Call external code to determine number of times to repeat expression

Question

I have a sequence that has an 'id' with a repeat expression that I need to repeat an unknown number times, and I'm not sure if this is currently supported.

data_channels:
  seq:
    - id: fast_data
      type: u2
      repeat: expr
      repeat-expr: ???

What I would like to be able to do is call an external method that calculates the correct number of repetitions, and returns that number to kaitai. Like:

data_channels:
  seq:
    - id: fast_data
      type: u2
      repeat: expr
      repeat-expr: CalulateRepetitions

However, even with "ks-opaque-types: true" in the meta data, I get an error that "CalculateRepetitions" cannot be accessed.

The number of repetitions depends on a number of things that just aren't accessible from kaitai, so an external method is a perfect solution.

Thanks in advance for any suggestions.

Answer 1

If you know number of repetitions prior to type invocation, you can declare that as a type parameter and pass it from your application:

data_channels:
  params:
    - id: num_items
      type: u4
  seq:
    - id: fast_data
      type: u2
      repeat: expr
      repeat-expr: num_items

If you don't know that before parsing, you can still use this trick to bridge into opaque type and back, i.e.:

meta:
  id: your_main_type
  ksc-opaque-types: true
seq:
  - id: something
    type: u1
  # at this point we don't know number of repetitions yet
  - id: data_channels
    type: opaque_data_channels_wrapper

After that, implement OpaqueDataChannelsWrapper class in your app. If there, it could be something like that:

class OpaqueDataChannelsWrapper {
  public OpaqueDataChannelsWrapper(KaitaiStream io) {
    // do something here to determine number of repetitions

    // dive back into KaitaiStruct parsing with that information
    DataChannels realDataChannels = new DataChannels(io, numRepetitions);
  }
}