Calculate percentaile values in an array of float values

percentile.rb

#!/usr/bin/env ruby
class Percentile
  ROUND_DIGIT = 1
  
  def pctile(data0)
    return [] if data0.empty?
    data = data0.map{|x|Integer(x.round(ROUND_DIGIT) * (10 ** ROUND_DIGIT))}
    sorted_data = data.sort
    index_sum = Hash.new(0)
    count = Hash.new(0)
    # calculate sum of index and count of each value
    sorted_data.each_with_index do |value, index|
      index_sum[value] += index
      count[value] += 1
    end
    # calculate mean indice of each value
    average_index = index_sum.map{|val, sum| [val , (sum.to_f / count[val])]}.to_h
    # calculate perventiles of each value
    data_percentile_ranks = data.map do |value|
      percentile_rank = average_index[value] / (data.length - 1) * 100
      [(value.to_f / (10 ** ROUND_DIGIT)), percentile_rank]
    end
    data_percentile_ranks
  end
  
  def run
    pp pctile([3.14, 1.4142, 3.14159, 2.2362])
    ## => [[3.1, 83.33333333333334], [1.4, 0.0], [3.1, 83.33333333333334], [2.2, 33.33333333333333]]
  end
end

if $0 == __FILE__
  Percentile.new.run
end