geohash.class.php
/**
- Geohash generation class
- http://blog.dixo.net/downloads/
*
- This file copyright (C) 2008 Paul Dixon ([email protected])
*
- This program is free software; you can redistribute it and/or
- modify it under the terms of the GNU General Public License
- as published by the Free Software Foundation; either version 3
- of the License, or (at your option) any later version.
*
- This program is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- GNU General Public License for more details.
*
- You should have received a copy of the GNU General Public License
- along with this program; if not, write to the Free Software
- Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
/**
- Encode and decode geohashes
*
*/
class Geohash
{
private $coding="0123456789bcdefghjkmnpqrstuvwxyz";
private $codingMap=array();
public function Geohash()
{
//build map from encoding char to 0 padded bitfield
for($i=0; $i<32; $i++)
{
$this->codingMap[substr($this->coding,$i,1)]=str_pad(decbin($i), 5, "0", STR_PAD_LEFT);
}
}
/**
* Decode a geohash and return an array with decimal lat,long in it
*/
public function decode($hash)
{
//decode hash into binary string
$binary="";
$hl=strlen($hash);
for($i=0; $i<$hl; $i++)
{
$binary.=$this->codingMap[substr($hash,$i,1)];
}
//split the binary into lat and log binary strings
$bl=strlen($binary);
$blat="";
$blong="";
for ($i=0; $i<$bl; $i++)
{
if ($i%2)
$blat=$blat.substr($binary,$i,1);
else
$blong=$blong.substr($binary,$i,1);
}
//now concert to decimal
$lat=$this->binDecode($blat,-90,90);
$long=$this->binDecode($blong,-180,180);
//figure out how precise the bit count makes this calculation
$latErr=$this->calcError(strlen($blat),-90,90);
$longErr=$this->calcError(strlen($blong),-180,180);
//how many decimal places should we use? There's a little art to
//this to ensure I get the same roundings as geohash.org
$latPlaces=max(1, -round(log10($latErr))) - 1;
$longPlaces=max(1, -round(log10($longErr))) - 1;
//round it
$lat=round($lat, $latPlaces);
$long=round($long, $longPlaces);
return array($lat,$long);
}
/**
* Encode a hash from given lat and long
*/
public function encode($lat,$long)
{
//how many bits does latitude need?
$plat=$this->precision($lat);
$latbits=1;
$err=45;
while($err>$plat)
{
$latbits++;
$err/=2;
}
//how many bits does longitude need?
$plong=$this->precision($long);
$longbits=1;
$err=90;
while($err>$plong)
{
$longbits++;
$err/=2;
}
//bit counts need to be equal
$bits=max($latbits,$longbits);
//as the hash create bits in groups of 5, lets not
//waste any bits - lets bulk it up to a multiple of 5
//and favour the longitude for any odd bits
$longbits=$bits;
$latbits=$bits;
$addlong=1;
while (($longbits+$latbits)%5 != 0)
{
$longbits+=$addlong;
$latbits+=!$addlong;
$addlong=!$addlong;
}
//encode each as binary string
$blat=$this->binEncode($lat,-90,90, $latbits);
$blong=$this->binEncode($long,-180,180,$longbits);
//merge lat and long together
$binary="";
$uselong=1;
while (strlen($blat)+strlen($blong))
{
if ($uselong)
{
$binary=$binary.substr($blong,0,1);
$blong=substr($blong,1);
}
else
{
$binary=$binary.substr($blat,0,1);
$blat=substr($blat,1);
}
$uselong=!$uselong;
}
//convert binary string to hash
$hash="";
for ($i=0; $icoding[$n];
}
return $hash;
}
/**
* What's the maximum error for $bits bits covering a range $min to $max
*/
private function calcError($bits,$min,$max)
{
$err=($max-$min)/2;
while ($bits--)
$err/=2;
return $err;
}
/*
* returns precision of number
* precision of 42 is 0.5
* precision of 42.4 is 0.05
* precision of 42.41 is 0.005 etc
*/
private function precision($number)
{
$precision=0;
$pt=strpos($number,'.');
if ($pt!==false)
{
$precision=-(strlen($number)-$pt-1);
}
return pow(10,$precision)/2;
}
/**
* create binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example
* removing the tail recursion is left an exercise for the reader
*/
private function binEncode($number, $min, $max, $bitcount)
{
if ($bitcount==0)
return "";
#echo "$bitcount: $min $max
";
//this is our mid point - we will produce a bit to say
//whether $number is above or below this mid point
$mid=($min+$max)/2;
if ($number>$mid)
return "1".$this->binEncode($number, $mid, $max,$bitcount-1);
else
return "0".$this->binEncode($number, $min, $mid,$bitcount-1);
}
/**
* decodes binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example
* removing the tail recursion is left an exercise for the reader
*/
private function binDecode($binary, $min, $max)
{
$mid=($min+$max)/2;
if (strlen($binary)==0)
return $mid;
$bit=substr($binary,0,1);
$binary=substr($binary,1);
if ($bit==1)
return $this->binDecode($binary, $mid, $max);
else
return $this->binDecode($binary, $min, $mid);
} }
?>
test.php
/**
- Geohash generation class test harness
- http://blog.dixo.net/downloads/
*
- This file copyright (C) 2008 Paul Dixon ([email protected])
*
- This program is free software; you can redistribute it and/or
- modify it under the terms of the GNU General Public License
- as published by the Free Software Foundation; either version 3
- of the License, or (at your option) any later version.
*
- This program is distributed in the hope that it will be useful,
- but WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
- GNU General Public License for more details.
*
- You should have received a copy of the GNU General Public License
- along with this program; if not, write to the Free Software
- Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
require_once('geohash.class.php');
$geohash=new Geohash;
//these test hashes were made on geohash.org
//and test various combinations of precision
//and range
$tests=array(
"ezs42"=>array(42.6,-5.6),
"mh7w"=>array(-20, 50),
"t3b9m"=>array(10.1, 57.2),
"c2b25ps"=>array(49.26, -123.26),
"80021bgm"=>array(0.005, -179.567),
"k484ht99h2"=>array(-30.55555, 0.2),
"8buh2w4pnt"=>array(5.00001, -140.6),);
foreach($tests as $actualhash=>$coords)
{
$computed_hash=$geohash->encode($coords[0], $coords[1]);
echo "Encode {$coords[0]}, {$coords[1]} as $actualhash : ";
if ($computed_hash==$actualhash)
{
echo "OK
";
}
else
{
echo "FAIL (got $computed_hash)
";
}
echo "
";
$computed_coords=$geohash->decode($actualhash);
echo "Decode $actualhash as {$coords[0]}, {$coords[1]} : ";
if (($computed_coords[0]==$coords[0]) && ($computed_coords[1]==$coords[1]))
{
echo "OK
";
}
else
{
echo "FAIL (got {$computed_coords[0]}, {$computed_coords[1]})
";
}
echo "
";}
?>