From 1117e29c697b54eb05279b73b62515de1595dbb2 Mon Sep 17 00:00:00 2001
From: Aaron Parecki <aaron@parecki.com>
Date: Sun, 28 Jan 2018 14:18:37 -0800
Subject: [PATCH] add geo functions

---
 README.md     |  1 +
 composer.json |  3 +-
 src/geo.php   | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 96 insertions(+), 1 deletion(-)
 create mode 100644 src/geo.php

diff --git a/README.md b/README.md
index 2193a22..09d3b71 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,7 @@ This project provides several helpful utilities for working with PHP projects, a
 * `p3k\url` - helpful functions for parsing and building URLs
 * `p3k\cache` - a simple caching helper backed by Redis
 * `p3k\utils` - miscellaneous functions
+* `p3k\geo` - geo functions for calculating distance and other things
 * `p3k\global` - sets the timezone to UTC (you're already storing all your dates in UTC, right?)
 
 PHP Support
diff --git a/composer.json b/composer.json
index 3b6e4b1..3f92399 100644
--- a/composer.json
+++ b/composer.json
@@ -23,7 +23,8 @@
       "src/url.php",
       "src/utils.php",
       "src/date.php",
-      "src/cache.php"
+      "src/cache.php",
+      "src/geo.php"
     ]
   },
   "autoload-dev": {
diff --git a/src/geo.php b/src/geo.php
new file mode 100644
index 0000000..ff265cf
--- /dev/null
+++ b/src/geo.php
@@ -0,0 +1,93 @@
+<?php
+namespace p3k\geo;
+
+//RamerDouglasPeucker
+//Reduces the number of points on a polyline by removing those that are closer to the line
+//than the distance $epsilon.
+//The polyline is provided as an array of arrays, where each internal array is one point on the polyline,
+//specified by easting (x-coordinate) with key "0" and northing (y-coordinate) with key "1".
+//It is assumed that the coordinates and distance $epsilon are given in the same units.
+//The result is returned as an array in a similar format.
+//Each point returned in the result array will retain all its original data, including its E and N
+//values along with any others.
+function ramerDouglasPeucker($pointList, $epsilon)
+{
+    if(count($pointList) == 0)
+      return array();
+
+    // Find the point with the maximum distance
+    $dmax = 0;
+    $index = 0;
+    $totalPoints = count($pointList);
+    for ($i = 1; $i < ($totalPoints - 1); $i++)
+    {
+        $d = $this->_perpendicularDistance($pointList[$i][0], $pointList[$i][1],
+                                   $pointList[0][0], $pointList[0][1],
+                                   $pointList[$totalPoints-1][0], $pointList[$totalPoints-1][1]);
+
+        if ($d > $dmax)
+        {
+            $index = $i;
+            $dmax = $d;
+        }
+    }
+
+    $resultList = array();
+
+    // If max distance is greater than epsilon, recursively simplify
+    if ($dmax >= $epsilon)
+    {
+        // Recursive call
+        $recResults1 = $this->_ramerDouglasPeucker(array_slice($pointList, 0, $index + 1), $epsilon);
+        $recResults2 = $this->_ramerDouglasPeucker(array_slice($pointList, $index, $totalPoints - $index), $epsilon);
+
+        // Build the result list
+        $resultList = array_merge(array_slice($recResults1, 0, count($recResults1) - 1),
+                                  array_slice($recResults2, 0, count($recResults2)));
+    }
+    else
+    {
+        $resultList = array($pointList[0], $pointList[$totalPoints-1]);
+    }
+    // Return the result
+    return $resultList;
+}
+
+
+// http://www.loughrigg.org/rdp/
+
+//The author has placed this work in the Public Domain, thereby relinquishing all copyrights.
+//You may use, modify, republish, sell or give away this work without prior consent.
+//This implementation comes with no warranty or guarantee of fitness for any purpose.
+
+//=========================================================================
+//An implementation of the Ramer-Douglas-Peucker algorithm for reducing
+//the number of points on a polyline
+//see http://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm
+//=========================================================================
+
+//Finds the perpendicular distance from a point to a straight line.
+//The coordinates of the point are specified as $ptX and $ptY.
+//The line passes through points l1 and l2, specified respectively with their
+//coordinates $l1x and $l1y, and $l2x and $l2y
+function perpendicularDistance($ptX, $ptY, $l1x, $l1y, $l2x, $l2y)
+{
+    $result = 0;
+    if ($l2x == $l1x)
+    {
+        //vertical lines - treat this case specially to avoid divide by zero
+        $result = abs($ptX - $l2x);
+    }
+    else
+    {
+        $slope = (($l2y-$l1y) / ($l2x-$l1x));
+        $passThroughY = (0-$l1x)*$slope + $l1y;
+        $result = (abs(($slope * $ptX) - $ptY + $passThroughY)) / (sqrt($slope*$slope + 1));
+    }
+    return $result;
+}
+
+// Calculate the Great Circle distance between two points, in meters
+function gc_distance($lat1, $lng1, $lat2, $lng2) {
+  return ( 6378100 * acos( cos( deg2rad($lat1) ) * cos( deg2rad($lat2) ) * cos( deg2rad($lng2) - deg2rad($lng1) ) + sin( deg2rad($lat1) ) * sin( deg2rad($lat2) ) ) );
+}