Extract class to manage GeozoneStats

Even if this class looks very simple now, we're trying a few things
related to these stats. Having a class for it makes future changes
easier and, if there weren't any future changes, at least it makes
current experiments easier.

Note we keep the method `participants_by_geozone` to return a hash
because we're caching the stats and storing GeozoneStats objects would
need a lot more memory and we would get an error.

app/models/concerns/statisticable.rb

@@ -69,19 +69,21 @@ module Statisticable
     end
 
     def participants_by_geozone
-      geozones.map do |geozone|
-        count = participants.where(geozone: geozone).count
-
+      geozone_stats.map do |stats|
         [
-          geozone.name,
+          stats.name,
           {
-            count: count,
-            percentage: calculate_percentage(count, total_participants)
+            count: stats.count,
+            percentage: stats.percentage
           }
         ]
       end.to_h
     end
 
+    def calculate_percentage(fraction, total)
+      PercentageCalculator.calculate(fraction, total)
+    end
+
     private
 
       def base_stats_methods
@@ -124,10 +126,8 @@ module Statisticable
         Geozone.all.order("name")
       end
 
-      def calculate_percentage(fraction, total)
-        return 0.0 if total.zero?
-
-        (fraction * 100.0 / total).round(3)
+      def geozone_stats
+        geozones.map { |geozone| GeozoneStats.new(geozone, participants) }
       end
 
       def range_description(start, finish)

lib/geozone_stats.rb

@@ -0,0 +1,24 @@
+class GeozoneStats
+  attr_reader :geozone, :participants
+
+  def initialize(geozone, participants)
+    @geozone = geozone
+    @participants = participants
+  end
+
+  def geozone_participants
+    participants.where(geozone: geozone)
+  end
+
+  def name
+    geozone.name
+  end
+
+  def count
+    geozone_participants.count
+  end
+
+  def percentage
+    PercentageCalculator.calculate(count, participants.count)
+  end
+end

lib/percentage_calculator.rb

@@ -0,0 +1,7 @@
+module PercentageCalculator
+  def self.calculate(fraction, total)
+    return 0.0 if total.zero?
+
+    (fraction * 100.0 / total).round(3)
+  end
+end

spec/lib/geozone_stats_spec.rb

@@ -0,0 +1,44 @@
+require "rails_helper"
+
+describe GeozoneStats do
+  let(:winterfell) { create(:geozone, name: "Winterfell") }
+  let(:riverlands) { create(:geozone, name: "Riverlands") }
+
+  describe "#name" do
+    let(:stats) { GeozoneStats.new(winterfell, []) }
+
+    it "returns the geozone name" do
+      expect(stats.name).to eq "Winterfell"
+    end
+  end
+
+  describe "#count" do
+    before do
+      2.times { create(:user, geozone: winterfell) }
+      1.times { create(:user, geozone: riverlands) }
+    end
+
+    let(:winterfell_stats) { GeozoneStats.new(winterfell, User.all) }
+    let(:riverlands_stats) { GeozoneStats.new(riverlands, User.all) }
+
+    it "counts participants from the geozone" do
+      expect(winterfell_stats.count).to eq 2
+      expect(riverlands_stats.count).to eq 1
+    end
+  end
+
+  describe "#percentage" do
+    before do
+      2.times { create(:user, geozone: winterfell) }
+      1.times { create(:user, geozone: riverlands) }
+    end
+
+    let(:winterfell_stats) { GeozoneStats.new(winterfell, User.all) }
+    let(:riverlands_stats) { GeozoneStats.new(riverlands, User.all) }
+
+    it "calculates percentage relative to the amount of participants" do
+      expect(winterfell_stats.percentage).to eq 66.667
+      expect(riverlands_stats.percentage).to eq 33.333
+    end
+  end
+end