How to use instances: in a loop, where each iteration has a different size?

[jeansanch]

How can I get the pos of an item inside a loop of blocks that have different sizes?

I've tried doing

seq:
  - id: skip_init
    size: 280060
  - id: run_data
    type: data
    repeat: expr 
    repeat-expr: 2 # just to test
instances:
  names:
    io: run_data[_index]._io
    pos: _io.pos
    size: run_data[_index].name_size

I have tried to add instances with a similar logic inside of the run_data block, but it would always point to the same block of data.

I'm doing this because I've a structure that looks something like this.

name[name_size]
zeroes[2] (that will only appear if (name_size / 2) % 2 == 1) + data[8]
name2[name_size]
zeroes[Value also based on the string size]
data[400]
name_size[2]
data[16]

so I have to pass through the "name" variable first, then I'm able to get its value (name_size).

Still I've managed to parse each block using another logic, that doesn't allow me to filter the "name" , that's why I was using instances for "post-processing"

What I would like to do is something like this.

seq:
  - id: skip_init
    size: 280060
  - id: run_data
    type: data
    repeat: expr 
    repeat-expr: 2 # just to test
instances:
  names:
    pos: Start of the block at each index
    size: name_size variable of each index

Am I taking the best approach for this? I've read the documentation, but I was not able to get an answer for this specific case.

[jeansanch]

I would like to add that the code referenced at http://doc.kaitai.io/user_guide.html#_relative_positioning is not possible to do here, because I don't have a fixed size as in "some_block.size" in the code on the example "Good (w/substream)" and I'm only able to get the block size, at the end of the block.