simonoff · November 18, 2013 21:22 · Feb 6, 2012 · Feb 6, 2012 · Feb 3, 2012
diff --git a/linearregression.rb b/linearregression.rb
@@ -1,6 +1,9 @@
 # Adapted from a C# example here:
 # http://stackoverflow.com/questions/43224/how-do-i-calculate-a-trendline-for-a-graph
 
+# And thanks to John Esser for helping figure out how to
+# calculate the targets to stabilize a negative slope!
+
 class LinearRegression
   attr_accessor :slope, :intercept
 

diff --git a/linearregression.rb b/linearregression.rb
@@ -45,5 +45,40 @@ def predict( x )
   # was continued one more element
   def next
     predict (@size + 1)
+  end
+
+  # Determine the target needed to stabilize
+  # the slope back to 0, assuming slope is
+  # negative. The target returned will be
+  # the number needed to have stabilized slope
+  # inclusive of all values in the set
+  def stabilize_over_all
+    target = stabilize @y_values
+    target
+  end 
+
+  # Determine the target needed to stabilize
+  # the slope back to 0, assuming slope is
+  # negative. The target returned will be
+  # the number needed to have stabilized slope
+  # over a set of values of size n (including
+  # the target value
+  def stabilize_over_n_values(n)
+    values = []
+    values = @y_values.last(n-1) if n - 1 >= 0
+    target = stabilize values
+    target
   end 
+
+  private
+  def stabilize(values)
+    n = values.length
+    sum = 0 
+    target = 0 
+    values.each_with_index do |value, i|
+        sum += (n - (2 * i)) * value
+    end 
+    target = 1.0 * sum / n if n > 0 
+    target
+  end
 end
diff --git a/linearregression.rb b/linearregression.rb
@@ -0,0 +1,49 @@
+# Adapted from a C# example here:
+# http://stackoverflow.com/questions/43224/how-do-i-calculate-a-trendline-for-a-graph
+
+class LinearRegression
+  attr_accessor :slope, :intercept
+
+  # Pass in an array of values to get the regression on
+  def initialize y_values
+    @y_values = y_values
+    @size = @y_values.size
+    @x_values = (1..@size).to_a
+
+    #initialize everything to 0
+    sum_x = 0 
+    sum_y = 0 
+    sum_xx = 0 
+    sum_xy = 0 
+
+    # calculate the sums
+    @y_values.zip(@x_values).each do |y, x|
+      sum_xy += x*y 
+      sum_xx += x*x 
+      sum_x  += x
+      sum_y  += y
+    end 
+
+    # calculate the slope
+    @slope = 1.0 * ((@size * sum_xy) - (sum_x * sum_y)) / ((@size * sum_xx) - (sum_x * sum_x))
+    @intercept = 1.0 * (sum_y - (@slope * sum_x)) / @size
+  end 
+
+  # Get the y-axis values of the trend line
+  def trend
+    return @x_values.map { |x| predict(x) }
+  end 
+
+  # Get the Y value for any given X value
+  # from y = mx + b, or
+  # y = slope * x + intercept
+  def predict( x ) 
+    y = @slope * x + @intercept
+  end 
+
+  # Get the "next" value if the sequence
+  # was continued one more element
+  def next
+    predict (@size + 1)
+  end 
+end