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Date: Tue, 8 Dec 2015 22:22:04 +0100
From: "Nicklas Karlsson (nicklas DOT karlsson17 AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] gsch2pcb after refdes-renum? (If implemented
 syncronization detail)
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> > Changes compare schematic file with pcb file so it is done manually
> > and I guess it is fine like this. For idempotent changes applying
> > them more than once make no difference.
> 
> What we have now breaks down in a simple case.  Consider: we want to
> rename U1, U2, and U3 to be U2, U3, and U1 (note the cyclic
> dependency).  ExecuteScript() would have something like this:
> 
> Rename(U1,U2)
> Rename(U2,U3)
> Rename(U3,U1)
> 
> There is no "correct" order to do these in, but even if the script is
> smart enough to use a temporary name for one of the symbols, or pcb
> was smart enough to know old vs new names, running the script twice
> would "rotate" the names again.
> 
> Hence, my position that something other than refdes is needed to
> identify "this symbol" or "that gate".  Tracking the lifetime of a
> symbol so you can refer to "the one I used back when" is the trick
> part I was asking about.
> 
> If we really want to cleanly support simple operations like "rename a
> part" we need to change gaf's dependency on the name identifying "the
> part", and start treating the refdes like any other generic attribute.

I could put it down like this:
  0. There bust be knowledge of link between symbol <--> footprint, even though it is not really a link.

Alternative 1 unique number is used to link:
  1. They have the same unique number.

Alternative 2 refdes is used to link:
  1. refdes must be unique.
  2. Old and new refdes must be known.
  2. Change need to be applied exactly once.

Alternative 2 topology is used to link:
  1. This type of part connected to these nets.
  2. There may be a lot of decoupling capacitors in parallel.

If all three where supported I would try them in the order Alternative 1,2,3 until link is found. If i should implement it in gEDA I would probably choose Alternative 2 to keep the file format, this is a little bit special case. To find out if a rename have occured the information is available then the rename happen or by comparing new schematic file with old schematic file.

Nicklas Karlsson