Compute stats for array of numbers

stats.rb

def percentile(arr, pcent)
  sorted = arr.sort

  # http://en.wikipedia.org/wiki/Percentile#Linear_interpolation_between_closest_ranks
  percent_ranks = (1..arr.length).map { |i| (100.0 / sorted.length) * ( i - 0.5) }

  if pcent < percent_ranks.first
    sorted[0]
  elsif pcent > percent_ranks.last
    sorted.last
  else
    idx = percent_ranks.find_index { |rank| rank == pcent }
    return sorted[idx] if idx != nil

    idx = rank_at = nil
    percent_ranks.each_with_index do |rank, i|
      if rank < pcent && pcent < percent_ranks[i+1]
        idx = i
        rank_at = rank
      end
    end
    sorted[idx] + (arr.length * (pcent-rank_at).to_f/100 * (sorted[idx+1] - sorted[idx]))
  end
end

def stats(arr)
  arr = arr.clone.extend(Stats)

  lowest = arr.min
  highest = arr.max
  total = arr.inject(:+)
  len = arr.length
  average = total.to_f / len # to_f so we don't get an integer result
  sorted = arr.sort
  variance = (arr.inject(0) {|accum, i| accum + (i-average)**2 }.to_f/(len))
  std_dev = Math.sqrt(variance)
  median = len % 2 == 1 ? sorted[len/2] : (sorted[len/2 - 1] + sorted[len/2]).to_f / 2
  {
    :min => lowest,
    :max => highest,
    :avg => average,
    :std_dev => std_dev,
    :med => median,
    :pct_50 => percentile(arr, 50),
    :pct_90 => percentile(arr, 90)
  }.tap do |stats|
    if (stats[:med]-stats[:pct_50]).abs > 0.000001
      raise "med was #{stats[:med].inspect} but 50% percenile was #{stats[:pct_50].inspect} for #{arr.inspect}}"
    end
  end
end


module Stats

  def sum
    self.inject(0){|accum, i| accum + i }
  end

  def mean
    self.sum/self.length.to_f
  end

  def sample_variance
    m = self.mean
    sum = self.inject(0){|accum, i| accum +(i-m)**2 }
    sum/(self.length - 1).to_f
  end

  def standard_deviation
    return Math.sqrt(self.sample_variance)
  end

end