Re: gEDA-dev: layers and layer groups

[der Mouse <mouse@xxxxxxxxxxxxxxxxxxxxxx>]

>> So, would it be fair to say that the layers are 0..max_layers-1,
>> with some places where layer numbers can appear giving special-case
>> semantics to the otherwise invalid values max_layers and
>> max_layers+1?
> Yes, pretty much like that.

Okay, then.  Question time. :)

In the lesstif HID, in lesstif_set_layer(), there is code that first
maps an incoming group number (ie, an argument in [0..max_layer)) to
its [0] member.  Then, if the result, idx, is in [0..max_layer+2), it
uses PCB->Data->Layer[idx].On as the return value.

This seems to imply that (a) max_layer or max_layer+1 can be the [0]
element of a group (rather than being a later member, just there to tag
the group as being the solder or component side copper), and (b)
PCB->Data->Layer[] has something useful in the [max_layer] and
[max_layer+1] entries.  However, it is not at all clear whether that's
actually so.

Of the other places in the lesstif HID that use ->Layer[]...
- lgbutton_expose special-cases max_layer and max_layer+1.
- lesstif_update_layer_groups likewise.
- SwapSides accesses those ->Layer[] values iff they are in the solder
   or component group's [0] elements.

Comparing with the GTK HID, I find that set_layer and SwapSides are in
this respect the same as their lesstif counterparts (and indeed in
set_layer there's a comment implying that its history involves the
lesstif version).  config_layers_apply uses only Layers[] for
[0..max_layers).  layer_name_entry_cb depends on code elsewhere;
looking for that code, I _think_ it uses only [0..max_layers).
ghid_config_groups_changed special-cases max_layer and max_layer+1.
layer_process, given how it's used, seems to access Layer[] for
[0..MAX_LAYER), oddly enough.  layer_button_enable_cb seems to use
[0..max_layer] (!).  ghid_layer_enable_buttons_update and
ghid_layer_buttons_update use [0..max_layer).  ToggleView uses the
.Name fields from [0..max_layer+2).

I find this all rather confusing.  It looks as though some fields are
valid and some aren't, and then there's the GTK layer_button_enable_cb,
which uses one of the extra "layers" but not the other, just to keep
things interesting.

All this makes it seem likely to me that PCB->Data->Layer[] has data
for [0..max_layer), with the code that uses other entries being buggy
(and getting away with it only by chance), with SwapSides living
dangerously by assuming that the layer groups are "well-formed".  Am I
right?  And, if not, what _is_ in PCB->Data->Layer[max_layer] and
[max_layer+1]?

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