Slot length calculation

slot_length.rb

# encoding: utf-8

require 'time'

class SlotLength
  # Contains the number of seconds in the slot
  attr_reader :seconds
  attr_reader :ordered_seconds

  # Contains the string for the slot length
  attr_reader :length

  # Contains the string for the ordered length
  attr_reader :ordered_length

  # Creates an instance of a SlotLength
  #
  # @param  [String] length
  #
  def initialize(length)
    unless length.match /^\d\d:\d\d:\d\d$/
      raise ArgumentError.new("Invalid length format")
    end

    # Do some calculations
    @length          = length
    @seconds         = calculate_seconds(@length)
    @ordered_seconds = calculate_ordered_seconds(@seconds)
    @ordered_length  = generate_ordered_length(@ordered_seconds)
  end

  # Return the length string
  #
  # @return [String]
  #
  def to_s
    length
  end

  private

  # Calculate the number of seconds
  #
  # @return [Fixnum]
  #
  def calculate_seconds(length)
    Time.parse(length).to_i - day_start_tick
  end

  # Calculate the number of seconds for the ordered length
  #
  # @return [Fixnum]
  #
  def calculate_ordered_seconds(seconds)
    seconds < 1110 ? seconds-60 : seconds-90
  end

  # Generate the ordered length string
  #
  # @return [String]
  #
  def generate_ordered_length(ordered_seconds)
    Time.at(day_start_tick+ordered_seconds).strftime('%H:%M:%S')
  end

  # Get the tick for the start of the day
  #
  # @return [Fixnum]
  #
  def day_start_tick
    Time.parse('00:00:00').to_i
  end
end

slot_length_spec.rb

# encoding: utf-8

require_relative "spec_helper"
require_relative "../lib/slot_length"

describe SlotLength do
  subject { SlotLength }

  it "raises an ArgumentError if the length string is invalid" do
    -> { s('foo') }.must_raise ArgumentError
  end

  describe "seconds" do
    it "returns the number of seconds in the slot" do
      s('00:08:00').seconds.must_equal 480
      s('00:30:00').seconds.must_equal 1800
      s('00:50:00').seconds.must_equal 3000
    end
  end

  describe "ordered_seconds" do
    it "returns the number of seconds for the ordered length" do
      s('00:05:00').ordered_seconds.must_equal 240  # 4 min
      s('00:10:00').ordered_seconds.must_equal 540  # 9 min
      s('00:15:00').ordered_seconds.must_equal 840  # 14 min
      s('00:20:00').ordered_seconds.must_equal 1110 # 18 min 30 sec
      s('00:30:00').ordered_seconds.must_equal 1710 # 28 min 30 sec
      s('01:00:00').ordered_seconds.must_equal 3510 # 58 min 30 sec
      s('02:00:00').ordered_seconds.must_equal 7110 # 1 hour 58 min 30 sec
    end
  end

  describe "ordered_length_string" do
    it "returns the string for the ordered length" do
      s('00:05:00').ordered_length.must_equal '00:04:00'
      s('00:10:00').ordered_length.must_equal '00:09:00'
      s('00:15:00').ordered_length.must_equal '00:14:00'
      s('00:20:00').ordered_length.must_equal '00:18:30'
      s('00:30:00').ordered_length.must_equal '00:28:30'
      s('01:00:00').ordered_length.must_equal '00:58:30'
      s('02:00:00').ordered_length.must_equal '01:58:30'
    end
  end

  describe "to_s" do
    it "returns the initial length string" do
      s('00:20:00').to_s.must_equal '00:20:00'
      "#{s('00:10:00')}".must_equal '00:10:00'
    end
  end
end