Show trends on some dashboard numbers

app/assets/javascripts/admin/models/report.js

@@ -1,7 +1,69 @@
 Discourse.Report = Discourse.Model.extend({
   reportUrl: function() {
     return("/admin/reports/" + this.get('type'));
-  }.property('type')
+  }.property('type'),
+
+  valueAt: function(numDaysAgo) {
+    if (this.data) {
+      var wantedDate = Date.create(numDaysAgo + ' days ago', 'en').format('{yyyy}-{MM}-{dd}');
+      var item = this.data.find( function(d, i, arr) { return d.x === wantedDate; } );
+      if (item) {
+        return item.y;
+      }
+    }
+    return 0;
+  },
+
+  sumDays: function(startDaysAgo, endDaysAgo) {
+    if (this.data) {
+      var earliestDate = Date.create(endDaysAgo + ' days ago', 'en');
+      var latestDate = Date.create(startDaysAgo + ' days ago', 'en');
+      var d, sum = 0;
+      this.data.each(function(datum){
+        d = Date.create(datum.x);
+        if(d >= earliestDate && d <= latestDate) {
+          sum += datum.y;
+        }
+      });
+      return sum;
+    }
+  },
+
+  yesterdayTrend: function() {
+    var yesterdayVal = this.valueAt(1);
+    var twoDaysAgoVal = this.valueAt(2);
+    if ( yesterdayVal > twoDaysAgoVal ) {
+      return 'trending-up';
+    } else if ( yesterdayVal < twoDaysAgoVal ) {
+      return 'trending-down';
+    } else {
+      return 'no-change';
+    }
+  }.property('data'),
+
+  sevenDayTrend: function() {
+    var currentPeriod = this.sumDays(1,7);
+    var prevPeriod = this.sumDays(8,14);
+    if ( currentPeriod > prevPeriod ) {
+      return 'trending-up';
+    } else if ( currentPeriod < prevPeriod ) {
+      return 'trending-down';
+    } else {
+      return 'no-change';
+    }
+  }.property('data'),
+
+  thirtyDayTrend: function() {
+    if( this.get('prev30Days') ) {
+      var currentPeriod = this.sumDays(1,30);
+      if( currentPeriod > this.get('prev30Days') ) {
+        return 'trending-up';
+      } else if ( currentPeriod < this.get('prev30Days') ) {
+        return 'trending-down';
+      }
+    }
+    return 'no-change';
+  }.property('data', 'prev30Days')
 });
 
 Discourse.Report.reopenClass({