Paddock Equality

paddock.rb

# In this example case when we create paddock objects we want them to be equal
# to each other if their name and area are the same and their lat/lng are "close
# enough" to each other. In our case we want "close enough" to mean the lat and
# lng match up to four decimal places.
#
# Download this file and the corresponding test case file tc_paddock.rb. Run the
# test case file like this `ruby tc_paddock.rb`. Currently all tests fail.

# Please make the changes you feel are necessary to make the tests pass, then
# make a gist with your version of paddock.rb and let us know.

require 'bigdecimal'

class Paddock
  attr_accessor :lat, :lng, :name, :area

  PRECISION = 4

  #
  # lat and lng attribute get shortened when initializing
  #
  def initialize(lat:, lng:, name:, area:)
    self.lat = shortDecimalOf lat
    self.lng = shortDecimalOf lng
    self.name = name
    self.area = area
  end

  #
  # redefine '==' method with custom behaviour
  # by comparing object attributes - NOT by object_id anymore
  #
  def ==(other_paddock)
    self.name == other_paddock.name and
    self.area == other_paddock.area and
    self.lat == shortDecimalOf(other_paddock.lat) and
    self.lng == shortDecimalOf(other_paddock.lng)
  end

  #
  # redefine 'eql?' method with redefined '==' method above
  #
  alias eql? ==

  def hash
    [lat, lng, name, area].hash
  end

  #
  # short out value to 4 decimal precision
  #
  def shortDecimalOf(value)
    BigDecimal::new(value, 0).truncate(PRECISION).to_f
  end
end

tc_paddock.rb

require 'test/unit'
require_relative 'paddock'

class TestPaddock < Test::Unit::TestCase
  def setup
    @p1 = Paddock.new(lat: -31.9415, lng: 115.8418, name: 'Agworld HQ', area: 10)
    @p2 = Paddock.new(lat: -31.94154, lng: 115.84183, name: 'Agworld HQ', area: 10)
    @p3 = Paddock.new(lat: -31.9405, lng: 115.8408, name: 'Agworld HQ', area: 11)
    @p4 = Paddock.new(lat: -31.9415, lng: 115.8418, name: 'Agworld Branch', area: 10)
  end

  def test_double_equals
    assert(@p1 == @p2, 'p1 should == p2')
    assert_equal(@p1 == @p3, false, 'p1 should not == p3')
    assert_equal(@p1 == @p4, false, 'p1 should not == p4')
  end

  def test_eql?
    assert(@p1.eql?(@p2), 'p1 should eql? p2')
    assert_equal(@p1.eql?(@p3), false, 'p1 should not eql? p3')
  end

  def test_uniq
    assert_equal([@p1, @p2].uniq.length, 1)
    assert_equal([@p1, @p3].uniq.length, 2)
  end
end