preg.rb

require 'date'

DUE_DATE = "2013-05-19"

#data taken from http://spacefem.com/pregnant/charts/duedate2.php
#starts at day 222
DATA = [
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  1, 2, 1, 4, 2, 2, 1, 4, 6, 7, 5, 1, 5, 8, 7, 9, 10, 11, 13, 13, 18,
  14, 13, 9, 27, 29, 27, 31, 27, 26, 36, 43, 43, 51, 67, 74, 60, 47,
  41, 44, 36, 37, 48, 30, 20, 22, 24, 16, 11, 12, 8, 4, 2, 0, 1, 0, 1
]

start_index = Date.today + 58 - Date.parse(DUE_DATE)
frequencies = DATA[start_index .. -1]
acc = 0
cumulative_frequencies = frequencies.map{|n| acc += n}
cumulative_probabilities = cumulative_frequencies.map{|n| n.to_f / cumulative_frequencies[-1]}

puts "Chance of baby"
cumulative_probabilities.each_with_index do |prob, i|
  puts "by #{(Date.today + i).to_s}: #{(prob*100).to_i}%"
end