decode

            pub fn decode(in: *Reader) DecodeError!Header {
                const descriptor: Descriptor = @bitCast(try in.takeByte());

                if (descriptor.reserved) return error.ReservedBitSet;

                const window_descriptor: ?u8 = if (descriptor.single_segment_flag) null else try in.takeByte();

                const dictionary_id: ?u32 = if (descriptor.dictionary_id_flag > 0) d: {
                    // if flag is 3 then field_size = 4, else field_size = flag
                    const field_size = (@as(u4, 1) << descriptor.dictionary_id_flag) >> 1;
                    break :d try in.takeVarInt(u32, .little, field_size);
                } else null;

                const content_size: ?u64 = if (descriptor.single_segment_flag or descriptor.content_size_flag > 0) c: {
                    const field_size = @as(u4, 1) << descriptor.content_size_flag;
                    const content_size = try in.takeVarInt(u64, .little, field_size);
                    break :c if (field_size == 2) content_size + 256 else content_size;
                } else null;

                return .{
                    .descriptor = descriptor,
                    .window_descriptor = window_descriptor,
                    .dictionary_id = dictionary_id,
                    .content_size = content_size,
                };
            }