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Revision 1.2 by root, Mon Mar 14 03:14:41 2022 UTC vs.
Revision 1.3 by root, Thu Mar 17 00:32:54 2022 UTC

… …
72 usually works best, so you usually do not specify this parameter. 72 usually works best, so you usually do not specify this parameter.
73 73
74 If something is found, the associated data blob (always a binary 74 If something is found, the associated data blob (always a binary
75 string) is returned, otherwise you receive "undef". 75 string) is returned, otherwise you receive "undef".
76 76
77 Unless you specify a cusotrm format, the data blob is actually a 77 Unless you specify a custom format/extractor when building your
78 UTF-8 string, so you might want to call "utf8::decode" on it to get 78 database, the data blob is actually a UTF-8 string, so you might
79 a unicode astring. 79 want to call "utf8::decode" on it to get a unicode string:
80 80
81 At the moment, the implementation is in pure perl, but will 81 my $res = $db->lookup (47, 37); # near mariupol, UA
82 eventually move to C. 82 if (defined $res) {
83 utf8::decode $res;
84 # $res now contains the unicode result
85 }
83 86
84ALGORITHM 87ALGORITHM
85 The algorithm that this module implements consists of two parts: binning 88 The algorithm that this module implements consists of two parts: binning
86 and weighting (done when writing the database) and then finding the 89 and weighting (done when writing the database) and then finding the
87 nearest point. 90 nearest point.
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96 99
97 It will then calculate the (squared) distance to the search coordinate 100 It will then calculate the (squared) distance to the search coordinate
98 using an approximate euclidean distance on an equireactangular 101 using an approximate euclidean distance on an equireactangular
99 projection. The squared distance is multiplied with a weight (1..25 for 102 projection. The squared distance is multiplied with a weight (1..25 for
100 the geonames database, based on population and adminstrative status, 103 the geonames database, based on population and adminstrative status,
101 always 1 for postcal codes), and the minimum distance wins. 104 always 1 for postal codes), and the minimum distance wins.
102 105
103 Binning should not introduce errors, but bigger bins can slow down 106 Binning should not introduce errors, but bigger bins can slow down
104 lookup times due to having to look at more places. The lookup assumes a 107 lookup times due to having to look at more places. The lookup assumes a
105 spherical shape for the earth, the equirectangular projection stretches 108 spherical shape for the earth, the equirectangular projection stretches
106 distances unevenly and the euclidean distance calculation introduces 109 distances unevenly and the euclidean distance calculation introduces
107 further errors. For typical distance (<< 100km) and the intended usage, 110 further errors. For typical distance (<< 100km) and the intended usage,
108 these errors should be considered negligible. 111 these errors should be considered negligible.
109 112
110SPEED 113SPEED
111 The current implementation is written in pure perl, and on my machine, 114 On my machine, "lookup" typically does more than a million lookups per
112 typically does 10000-200000 lookups per second. The goal for version 1.0 115 second - performance varies depending on result density and number of
113 is to move the lookup to C. 116 indexed points.
114 117
115TENTATIVE ROADMAP 118TENTATIVE ROADMAP
116 The database writer should be accessible via a module, so you cna easily 119 The database writer should be accessible via a module, so you can easily
117 generate your own databases without having to run an external command. 120 generate your own databases without having to run an external command.
118 121
119 The api might be extended to allow for multiple returns, or nearest 122 The API might be extended to allow for multiple lookups, multiple
120 neighbour search. 123 returns, or nearest neighbour search, or more return values (distance,
124 coordinates).
125
126 Longer lookups will take advantage of perlmulticore.
127
128PERL MULTICORE SUPPORT
129 This is not yet implemented:
130
131 This module supports the perl multicore specification
132 (<http://perlmulticore.schmorp.de/>) when doing lookups.
121 133
122SEE ALSO 134SEE ALSO
123 geo-latlon2place-makedb to create databases from common formats. 135 geo-latlon2place-makedb to create databases from common formats.
124 136
125AUTHOR 137AUTHOR

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