compounding.rb

# To demonstrate what a "Bank of Dad" would be like

require "date"

INTEREST_PER_MONTH = 0.1

birthday = Date.today # replace this with the child's birthday
twentyone = 21.times.inject(birthday) { |d, _| d.next_year }

balance = 0.0

now = Date.today
while now < twentyone
  show = false
  if now.day == 1
    balance = balance * (1.0 + INTEREST_PER_MONTH)
    show = true
  end
  if now.sunday?
    balance += 1.00
    show = true
  end
  if show
    puts "#{now} $#{'%.2f' % balance}"
  end
  now = now.next_day
end