der

pub const der = struct {
    pub const Class = enum(u2) {
        universal,
        application,
        context_specific,
        private,
    };

    pub const PC = enum(u1) {
        primitive,
        constructed,
    };

    pub const Identifier = packed struct(u8) {
        tag: Tag,
        pc: PC,
        class: Class,
    };

    pub const Tag = enum(u5) {
        boolean = 1,
        integer = 2,
        bitstring = 3,
        octetstring = 4,
        null = 5,
        object_identifier = 6,
        sequence = 16,
        set = 17,
        utc_time = 23,
        generalized_time = 24,
        _,
    };

    pub const Element = struct {
        identifier: Identifier,
        slice: Slice,

        pub const Slice = struct {
            start: u32,
            end: u32,

            pub const empty: Slice = .{ .start = 0, .end = 0 };
        };

        pub const ParseError = error{CertificateFieldHasInvalidLength};

        pub fn parse(bytes: []const u8, index: u32) Element.ParseError!Element {
            var i = index;
            const identifier: Identifier = @bitCast(bytes[i]);
            i += 1;
            const size_byte = bytes[i];
            i += 1;
            if ((size_byte >> 7) == 0) {
                return .{
                    .identifier = identifier,
                    .slice = .{
                        .start = i,
                        .end = i + size_byte,
                    },
                };
            }

            const len_size: u7 = @truncate(size_byte);
            if (len_size > @sizeOf(u32)) {
                return error.CertificateFieldHasInvalidLength;
            }

            const end_i = i + len_size;
            var long_form_size: u32 = 0;
            while (i < end_i) : (i += 1) {
                long_form_size = (long_form_size << 8) | bytes[i];
            }

            return .{
                .identifier = identifier,
                .slice = .{
                    .start = i,
                    .end = i + long_form_size,
                },
            };
        }
    };
}