geohash算法原理及实现方式

  1 <?php
  2 /**
  3  * Geohash generation class
  4  * http://blog.dixo.net/downloads/
  5  *
  6  * This file copyright (C) 2008 Paul Dixon (paul@elphin.com)
  7  *
  8  * This program is free software; you can redistribute it and/or
  9  * modify it under the terms of the GNU General Public License
 10  * as published by the Free Software Foundation; either version 3
 11  * of the License, or (at your option) any later version.
 12  *
 13  * This program is distributed in the hope that it will be useful,
 14  * but WITHOUT ANY WARRANTY; without even the implied warranty of
 15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 16  * GNU General Public License for more details.
 17  *
 18  * You should have received a copy of the GNU General Public License
 19  * along with this program; if not, write to the Free Software
 20  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 21  */
 22 
 23 
 24 
 25 /**
 26 * Encode and decode geohashes
 27 *
 28 */
 29 class Geohash
 30 {
 31     private $coding="0123456789bcdefghjkmnpqrstuvwxyz";
 32     private $codingMap=array();
 33     
 34     public function Geohash()
 35     {
 36         //build map from encoding char to 0 padded bitfield
 37         for($i=0; $i<32; $i++)
 38         {
 39             $this->codingMap[substr($this->coding,$i,1)]=str_pad(decbin($i), 5, "0", STR_PAD_LEFT);
 40         }
 41         
 42     }
 43     
 44     /**
 45     * Decode a geohash and return an array with decimal lat,long in it
 46     */
 47     public function decode($hash)
 48     {
 49         //decode hash into binary string
 50         $binary="";
 51         $hl=strlen($hash);
 52         for($i=0; $i<$hl; $i++)
 53         {
 54             $binary.=$this->codingMap[substr($hash,$i,1)];
 55         }
 56         
 57         //split the binary into lat and log binary strings
 58         $bl=strlen($binary);
 59         $blat="";
 60         $blong="";
 61         for ($i=0; $i<$bl; $i++)
 62         {
 63             if ($i%2)
 64                 $blat=$blat.substr($binary,$i,1);
 65             else
 66                 $blong=$blong.substr($binary,$i,1);
 67             
 68         }
 69         
 70         //now concert to decimal
 71         $lat=$this->binDecode($blat,-90,90);
 72         $long=$this->binDecode($blong,-180,180);
 73         
 74         //figure out how precise the bit count makes this calculation
 75         $latErr=$this->calcError(strlen($blat),-90,90);
 76         $longErr=$this->calcError(strlen($blong),-180,180);
 77                 
 78         //how many decimal places should we use? There‘s a little art to
 79         //this to ensure I get the same roundings as geohash.org
 80         $latPlaces=max(1, -round(log10($latErr))) - 1;
 81         $longPlaces=max(1, -round(log10($longErr))) - 1;
 82         
 83         //round it
 84         $lat=round($lat, $latPlaces);
 85         $long=round($long, $longPlaces);
 86         
 87         return array($lat,$long);
 88     }
 89 
 90     
 91     /**
 92     * Encode a hash from given lat and long
 93     */
 94     public function encode($lat,$long)
 95     {
 96         //how many bits does latitude need?    
 97         $plat=$this->precision($lat);
 98         $latbits=1;
 99         $err=45;
100         while($err>$plat)
101         {
102             $latbits++;
103             $err/=2;
104         }
105         
106         //how many bits does longitude need?
107         $plong=$this->precision($long);
108         $longbits=1;
109         $err=90;
110         while($err>$plong)
111         {
112             $longbits++;
113             $err/=2;
114         }
115         
116         //bit counts need to be equal
117         $bits=max($latbits,$longbits);
118         
119         //as the hash create bits in groups of 5, lets not
120         //waste any bits - lets bulk it up to a multiple of 5
121         //and favour the longitude for any odd bits
122         $longbits=$bits;
123         $latbits=$bits;
124         $addlong=1;
125         while (($longbits+$latbits)%5 != 0)
126         {
127             $longbits+=$addlong;
128             $latbits+=!$addlong;
129             $addlong=!$addlong;
130         }
131         
132         
133         //encode each as binary string
134         $blat=$this->binEncode($lat,-90,90, $latbits);
135         $blong=$this->binEncode($long,-180,180,$longbits);
136         
137         //merge lat and long together
138         $binary="";
139         $uselong=1;
140         while (strlen($blat)+strlen($blong))
141         {
142             if ($uselong)
143             {
144                 $binary=$binary.substr($blong,0,1);
145                 $blong=substr($blong,1);
146             }
147             else
148             {
149                 $binary=$binary.substr($blat,0,1);
150                 $blat=substr($blat,1);
151             }
152             $uselong=!$uselong;
153         }
154         
155         //convert binary string to hash
156         $hash="";
157         for ($i=0; $i<strlen($binary); $i+=5)
158         {
159             $n=bindec(substr($binary,$i,5));
160             $hash=$hash.$this->coding[$n];
161         }
162         
163         
164         return $hash;
165     }
166     
167     /**
168     * What‘s the maximum error for $bits bits covering a range $min to $max
169     */
170     private function calcError($bits,$min,$max)
171     {
172         $err=($max-$min)/2;
173         while ($bits--)
174             $err/=2;
175         return $err;
176     }
177     
178     /*
179     * returns precision of number
180     * precision of 42 is 0.5
181     * precision of 42.4 is 0.05
182     * precision of 42.41 is 0.005 etc
183     */
184     private function precision($number)
185     {
186         $precision=0;
187         $pt=strpos($number,‘.‘);
188         if ($pt!==false)
189         {
190             $precision=-(strlen($number)-$pt-1);
191         }
192         
193         return pow(10,$precision)/2;
194     }
195     
196     
197     /**
198     * create binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example
199     * removing the tail recursion is left an exercise for the reader
200     */
201     private function binEncode($number, $min, $max, $bitcount)
202     {
203         if ($bitcount==0)
204             return "";
205         
206         #echo "$bitcount: $min $max<br>";
207             
208         //this is our mid point - we will produce a bit to say
209         //whether $number is above or below this mid point
210         $mid=($min+$max)/2;
211         if ($number>$mid)
212             return "1".$this->binEncode($number, $mid, $max,$bitcount-1);
213         else
214             return "0".$this->binEncode($number, $min, $mid,$bitcount-1);
215     }
216     
217 
218     /**
219     * decodes binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example
220     * removing the tail recursion is left an exercise for the reader
221     */
222     private function binDecode($binary, $min, $max)
223     {
224         $mid=($min+$max)/2;
225         
226         if (strlen($binary)==0)
227             return $mid;
228             
229         $bit=substr($binary,0,1);
230         $binary=substr($binary,1);
231         
232         if ($bit==1)
233             return $this->binDecode($binary, $mid, $max);
234         else
235             return $this->binDecode($binary, $min, $mid);
236     }
237 }
238 
239 
240 
241 
242 
243 
244 ?>