module BinData::CheckOrAdjustOffsetPlugin::CheckOffsetMixin

def check_offset(io)
  actual_offset = io.offset
  expected = eval_parameter(:check_offset, :offset => actual_offset)
  if not expected
    raise ValidityError, "offset not as expected for #{debug_name}"
  elsif actual_offset != expected and expected != true
    raise ValidityError,
          "offset is '#{actual_offset}' but " +
          "expected '#{expected}' for #{debug_name}"
  end
end