Phone Format test

phone_format.rb

#! /usr/bin/ruby

#      _             _
#  ___| |_ ___ _ __ | |__   __ _ _ __    ___ ___  _ __ ___
# / __| __/ _ \ '_ \| '_ \ / _` | '_ \  / __/ _ \| '_ ` _ \
# \__ \ ||  __/ |_) | | | | (_| | | | || (_| (_) | | | | | |
# |___/\__\___| .__/|_| |_|\__,_|_| |_(_)___\___/|_| |_| |_|
#             |_|

# coding challenge for Simplero
# https://engineering.simplero.com/coding-challenge

# phone_format formats a phone number, discarding all non-digit characters
# and formatting it in groups of 3 digits, separated by hyphens, such that
# the last group will never have a single digit.
# phone_format("555 123 1234") => "555-123-12-34"
# phone_format("(+1) 888 33x19") => "188-833-19"

# I tried initially solving this with just a regexp.
# I was quickly reminded of this quote:
# > Some people, when confronted with a problem, think "I know,
# > I'll use regular expressions." Now they have two problems.
#                             - Jamie Zawinski
# https://en.wikiquote.org/wiki/Jamie_Zawinski#Attributed

# monkey patch string
# cribbed from https://apidock.com/rails/String/last
class String
  def first(limit = 1)
    return '' if limit.zero?
    return dup if limit >= size

    self[0..(limit - 1)]
  end

  def last(limit = 1)
    return '' if limit.zero?
    return dup if limit >= size

    self[-limit..-1]
  end
end

# gratuitous use of lambdas
def maketest(method)
  lambda do |input, result|
    if (actual = send(method, input)) == result
      "OK: success with #{input} == #{result}"
    else
      "#{input} failed with #{actual}"
    end
  end
end

# why, yes, you should be able to generalize group size and minimum.
# but I did not.
def phone_format(string)
  str = string.tr('^0-9', '')
  return str if str.length < 4

  if str.length % 3 == 1
    str.first(-4).scan(/.{3}/) + str.last(4).scan(/.{2}/)
  else
    str.scan(/.{2,3}/)
    # to put the smaller group at the beginning
    # str.reverse.scan(/.{2,3}/).map(&:reverse).reverse
  end.join('-')
end

pftest = maketest(:phone_format)
puts pftest.call('555 123 1234', '555-123-12-34')
puts pftest.call('(+1) 888 33x19', '188-833-19')
puts pftest.call('310.384.3771', '310-384-37-71') # my cell
puts pftest.call('12!345...6, 7, 8.', '123-456-78') # abomination
puts pftest.call('88888888', '888-888-88') # I prefer 88-888-888
puts pftest.call('7777777', '777-77-77')
puts pftest.call('666665', '666-665') # the beast, and his neighbor
puts pftest.call('55555', '555-55') # but I prefer 55-555
puts pftest.call('4444', '44-44')
puts pftest.call('333', '333')
puts pftest.call('22', '22')
puts pftest.call('1', '1') # mind, blown.
puts pftest.call('', '')