lib/field_test/experiment.rb in field_test-0.1.2 vs lib/field_test/experiment.rb in field_test-0.2.0

@@ -1,13 +1,14 @@
 module FieldTest
   class Experiment
-    attr_reader :id, :name, :variants, :weights, :winner, :started_at, :ended_at
+    attr_reader :id, :name, :description, :variants, :weights, :winner, :started_at, :ended_at
 
     def initialize(attributes)
       attributes = attributes.symbolize_keys
       @id = attributes[:id]
       @name = attributes[:name] || @id.to_s.titleize
+      @description = attributes[:description]
       @variants = attributes[:variants]
       @weights = @variants.size.times.map { |i| attributes[:weights].to_a[i] || 1 }
       @winner = attributes[:winner]
       @started_at = Time.zone.parse(attributes[:started_at].to_s) if attributes[:started_at]
       @ended_at = Time.zone.parse(attributes[:ended_at].to_s) if attributes[:ended_at]
@@ -31,10 +32,21 @@
       membership.participant = participants.first
 
       if membership.changed?
         begin
           membership.save!
+
+          # log it!
+          info = {
+            experiment: id,
+            variant: membership.variant,
+            participant: membership.participant
+          }.merge(options.slice(:ip, :user_agent))
+
+          # sorta logfmt :)
+          info = info.map { |k, v| v = "\"#{v}\"" if k == :user_agent; "#{k}=#{v}" }.join(" ")
+          Rails.logger.info "[field test] #{info}"
         rescue ActiveRecord::RecordNotUnique
           membership = memberships.find_by(participant: participants.first)
         end
       end
 
@@ -73,14 +85,14 @@
           converted: converted,
           conversion_rate: participated > 0 ? converted.to_f / participated : nil
         }
       end
       case variants.size
-      when 1
-        results[variants[0]][:prob_winning] = 1
-      when 2, 3
-        variants.size.times do |i|
+      when 1, 2, 3
+        total = 0.0
+
+        (variants.size - 1).times do |i|
           c = results.values[i]
           b = results.values[(i + 1) % variants.size]
           a = results.values[(i + 2) % variants.size]
 
           alpha_a = 1 + a[:converted]
@@ -88,17 +100,27 @@
           alpha_b = 1 + b[:converted]
           beta_b = 1 + b[:participated] - b[:converted]
           alpha_c = 1 + c[:converted]
           beta_c = 1 + c[:participated] - c[:converted]
 
-          results[variants[i]][:prob_winning] =
+          # TODO calculate this incrementally by caching intermediate results
+          prob_winning =
             if variants.size == 2
-              prob_b_beats_a(alpha_b, beta_b, alpha_c, beta_c)
+              cache_fetch ["field_test", "prob_b_beats_a", alpha_b, beta_b, alpha_c, beta_c] do
+                Calculations.prob_b_beats_a(alpha_b, beta_b, alpha_c, beta_c)
+              end
             else
-              prob_c_beats_a_and_b(alpha_a, beta_a, alpha_b, beta_b, alpha_c, beta_c)
+              cache_fetch ["field_test", "prob_c_beats_a_and_b", alpha_a, beta_a, alpha_b, beta_b, alpha_c, beta_c] do
+                Calculations.prob_c_beats_a_and_b(alpha_a, beta_a, alpha_b, beta_b, alpha_c, beta_c)
+              end
             end
+
+          results[variants[i]][:prob_winning] = prob_winning
+          total += prob_winning
         end
+
+        results[variants.last][:prob_winning] = 1 - total
       end
       results
     end
 
     def active?
@@ -138,35 +160,14 @@
           return variants[i] if n >= pick
         end
         variants.last
       end
 
-      # formula from
-      # http://www.evanmiller.org/bayesian-ab-testing.html
-      def prob_b_beats_a(alpha_a, beta_a, alpha_b, beta_b)
-        total = 0.0
-
-        0.upto(alpha_b - 1) do |i|
-          total += Math.exp(Math.logbeta(alpha_a + i, beta_b + beta_a) -
-            Math.log(beta_b + i) - Math.logbeta(1 + i, beta_b) -
-            Math.logbeta(alpha_a, beta_a))
+      def cache_fetch(key)
+        if FieldTest.cache
+          Rails.cache.fetch(key.join("/")) { yield }
+        else
+          yield
         end
-
-        total
-      end
-
-      def prob_c_beats_a_and_b(alpha_a, beta_a, alpha_b, beta_b, alpha_c, beta_c)
-        total = 0.0
-        0.upto(alpha_a - 1) do |i|
-          0.upto(alpha_b - 1) do |j|
-            total += Math.exp(Math.logbeta(alpha_c + i + j, beta_a + beta_b + beta_c) -
-              Math.log(beta_a + i) - Math.log(beta_b + j) -
-              Math.logbeta(1 + i, beta_a) - Math.logbeta(1 + j, beta_b) -
-              Math.logbeta(alpha_c, beta_c))
-          end
-        end
-
-        1 - prob_b_beats_a(alpha_c, beta_c, alpha_a, beta_a) -
-          prob_b_beats_a(alpha_c, beta_c, alpha_b, beta_b) + total
       end
   end
 end