naive amortization

naive_amortization.rb

# naive amortization implementation using enumerators.
# used for a possible answer to compare against candidate solutions

# intended to determine how a candidate breaks up a problem

# similar haskell implementation - https://gist.github.com/tippenein/8dceb2d50272ec24ce13fe83f3a57bfa

module Amortizable
  include Enumerable

  def estimated_monthly
    r = apr / 100 / 12  # percent per period
    top = r * (1 + r) ** term
    bottom = ((1 + r) ** term) - 1
    principal * (top / bottom)
  end

  def amortization
    principal_month_pairs.map do |pair|
      Amortization.new(*pair, apr)
    end
  end

  private
  def principal_month_pairs
    principals = principal_enumerator_from(principal).take(self.term)
    months = month_enumerator_from(self.created_at).take(self.term)
    principals.zip(months)
  end

  def principal_enumerator_from(initial_principal)
    Enumerator.new do |g|
      s = initial_principal
      while s > 0 do
        next_principal = s - estimated_monthly
        g.yield next_principal
        s = next_principal
      end
    end
  end

  def month_enumerator_from(start)
    Enumerator.new do |g|
      s = start
      loop do
        next_month = 1.month.since(s)
        g.yield next_month
        s = next_month
      end
    end
  end


  class Amortization
    attr_accessor :date, :principal, :interest, :apr
    def initialize(principal, date, apr)
      @principal = principal > 0 ? principal : 0
      @date = date
      @apr = apr
      @interest = interest_from(@principal)
    end

    def interest_from(principal)
      apr_percent = apr / 100
      (principal * (apr_percent / 365) * 30).round(2)
    end
  end
end