daily_cost.rb

require 'date'

def daily_cost(start_date, end_date, time_period_costs)
  daily_cost = {}

  if start_date > end_date
    raise "Wrong time period given."
  end
  
  (start_date..end_date).each do |date|
    time_period_costs.each do |time_period_cost|
      unless time_period_cost[:cost].is_a?(Float)
        raise "Wrong cost given." 
      end

      daily_cost[date] ||= 0

      if time_period_cost[:time_period] == :daily
        daily_cost[date] = time_period_cost[:cost]
      elsif time_period_cost[:time_period] == :weekly
        daily_cost[date] += time_period_cost[:cost] / 7
      elsif time_period_cost[:time_period] == :monthly
        daily_cost[date] += time_period_cost[:cost] / days_in_month(date.year, date.month)
      else
        raise "Wrong time interval given."
      end
    end
  end

  daily_cost
end

def days_in_month(year, month)
  Date.new(year, month, -1).day
end

daily_cost_spec.rb

require 'spec_helper'
require_relative '../daily_cost.rb'

describe '#daily_cost' do
  it 'should work when empty time periods passed in' do
    expect(daily_cost(Date.new, Date.new, {})).to eq({})
  end

  describe "error handling" do
    it 'raises wrong time period' do
      start_date = Date.new(2020, 9, 2)
      end_date = Date.new(2020, 9, 1)
      expect {daily_cost(start_date, end_date, {})}
        .to raise_error(RuntimeError, "Wrong time period given.")
    end

    it 'raises wrong time interval' do
      start_date = Date.new(2020, 9, 1)
      end_date = Date.new(2020, 9, 2)

      time_period_costs = [
        {
          time_period: :yearly,
          cost: 30.0
        }
      ]

      expect {daily_cost(start_date, end_date, time_period_costs)}
        .to raise_error(RuntimeError, "Wrong time interval given.")
    end
  end

  context 'when mixed time periods passed in' do
    it 'returns correct value for all time periods' do
      start_date = Date.new(2019, 10, 1)
      end_date = Date.new(2019, 10, 3)

      time_period_costs = [
        {
          time_period: :daily,
          cost: 10.0
        },
        {
          time_period: :weekly,
          cost: 70.0
        },
        {
          time_period: :monthly,
          cost: 31.0
        }
      ]

      expect(daily_cost(start_date, end_date, time_period_costs))
        .to eq({
          Date.new(2019, 10, 1) => 21.0,
          Date.new(2019, 10, 2) => 21.0,
          Date.new(2019, 10, 3) => 21.0
        })
    end

    it 'returns correct value for daily and weekly' do
      start_date = Date.new(2019, 10, 1)
      end_date = Date.new(2019, 10, 3)

      time_period_costs = [
        {
          time_period: :daily,
          cost: 10.0
        },
        {
          time_period: :weekly,
          cost: 70.0
        }
      ]

      expect(daily_cost(start_date, end_date, time_period_costs))
        .to eq({
          Date.new(2019, 10, 1) => 20.0,
          Date.new(2019, 10, 2) => 20.0,
          Date.new(2019, 10, 3) => 20.0
        })
    end
  end

  context 'when single time period passed in' do
    it 'works with daily time period' do
      start_date = Date.new(2019, 10, 1)
      end_date = Date.new(2019, 10, 3)

      time_period_costs = [
        {
          time_period: :daily,
          cost: 10.0
        }
      ]

      expect(daily_cost(start_date, end_date, time_period_costs))
        .to eq({
          Date.new(2019, 10, 1) => 10.0,
          Date.new(2019, 10, 2) => 10.0,
          Date.new(2019, 10, 3) => 10.0
        })
    end

    it 'works with weekly time period' do
      start_date = Date.new(2019, 10, 1)
      end_date = Date.new(2019, 10, 3)

      time_period_costs = [
        {
          time_period: :weekly,
          cost: 70.0
        }
      ]

      expect(daily_cost(start_date, end_date, time_period_costs))
        .to eq({
          Date.new(2019, 10, 1) => 10.0,
          Date.new(2019, 10, 2) => 10.0,
          Date.new(2019, 10, 3) => 10.0
        })
    end

    it 'works with monthly time period' do
      start_date = Date.new(2019, 10, 1)
      end_date = Date.new(2019, 10, 3)

      time_period_costs = [
        {
          time_period: :monthly,
          cost: 31.0
        }
      ]

      expect(daily_cost(start_date, end_date, time_period_costs))
        .to eq({
          Date.new(2019, 10, 1) => 1.0,
          Date.new(2019, 10, 2) => 1.0,
          Date.new(2019, 10, 3) => 1.0
        })
    end
  end
end