solution.rb

require 'date'

class DataPoint
  # Create a new data point.
  #
  # tag_name      - A String containing a name or identifier for the
  #                 data being measured.
  # time_recorded - A DateTime object for the recording time.
  # data          - A String or Float which is the recorded data.
  #
  # Returns a DataPoint object.
  def initialize(tag_name, time_recorded, data)
    @tag_name, @time_recorded, @data = tag_name.to_sym, time_recorded, data
  end

  # Returns a Symbol representing the name of the data.
  attr_reader :tag_name

  # Returns a DateTime object containing the time recorded for the data.
  attr_reader :time_recorded

  # Returns a String or Float containing the DataPoint's measured data.
  attr_reader :data

  # Non-destructively add data to a DataPoint instance's @data
  # attribute. It will add Floats together or concatenate Strings with a
  # newline character between them.
  #
  # external_data - A String or Float to add to the stored data.
  #
  # Examples
  #
  #   @data = 1.0
  #   accumulate(2.0) # => 3.0
  #
  #   @data = "bar"
  #   accumulate("foo") # => "foo\nbar"
  #
  # Returns a String or Float of the combined data.
  def accumulate(external_data)
    if external_data.nil?
      @data
    else
      external_data + "\n" + @data rescue external_data + @data
    end
  end
end


# Collate DataPoints into a summary of today's data.
#
# data - an Array of DataPoints
#
# Returns a Hash containing today's collated data points.
def run_program(data)
  today = Date.today
  accumulated_data = {}

  data.each do |d|
    next unless (d.time_recorded === today)
    accumulated_data[d.tag_name] = d.accumulate(accumulated_data[d.tag_name])
  end

  accumulated_data
end

solution_test.rb

require_relative 'solution'
require 'minitest/autorun'

class SolutionTest < MiniTest::Unit::TestCase
  def setup
    @dp1 = DataPoint.new("generator status", DateTime.now, "active")
    @dp2 = DataPoint.new("fan rpm", DateTime.now, 1.0)
    @dp3 = DataPoint.new("generator status", DateTime.now, "idle")
    @dp4 = DataPoint.new("fan rpm", DateTime.now, 2.0)
    @dp5 = DataPoint.new("generator status", DateTime.now, "skipped")

    @dp_old_1 = DataPoint.new("generator status", DateTime.new, "overloaded")
    @dp_old_2 = DataPoint.new("fan rpm", DateTime.new, 10.0)
  end

  def test_data_point_can_be_initialized
    assert_instance_of(DataPoint, @dp1)
  end

  def test_attributes_can_be_read
    assert_equal(:"generator status", @dp1.tag_name)
    assert_equal("active", @dp1.data)
    assert_instance_of(DateTime, @dp1.time_recorded)
  end

  def test_accumulate_concatenates_strings_with_instance_data_at_the_end
    assert_equal("idle\nactive", @dp1.accumulate("idle"))
  end

  def test_accumulate_adds_floats_together
    assert_equal(2.0, @dp2.accumulate(1.0))
  end

  def test_run_program_collates_todays_data_only
    data_array = [@dp1, @dp2, @dp_old_1, @dp_old_2, @dp3, @dp4, @dp5]
    data_hash = run_program(data_array)
    assert_equal("active\nidle\nskipped", data_hash[:"generator status"])
    assert_equal(3.0, data_hash[:"fan rpm"])
  end
end