Add bin and edit workflow

bin/gems/facets-3.1.0/lib/standard/facets/math/gini_coefficient.rb

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+require 'facets/math/approx_equal'
+
+module Math
+
+  # Calculates the Gini Coefficient (a measure of inequality of a distribution
+  # based on the area between the Lorenz curve and the uniform curve).
+  #
+  # http://en.wikipedia.org/wiki/Gini_coefficient
+  #
+  # This is a slightly cleaner way of calculating the Gini Coefficient then
+  # the previous implementationj.
+  #
+  #   GC = \frac{\sum_{i=1}^N (2i-N-1)x_i}{N^2-\bar{x}}
+  #
+  def self.gini_coefficient(array)
+    return -1 if size <= 0 or any? { |x| x < 0 }
+    return 0 if size < 2 or all? { |x| approx_equal(x,0) }
+    s = 0
+    sort.each_with_index { |li,i| s += (2*i+1-size)*li }
+    s.to_f/(size**2*mean).to_f
+  end
+
+  ## OLD WAY
+  ##   GC = \frac{1}{N} \left ( N+1-2\frac{\sum_{i=1}^N (N+1-i)y_i}{\sum_{i=1}^N y_i} \right )
+  ## def self.gini_coefficient2(array)
+  ##   return -1 if size <= 0 or any? { |x| x < 0 }
+  ##   return 0 if size < 2 or all? { |x| Math::float_equal(x,0) }
+  ##   s = 0
+  ##   sort.each_with_index { |yi,i| s += (size - i)*yi }
+  ##   (size+1-2*(s.to_f/sum.to_f)).to_f/size.to_f
+  ## end
+
+end