Something I want to do with gimp

But don't know how.

Gimp has various useful tools:
It has select by color.
It has invert selection.
It has shrink selection. (with options to shrink from image border or not)
It has save a selection to path
It has load a selection from path
It has fill by color.


What I want do do with it, in pseudo-code because I haven't coded in forever remember little if anything, is this:



User inputs integer A, which is greater than or equal to 1.

SubThing(Int X, Path){
int Z==X/1 (rounding down)
int J==1

Fill white

For{int stop==0; stop=0;}
Selection from Path
Shrink selection by Z (not from border)
If selection nonexistant {stop==1
}
Else  {If J=0  {Fill white; J==1
  }
else  {Fill black; J==0
}
Z==Z+X
}
}
}

Main(int A){
Select Black
Path == Selection to Path

Subthing(A, Path)

Selection from Path
Invert selection
Path == Selection to Path

Subthing(A, Path)
}

-

Or something like that, it's been ages since I looked at code.

Anyway, trouble is I have no idea how to do it.

Anyone able to help?

[Added:]
On further examination, saving the selection to a channel might be a better ideal for reasons I don't fully understand but none the less appear to be real