editing Wikipedia ISBN calculation code

.rbenv-version

1.9.3-p194

about.txt

editing http://en.wikipedia.org/w/index.php?title=International_Standard_Book_Number, section “ISBN-13 check digit calculation”

page edits:
“/* ISBN-13 check digit calculation */ made Ruby checksum code more idiomatic (and easier-to-read)”
http://en.wikipedia.org/w/index.php?title=International_Standard_Book_Number&diff=prev&oldid=510791284

“ refactored Ruby code for clarity”
http://en.wikipedia.org/w/index.php?title=International_Standard_Book_Number&diff=510791920&oldid=510791284

helper.rb

# helper methods not worth the space for the Wikipedia example
def String.split_into_chars
  self.split(//)
end
def Numeric.divisible_by(num)
  self.modulo(num).zero?
end
def Enumerable.sum #implemented in Rails ActiveSupport
  self.reduce(:+)
end

idiomatic.rb

# idiomatic
# Ruby
def isbn_checksum(isbn_string)
  digits = isbn_string.split(//).map(&:to_i)
  transformed_digits = digits.each_with_index.map do |digit, digit_index|
    digit_index.modulo(2).zero? ? digit : digit*3
  end
  sum = transformed_digits.reduce(:+)
end

def is_valid_isbn13?(isbn13)
  checksum = isbn_checksum(isbn13)
  checksum.modulo(10).zero?
end

def isbn13_checksum_digit(isbn12)
  checksum = isbn_checksum(isbn12)
  10 - checksum.modulo(10)
end

original.rb

# original
# Ruby
def is_valid_isbn13?(isbn13)
  sum = 0
  13.times { |i| sum += i.modulo(2)==0 ? isbn13[i].to_i : isbn13[i].to_i*3 }
  0 == sum.modulo(10)
end

def isbn13_checksum(isbn12)
  sum = 0
  12.times { |i| sum += i.modulo(2)==0 ? isbn12[i].to_i : isbn12[i].to_i*3 }
  10 - sum.modulo(10)
end

tests.rb

require 'test/unit'

# require './original'
require './idiomatic'

class TestIsbn < Test::Unit::TestCase
  def test_verifies_isbn13s
    assert is_valid_isbn13?("9780306406157")
    assert ! is_valid_isbn13?("1234567890123")
  end

  def test_calculates_isbn12_checksums
    assert_equal 7, isbn13_checksum_digit("978030640615")
    assert_equal 8, isbn13_checksum_digit("123456789012")
  end
end