[Xapian-tickets] [Xapian] #481: Merge geospatial branch

[Xapian]

#481: Merge geospatial branch
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 Reporter:  richard      |       Owner:  richard  
     Type:  enhancement  |      Status:  assigned 
 Priority:  highest      |   Milestone:  1.3.0    
Component:  Library API  |     Version:  SVN trunk
 Severity:  normal       |    Keywords:           
Blockedby:               |    Platform:  All      
 Blocking:               |  
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Comment(by richard):

 The special cases are fragile, as implemented; I think that something at
 latitude -89.999999999 will be rounded to having latitude -90, but not
 trigger the special case (so the longitude will be stored, too).  This
 should be easy to fix.

 In performance testing, I noticed that a significant part of the time
 taken by geo_decode (around 30%) is doing the string comparison for "if
 (value == "\xff\xff")".  It occurs to me that it might be better not to
 have the special short representations of -90 and +90: instead, always use
 6 bytes for the representation.  This has the advantage that multiple
 values could be stored in the slot with no need to mark terminators; and
 that when encoding multiple values, a 6 byte fixed buffer could be used,
 instead of a string (which would probably be faster).  We could also just
 do the special case handling at encoding time, checking the integer
 representation of the number of 16ths of a second in the latitude, and
 setting the longitude to zero if that is the value for -90 or +90.

 It also occurs to me that it might be better to store longitude values as
 more significant that latitude.  With the current code, a range query on
 the encoded values can be used to check if the coordinate is in a given
 degree range of latitude values, but I suspect being able to do a quick
 range query to check a range of longitudes would be more useful in many
 cases; for example, it would enable US and european coordinates to be
 distinguished quickly; there is probably a greater range of longitude
 degree values in use than latitudes.

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Ticket URL: <http://trac.xapian.org/ticket/481#comment:22>
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