How to skip length indicator of a LL subfield

[kamenlitchev]

I have an ans..63 LL field that has three subfields as follows:

1. ans..50
2. ans 10
3. ans 3

Subfield 1, although variable, shall not be prefixed with size bytes. The specific ISO server relies on subtracting the fixed portions size (10+3) from the overall main LL field size to "know" the size of Subfield 1. How can I achieve this?

The code below does not work, since it inserts length LL bytes in subfield 1:

mainFieldSpec = &field.Spec{
		Length:      63,
		Description: "Main Field",
		Pref:        prefix.ASCII.LL,
		Tag: &field.TagSpec{
			Sort: sort.StringsByInt,
		},
		Subfields: map[string]field.Field{
			"1": field.NewString(&field.Spec{
				Length:      50,
				Description: `Subfield 1`,
				Enc:         encoding.ASCII,
				Pref:        prefix.ASCII.LL,
			}),
			"2": field.NewString(&field.Spec{
				Length:      10,
				Description: "Subfield 2",
				Enc:         encoding.ASCII,
				Pref:        prefix.ASCII.Fixed,
				Pad:         padding.Right(' '),
			}),
			"3": field.NewString(&field.Spec{
				Length:      3,
				Description: "Subfield 3",
				Enc:         encoding.ASCII,
				Pref:        prefix.ASCII.Fixed,
				Pad:         padding.Right(' '),
			}),
		},

Thanks in advance!

P.S. I tried Pref: prefix.None.LL, but this throws an exception

[pratikrath126]

Hey @kamenlitchev, interesting use case. In many ISO8583 libraries, the length indicator is often coupled with the field definition (e.g., LVAR, LLVAR). If you need to skip it for a specific subfield, you might need to define a custom field packer/unpacker that treats the length as part of the data or simply ignores the length prefix logic. Check if Moov's iso8583 allows for custom encoders for subfields in a spec. That's usually where you can override the default length-prefixed behavior!

[kamenlitchev]

This library is AMAZING!!! The solution was Pref: prefix.None.Fixed. This skipped the prefix while still encoding the data as variable length field.

"1": field.NewString(&field.Spec{
	Length:      50,
	Description: `Subfield 1`,
	Enc:         encoding.ASCII,
	Pref:        prefix.None.Fixed,
}),