substring_match.rb

require 'test/unit'

def find substr, str
  str.split('').each_with_index do |char, index|
    if substr.chr == char
      if substr.length == 1 or find(substr[1..-1], str[index+1..-1]) == 0
        return index
      end
    end
  end
  return nil
end

class FindTest < Test::Unit::TestCase
  # should return index of first letter in matched string
  def test_found
    assert_equal 1, find('bcd', 'abcdefg')
  end

  # should return nil if the substring is not matched
  def test_not_found
    assert_equal nil, find('x', 'abcdefg')
  end

  # should accurately match from the beginning after a partial match
  def test_consumes_partial_matches
    assert_equal 5, find('bcd', 'abcbabcd')
  end

  # should return nil when attempting to match for or against the empty string
  def test_empty_string
    assert_equal nil, find('', 'abcdabe')
    assert_equal nil, find('abc', '')
  end
end