perdiem.rb

class Budget < ActiveRecord::Base
  attr_accessible :budget, :expenses, :start_date, :end_date, :purchases_attributes

  has_many :purchases
  accepts_nested_attributes_for :purchases, :allow_destroy => true

  def perdiem
    if remaining_days > 0
      ((projected_spend_so_far - actual_spend_so_far) / remaining_days) + initial_perdiem
    else
      available_funds - actual_spend_so_far
    end
  end

  def initial_perdiem
    available_funds / total_days.to_f
  end

  def available_funds
    budget - expenses
  end

  def actual_spend_so_far
    purchases.sum(&:amount) - spent_today
  end

  def projected_spend_so_far
    initial_perdiem * (total_days - remaining_days)
  end

  def remaining_budget
    available_funds - actual_spend_so_far - spent_today
  end

  def remaining_days
    if now_utc.beginning_of_day < start_date.utc.beginning_of_day
      total_days
    else
      (end_date.utc.beginning_of_day.to_i - now_utc.beginning_of_day.to_i) / 86400
    end
  end

  def total_days
    (end_date.utc.beginning_of_day.to_i - start_date.utc.beginning_of_day.to_i) / 86400
  end

  def title
    "#{start_date.utc.strftime("%m/%d/%Y")} - #{end_date.utc.strftime("%m/%d/%Y")}"
  end

  def spent_today
    purchases.select{|p| p.created_at >= (now_utc.beginning_of_day)}.sum(&:amount)
  end

  def now_utc
    Time.now.utc
  end
end