Ruby rounds_to? Allows for a rough equality check to an implicit precision provided by the input `numeric` argument

0-rounds_to.rb

class Numeric
  # Returns true if for any i, self.round(i) == numeric
  #
  # Usage:
  #
  # 149.835.rounds_to?(0)       #=> true
  # 149.835.rounds_to?(100)     #=> true
  # 149.835.rounds_to?(150)     #=> true
  # 149.835.rounds_to?(149.8)   #=> true
  # 149.835.rounds_to?(149.84)  #=> true
  # 149.835.rounds_to?(149.835) #=> true
  #
  def rounds_to?(numeric)
    return true if self == numeric || zero?

    diff = (self - numeric).abs
    round(-diff.order_of_magnitude - 1) == numeric
  end

  # See http://en.wikipedia.org/wiki/Order_of_magnitude (Order of magnitude of a number)
  #
  # Usage:
  #
  # 100.order_of_magnitude  #=> 2
  # 10.order_of_magnitude   #=> 1
  # 1.order_of_magnitude    #=> 0
  # 0.order_of_magnitude    #=> 0
  # 0.1.order_of_magnitude  #=> -1
  # 0.01.order_of_magnitude #=> -2
  #
  def order_of_magnitude
    zero? ? 0 : Math.log10(abs).floor
  end
end

1-matcher_round_to.rb

require 'rspec/expectations'

# Made as a simpler to use specific case implemention of https://www.relishapp.com/rspec/rspec-expectations/v/2-14/docs/built-in-matchers/be-within-matcher
#
# Usage:
#
#  describe 149.835 do
#    it { should round_to(149.84) }
#  end
#
#  # deliberate failure
#  describe 149.835 do
#    it { should round_to(149) }
#  end
#  #=> expected 149.835 to round to 149
#
RSpec::Matchers.define :round_to do |numeric|
  match do |actual|
    actual.rounds_to?(numeric)
  end
end