abs_offset and adjust_offset not playing nicely

[aidansteele]

Hi,

Love the library. I've been playing with the :adjust_offset option and ran into an issue. The issue may be with my understanding or the code. :) I'll demonstrate with an example

require 'bindata'
io = ["6B7963680001000000000010000000140000000000006A640000000B"].pack "H*"

class KeychainFile < BinData::Record
  endian :big
  string :magic, length: 4, asserted_value: 'kych'
  string :padding, read_length: 8
  uint32 :schema_offset # in this example, the value is 20
  uint32 :table_count, adjust_offset: lambda { schema_offset + 4 }
end

kf = KeychainFile.read io
kf.schema_offset.abs_offset # is 12, as expected
kf.table_count.abs_offset # is 16, where i would expect 24

It appears that abs_offset does not take into account the value returned by adjust_offset. The value of kf.table_count is located at offset 24 and my understanding of abs_offset is that it returns the location within the IO that houses the data. Is this a correct understanding? If so, I think there may be a bug but I am not clever enough to fix it :)

[aidansteele]

I banged away at my keyboard and eventually ended up with this:

module BinData
  class Base
    def real_abs_offset
      if @params[:adjust_offset].nil?
        parent.real_abs_offset + parent.offset_of(self)
      else
        @lazy.lazy_eval @params[:adjust_offset]
      end
    end
  end
end

I can't really explain how/why it works (and I feel like a monkey at a typewriter) or it's terribly wrong, but it's allowed me to continue on with my current task. I thought it would be worth posting here to see your thoughts on it.

[dmendel]

:adjust_offset is an experimental feature (which is why it is undocumented see #52). I'm still working out how to achieve seeking fuctionality in a structured way. :adjust_offset, as it currently stands, is analogous to using goto to achieve looping.

:adjust_offset will eventually be replaced with a better solution. Your above monkey patch looks like a good fix to your problem.

Do be aware that :adjust_offset is only in effect when reading. It will not automatically do any seeking when writing.

[dmendel]

BinData::Skip now has the :to_abs_offset option. This is the official way to perform seeking in a stream. Skip will ensure that #abs_offset is calculated correctly.

:adjust_offset remains for backwards compatibility.

Your example can be written as:

class KeychainFile < BinData::Record
  endian :big
  string :magic, length: 4, asserted_value: 'kych'
  string :padding, read_length: 8
  uint32 :schema_offset # in this example, the value is 20
  skip   to_abs_offset: lambda { schema_offset + 4 }
  uint32 :table_count
end