X-Git-Url: https://git.openstreetmap.org/rails.git/blobdiff_plain/04a6c41be94cea2175e49aa290bf204e69c41a5c..6ad1642dae301de389bdfdaacc5d88c83277b60a:/public/javascripts/site.js

diff --git a/public/javascripts/site.js b/public/javascripts/site.js
index b11a2e012..840617e17 100644
--- a/public/javascripts/site.js
+++ b/public/javascripts/site.js
@@ -254,23 +254,35 @@ function i18n(string, keys) {
   return string;
 }
 
+/*
+ * Called to interlace the bits in x and y, making a Morton code.
+ */
+function interlace(x, y) {
+    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 makeShortCode(lat, lon, zoom) {
     char_array = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_@";
     var x = Math.round((lon + 180.0) * ((1 << 30) / 90.0));
     var y = Math.round((lat +  90.0) * ((1 << 30) / 45.0));
-    // hack around the fact that JS apparently only allows 53-bit integers?!?
-    // note that, although this reduces the accuracy of the process, it's fine for
-    // z18 so we don't need to care for now.
-    var c1 = 0, c2 = 0;
-    for (var i = 31; i > 16; --i) {
-        c1 = (c1 << 1) | ((x >> i) & 1);
-        c1 = (c1 << 1) | ((y >> i) & 1);
-    }
-    for (var i = 16; i > 1; --i) {
-        c2 = (c2 << 1) | ((x >> i) & 1);
-        c2 = (c2 << 1) | ((y >> i) & 1);
-    }
+    // JavaScript only has to keep 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.
     var str = "";
+    var c1 = interlace(x >>> 17, y >>> 17), c2 = interlace((x >>> 2) & 0x7fff, (y >>> 2) & 0x7fff);
     for (var i = 0; i < Math.ceil((zoom + 8) / 3.0) && i < 5; ++i) {
         digit = (c1 >> (24 - 6 * i)) & 0x3f;
         str += char_array.charAt(digit);