Adding missing seasonal components files.

lib/tealeaves/seasonal_components.rb

@@ -0,0 +1,28 @@
+module TeaLeaves
+  class SeasonalComponents
+    def initialize(period, data)
+      @period = period
+      @data = data
+    end
+
+    def seasonal_averages
+      @seasonal_averages ||= seasonal_groups.map do |group| 
+        group.inject(&:+) / group.size.to_f
+      end
+    end
+
+    def seasonal_factors(operation = :-)
+      @seasonal_factors ||= seasonal_averages.map {|i| i.send(operation, avg) }
+    end
+
+    private
+
+    def avg
+      @avg ||= seasonal_averages.inject(&:+) / @period.to_f
+    end
+
+    def seasonal_groups
+      @data.take(@period).zip( *(@data.drop(@period).each_slice(@period)) ).map(&:compact)
+    end
+  end
+end

spec/seasonal_components_spec.rb

@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe TeaLeaves::SeasonalComponents do
+  it "returns seasonal averages for a period" do
+    described_class.new(4, [1,2,1,3,2,4,1,9]).seasonal_averages.
+      should == [1.5, 3, 1, 6]
+  end
+
+  it "returns seasonal averages for a period, with ragged data" do
+    described_class.new(4, [1,2,1,3,2,4,1,9, 3]).seasonal_averages.
+      should == [2, 3, 1, 6]
+  end
+
+  it "returns seasonal weights" do
+    described_class.new(4, [1,2,1,3,2,4,1,9,3]).seasonal_factors.
+      should == [-1.0, 0.0, -2.0, 3.0]
+  end
+end