A couple of related issues come up when considering implementing a dimensionality reducing encoding -- just want to toss those out to see what people think the interface might look like.
There's a couple of aspects that could be brought in here, but lets keep it simple and considering the original question: (lat/lon) -> number. --Desired precision of the binning process The more bits we add to the z-curve, the more precise our comparison - i.e. a 63 bit key would have more "locations" to sort by than a 24 bit key. Would you see a reasonable default getting picked, make this user configurable, or both? (i.e. default to a value, extended options with a new constructor?) --Semantics for turning two lat/long pairs into a range I'm extrapolating here, but the only reason I see that locality matters is if we want to preserve locality for range searches. The internal implementation of the encoding/lexicoding process is going to directly impact the implementation of the range query. Now sure, someone could encode the lower left point, encode the upper right point, and construct a range out of that to pass for a scan, but that's going to be wildly inefficient in most cases. See: https://dl.dropboxusercontent.com/u/6649380/bbox.png If we just lexicode the lower left and upper right we traverse across the entire curve - hitting lots of areas that aren't actually in the original range. Now we can turn a single 2D range into a set of 1D ranges. There is some potential tuning here now, as the algorithm has a tradeoff on time to compute the ranges (and number of ranges) vs. "slop" (or inclusion of ranges which aren't actually in the original query). Would you see a static method perhaps on the z-curve lexicoder that returns a series of ranges based on an input window? Some other mechanism? And in the case of "slop" - would we just document that the ranges could actually include values not expected - or would we always fully decompose? ------------------ The other, "making it more complicated" questions would probably resolve around generalizing this to a multi-dimensional lexicoder. I would expect the WGS84 (lat/long) encoder would just be a 2D instance with -180/+180 -90/+90 bounds. There are probably completely different cases where it would be useful to have a locality sensitive hash. But with the big caveat that this requires more methods - I now need a way of defining a range for each of the dimensions (which before were defined as lat/lon), and I need an interface/class to pass those dimensions to the encoder - and should be able to use those same definitions to decode my values on the way out. I'm not trying to make a mountain out of a molehill - one could definitely pick some sane defaults, document behaviour, and put in a WGS84 specific implementation. I'm just thinking what's the minimum viable bit that's needed for this to be useful - and I suspect it's also the range decomposition piece - as I suspect *everyone* would really need that (if they cared about locality in the first place). My main question here would be to what extent would you see this going? A one off for WGS84, or a more generic multi-dimensional lexicoder; and in either case, what would the thoughts/consensus be for implementation of the additional methods (and possibly classes) required? On Fri, Jul 25, 2014 at 12:37 PM, Sean Busbey <[email protected]> wrote: > > On Fri, Jul 25, 2014 at 8:45 AM, Anthony F <[email protected]> wrote: > >> >> GeoMesa plans to update to 1.6 as soon as we get past a 1.0 RELEASE >> (which is waiting on approval by the Eclipse Foundation's IP review process >> and should happen in a month or so). I think we could break out a z-curve >> lexicoder and contribute before then though. I'll poke around at the >> lexicoder stuff in 1.6 and see what it would take to use that interface now. >> >> > > That sounds great Anthony. Let us know if we can help with anything! > > > > -- > Sean >
