implementing the soundex algorithm for fun

soundex.rb

require 'minitest/autorun'
require 'text'

# An exercise to implement the 'soundex' algorithm
#
# See https://en.wikipedia.org/wiki/Soundex for details
#
# Compare with 'text' gem impl: https://github.com/threedaymonk/text/blob/master/lib/text/soundex.rb
class Soundex
  def process(input)
    return nil unless input.is_a?(String)
    return nil unless input =~ /^[A-Za-z]/

    input = input.downcase.gsub(/hw/, '').split('')

    result = input.each_with_object("") do |letter, acc|
      case letter
      when /[aeiouy]/
        acc << '0' # insert vowel spacer
      when /[bfpv]/
        acc << '1'
      when /[cgjkqsxz]/
        acc << '2'
      when /[dt]/
        acc << '3'
      when /[l]/
        acc << '4'
      when /[mn]/
        acc << '5'
      when /[r]/
        acc << '6'
      end
    end

    # Rule #3 (remove duplicate numbers including first position)
    result.gsub!(/(\d)\1/, '\\1')

    # Place first letter of input in first position
    result[0] = input[0].upcase

    # Remove any vowel spacers
    result.delete!('0')

    # Return four character representation (first letter + 3 numbers)
    result.ljust(4, '0')[0, 4]
  end
end

class TestSoundex < Minitest::Test
  def setup
    @soundex = Soundex.new
  end

  def test_non_words
    assert_nil @soundex.process(nil)
    assert_nil @soundex.process('')
    assert_nil @soundex.process('123')
  end

  def test_short_words
    assert_equal 'A000', @soundex.process('a')
    assert_equal 'A500', @soundex.process('am')
  end

  def test_words
    assert_equal 'F623', @soundex.process('first')
    assert_equal 'T623', @soundex.process('thirst')
    assert_equal 'W623', @soundex.process('worst')
    assert_equal 'C623', @soundex.process('cursed')
  end

  def test_hw_rule
    assert_equal 'A261', @soundex.process('Ashcroft')
    assert_equal 'A261', @soundex.process('Ashcraft')
  end

  def test_tricky_words
    assert_equal 'T522', @soundex.process('Tymczak')
    assert_equal 'P236', @soundex.process('Pfister')
    assert_equal 'H555', @soundex.process('Honeymoon')
  end
end