
            {
                const header = it.next().?;
                try testing.expectEqualStrings("content-tYpe", header.name);
                try testing.expectEqualStrings("text/plain", header.value);
                try testing.expect(!it.is_trailer);
            }
            {
                const header = it.next().?;
                try testing.expectEqualStrings("content-disposition", header.name);
                try testing.expectEqualStrings("attachment; filename=example.txt", header.value);
                try testing.expect(!it.is_trailer);
            }
            {
                const header = it.next().?;
                try testing.expectEqualStrings("content-Length", header.name);
                try testing.expectEqualStrings("10", header.value);
                try testing.expect(!it.is_trailer);
            }
            {
                const header = it.next().?;
                try testing.expectEqualStrings("TRansfer-encoding", header.name);
                try testing.expectEqualStrings("deflate, chunked", header.value);
                try testing.expect(!it.is_trailer);
            }
            {
                const header = it.next().?;
                try testing.expectEqualStrings("connectioN", header.name);
                try testing.expectEqualStrings("keep-alive", header.value);
                try testing.expect(!it.is_trailer);
            }
            try testing.expectEqual(null, it.next());
        }

        inline fn int64(array: *const [8]u8) u64 {
            return @bitCast(array.*);
        }

        fn parseInt3(text: *const [3]u8) u10 {
            const nnn: @Vector(3, u8) = text.*;
            const zero: @Vector(3, u8) = .{ '0', '0', '0' };
            const mmm: @Vector(3, u10) = .{ 100, 10, 1 };
            return @reduce(.Add, (nnn -% zero) *% mmm);
        }

        test parseInt3 {
            const expectEqual = testing.expectEqual;
            try expectEqual(@as(u10, 0), parseInt3("000"));
            try expectEqual(@as(u10, 418), parseInt3("418"));
            try expectEqual(@as(u10, 999), parseInt3("999"));
        }

        /// Help the programmer avoid bugs by calling this when the string
        /// memory of `Head` becomes invalidated.
        fn invalidateStrings(h: *Head) void {
            h.bytes = undefined;
            h.reason = undefined;
            if (h.location) |*s| s.* = undefined;
            if (h.content_type) |*s| s.* = undefined;
            if (h.content_disposition) |*s| s.* = undefined;
        }
    }