add tests around mbTile SQKM calculation

lib/mbtiles.js

@@ -11,6 +11,7 @@ var tiletype = require('@mapbox/tiletype');
 var ZXYStream = require('./zxystream');
 var queue = require('d3-queue').queue;
 var os = require('os');
+var utils = require('./utils.js');
 
 if (process.env.BRIDGE_LOG_MAX_VTILE_BYTES_COMPRESSED) {
     var stats = { max:0, total:0, count:0 };
@@ -192,6 +193,15 @@ MBTiles.prototype.getTile = function(z, x, y, callback) {
                 if (stats.max < tileDataLength) {
                     stats.max = tileDataLength;
                 }
+                var area = stats.hasOwnProperty(z)
+                    ? stats[z] + utils.calculateTileArea(z, x, y)
+                    : utils.calculateTileArea(z, x, y);
+
+                stats = {
+                    ...stats,
+                    [z]: area
+                };
+
             }
             return callback(null, row.tile_data, headers);
         }

lib/utils.js

@@ -0,0 +1,35 @@
+'use strict'
+
+var EARTH_RADIUS = 6371.0088;
+
+function degToRad(degrees) {
+    return degrees * (Math.PI / 180);
+}
+
+function tileToLon(tileX, zoom) {
+    return ((tileX / 2**zoom) * 360.0) - 180.0;
+}
+
+function tileToLat(tileY, zoom) {
+    var n = Math.PI - 2 * Math.PI * tileY / 2**zoom;
+    return (180.0 / Math.PI) * Math.atan(0.5 * (Math.exp(n) - Math.exp(-n)));
+}
+
+/**
+ *
+ * @param {Number} zoom
+ * @param {Number} tileX
+ * @param {Number} tileY
+ * @returns {Number} SqKM of a given tile
+ */
+function calculateTileArea(zoom, tileX, tileY) {
+    var left = degToRad(tileToLon(tileX, zoom));
+    var top = degToRad(tileToLat(tileY, zoom));
+    var right = degToRad(tileToLon(tileX + 1, zoom));
+    var bottom = degToRad(tileToLat(tileY + 1, zoom));
+    return (Math.PI / degToRad(180)) * EARTH_RADIUS**2 * Math.abs(Math.sin(top) - Math.sin(bottom)) * Math.abs(left - right);
+}
+
+module.exports = {
+    calculateTileArea
+};