Several examples how to use Ruby RSpec 3 `be_within` matcher for delta compare

rspec-be-within-matcher.rb

require 'rspec'
require 'time'
require 'active_support/time'

class Foo
  def float_range_example
    33 * 5 * Math::PI
  end

  def time_range
    Time.now
  end

  def hash_with_range_value
    {
      id: 1,
      created_at: Time.now,
      city: "Banska Bystrica"
    }
  end

  def array_with_range_values
    [Time.now, Time.now.midnight, Time.now.midday, 3.days.ago]
  end

  def array_with_hash_with_range_value
    [
      { id: 12, created_at: Time.now },
      { id: 33, created_at: Time.now.midnight }
    ]
  end
end

RSpec.describe(Foo) do
  subject { described_class.new }

  describe '#float_range_example' do
    it do
      # if the value is from e.g.DB than this may not be true
      #
      #    expect(subject.float_range_example).to eq(518.3627878423158)
      #
      # therefore we must define range of value where that float number
      # value is acceptable

      expect(subject.float_range_example).to be_within(0.01).of(518.3627878423158)
    end
  end

  describe '#time_range' do
    context 'in pure Ruby' do
      it do
        expect(subject.time_range).to be_within(1).of(Time.now)
      end
    end

    context 'in Rails' do
      it do
        expect(subject.time_range).to be_within(1.second).of(Time.now)
      end
    end
  end

  describe '#hash_with_range_value' do
    it do
      expect(subject.hash_with_range_value).to match({
        id: 1,
        created_at: be_within(1.second).of(Time.now),
        city: /[bB]an/
      })
    end
  end

  describe '#array_with_range_values' do
    it do
      expect(subject.array_with_range_values).to match_array([
        be_within(2.seconds).of(Time.now),
        be_within(2.seconds).of(Time.now.midnight),
        be_within(2.seconds).of(Time.now.midday),
        be_within(2.seconds).of(Time.now - 3.days),
      ])
    end
  end

  describe '#array_with_hash_with_range_value' do
    it do
      expect(subject.array_with_hash_with_range_value).to match_array([
          { id: 12, created_at: be_within(2.seconds).of(Time.now) },
          { id: 33, created_at: be_within(2.seconds).of(Time.now.midnight) }
        ])
    end
  end
end


# Resources
#
# * RSpec version 3.4
# * Ruby 2.3.0
# * https://www.relishapp.com/rspec/rspec-expectations/v/2-8/docs/built-in-matchers/be-within-matcher