osmGeoHash.php

<?php

/*

 OpenStreetMap GeoHash generator and direct link generator

 Usage examples:

 OsmGeoHash\getHash(40.689167, -74.044444, 13) -> returns "Zctz1Jf"
 OsmGeoHash\getLink(40.689167, -74.044444, 13) -> returns "https://osm.org/go/Zctz1Jf?m="


 This is a direct translation of https://github.com/openstreetmap/openstreetmap-website/blob/e84b2bd22f7c92fb7a128a91c999f86e350bf04d/app/assets/javascripts/application.js
 As linked from https://wiki.openstreetmap.org/wiki/Shortlink

*/

namespace OsmGeoHash;

/*
 * Called to interlace the bits in x and y, making a Morton code.
 */

function interlace($x, $y): int
{
  $x = ($x | ($x << 8)) & 0x00ff00ff;
  $x = ($x | ($x << 4)) & 0x0f0f0f0f;
  $x = ($x | ($x << 2)) & 0x33333333;
  $x = ($x | ($x << 1)) & 0x55555555;

  $y = ($y | ($y << 8)) & 0x00ff00ff;
  $y = ($y | ($y << 4)) & 0x0f0f0f0f;
  $y = ($y | ($y << 2)) & 0x33333333;
  $y = ($y | ($y << 1)) & 0x55555555;

  return ($x << 1) | $y;
}

/*
 * Called to create a short code for the short link.
 */
function getHash(float $lat, float $lon, float $zoom): string
{
  $char_array = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_~";
  $x = round(($lon + 180.0) * ((1 << 30) / 90.0));
  $y = round(($lat +  90.0) * ((1 << 30) / 45.0));

  // PHP might use 32 bits of bitwise operators, so this has to be
  // done in two parts. each of the parts c1/c2 has 30 bits of the total in it
  // and drops the last 4 bits of the full 64 bit Morton code.
  $str = "";
  $c1 = interlace($x >> 17, $y >> 17);
  $c2 = interlace(($x >> 2) & 0x7fff, ($y >> 2) & 0x7fff);
  for ($i = 0; $i < ceil(($zoom + 8) / 3.0) && $i < 5; ++$i) {
    $digit = ($c1 >> (24 - 6 * $i)) & 0x3f;
    $str .= $char_array[$digit];
  }
  for ($i = 5; $i < ceil(($zoom + 8) / 3.0); ++$i) {
    $digit = ($c2 >> (24 - 6 * ($i - 5))) & 0x3f;
    $str .= $char_array[$digit];
  }
  for ($i = 0; $i < (($zoom + 8) % 3); ++$i)
    $str .= "-";
  return $str;
}

// Get osm.org/go link
function getLink(float $lat, float $lon, float $zoom, bool $showMarker = true): string
{
  return 'https://osm.org/go/' . getHash($lat, $lon, $zoom) . ($showMarker ? '?m=' : '');
}