geohash.php

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<?php
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);
      }
      
   }
   
   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);
   }

   
   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; $i<strlen($binary); $i+=5)
      {
         $n=bindec(substr($binary,$i,5));
         $hash=$hash.$this->coding[$n];
      }
      
      
      return $hash;
   }
   
   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;
   }
   
   
   private function binEncode($number, $min, $max, $bitcount)
   {
      if ($bitcount==0)
         return "";
      
      #echo "$bitcount: $min $max<br>";
         
      //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);
   }
   

   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);
   }
}






?>