Walls data: Explicit null prefix2 and prefix3 in shot data station names does not properly override #PREFIX2 and #PREFIX3 directives

[Joe Walko]

If prefix/prefix2/prefix3 is set via #PREFIX/#PREFIX2/PREFIX3 directive, cavern does not override #PREFIXn directive with blank prefix when a station name contains explicitly blank prefix/prefix2/prefix3.

The attached SRV file shows it better than I can describe it. The SRV contains Walls test data, and, in comments, station listing from Walls, station list from dump3d, and relevant 'warning' messages from cavern log file.

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I noticed some station names were not matching up between survex and Walls for the large dataset I've been testing with. PREFIX2/PREFIX3 are yet another somewhat esoteric feature we use; I assign unique PREFIX2 to each of four datasets when merging into one, hence most every station has PREFIX2. along with limited use of PREFIX3. There are certain cases where we have explicitly blank prefix names as part of the station names, thus causing the station name mismatches.

[Joe Walko]

Sample Walls data showing explicit prefix issue; diagnostic walls and cavern info embedded as comments in srv

[Olly Betts]

We map Walls prefix levels into Survex's prefix levels, but Survex doesn't allow an empty prefix level so we currently "squash away" empty levels. I'd thought Eric said this didn't look like it would cause problems in practice, but it seems it does.

We handle an empty station name by replacing it with (literally) empty name, which contains a space so cannot collide with a "real" Walls station name. E.g. PEP: becomes PEP:empty name. I didn't find anything saying the name could be empty, though also nothing which clearly says it can't be, but Walls allows it in practice.

We can handle empty prefix levels similarly. Probably we want something shorter than empty prefix though since empty prefix levels are actually documented as a feature, and seem less weird than an empty station name, so they're probably more common. Also there can be two empty prefix levels which would result in e.g. X:::1 -> X:empty prefix:empty prefix:1.

The manual implies the same characters are allowed in a prefix as a station name, and the documented disallowed characters in a station name are:

• : - but this is the prefix level separator so we can't use it here
• ; too visually similar to : I think - consider X:;:;:1
• ,
• '#' (but in fact this is allowed in station names)
• tab
• space

I think the least terrible option is to use a single space (which doesn't work for an empty station name as it's invisible at the end of a name when displayed, but a single space for an empty prefix level will always be between two :).

We can omit any leading empty prefix levels without ambiguity (like Walls does), so your stations would be:

:::d1                        -> d1                           (same as Walls)
Prefix3::shotpref:d1         -> Prefix3: :shotpref:d1
Prefix3:Prefix2:shotpref:d1  -> Prefix3:Prefix2:shotpref:d1  (same as Walls)
Prefix3:shotpref::d1         -> Prefix3:shotpref: :d1
::shotpref:d1                -> shotpref:d1                  (same as Walls)
:shotpref::d1                -> shotpref: :d1
shotpref:::d1                -> shotpref: : :d1

And examples with empty station names:

::PEP:    -> PEP:empty name
:PEP::    -> PEP: :empty name
PEP:::    -> PEP: : :empty name

[Olly Betts]

Only "Unused fixed point" warnings remain, and the output seems to be consistent with Walls:

NODE 586700.00 4116200.00 300.00 [shotpref: : :d1] EXPORTED FIXED
NODE 586700.00 4126300.00 300.00 [Prefix3:shotpref: :d1] EXPORTED FIXED
NODE 586700.00 4136400.00 300.00 [Prefix3:Prefix2:shotpref:d1] EXPORTED FIXED
NODE 586700.00 4136500.00 300.00 [Prefix3: :shotpref:d1] EXPORTED FIXED
NODE 586700.00 4136500.00 300.00 [shotpref:d1] EXPORTED FIXED
NODE 586700.00 4136500.00 300.00 [shotpref: :d1] EXPORTED FIXED
NODE 586700.00 4136500.00 300.00 [d1] EXPORTED FIXED

There's another wrinkle in this area though.

Consider:

#fix :::X  0 0 0
#fix ::X:Y 1 1 1

Currently those map to X and X:Y in Survex, but that means that X is both a station and a survey in Survex's name hierarchy, which is not allowed.

I don't currently have an idea I like for addressing this. A key problem is that it probably rarely occurs in practice so we really don't want to change name mapping of Walls stations in ways that affect common use patterns (e.g. we could map :::X to, say, empty 3:X but that would affect the names of all stations with leading empty prefix levels).

There's a secondary problem with different prefix levels getting conflated - in your example ::shotpref:d1, :shotpref::d1 and shotpref:::d1 all become a name with the first component shotpref, but that's a different shotpref in each case! Survex uses the names for surveys, so you might have cave.area.survey.1 and tools working with Survex names will typically assume such a naming scheme, which might not work well here. I don't really know how Walls users use the prefix levels in practice - perhaps because of the convention of omitting leading empty prefix level these would be regarded as the same shotpref by a Walls user.

[Olly Betts]

Reported problem fixed by 14a55b3cd152b5cb5e9752e6ddea292ae15111c1.