## Solution by James
def matches(index, base)
  places = 4
  permutation = "%0#{places * 2}d" % index.to_s(base)
  original = (0...places).map { |i| permutation[i].chr }
  guess = (places...(places * 2)).map { |i| permutation[i].chr }
  matches = 0 
  guess.each do |g| 
    (0...places).each do |i| 
      if g == original[i]
        original[i] = nil 
        matches += 1
        break
      end 
    end 
  end 
  matches
end

## Solution by Nathan
def matches(index, base)
  counts = [0] * base
  4.times do
    counts[index % base] += 1
    index /= base
  end
  count = 0
  4.times do
    count += counts[index % base]
    index /= base
  end
  count
end

## Tests

require 'test/unit'

class MatchesTest < Test::Unit::TestCase
  
  def test_should_have_no_matches
    assert_equal 0, matches(12345678, 10)
  end
  
  def test_should_have_one_match_in_same_positiion
    assert_equal 1, matches(12341678, 10)
  end
  
  def test_should_have_one_match_in_different_positiion
    assert_equal 1, matches(12345671, 10)
  end
  
  def test_should_have_two_matches_in_same_positiion
    assert_equal 2, matches(12341278, 10)
  end
  
  def test_should_have_two_duplicate_matches_in_same_positiion
    assert_equal 2, matches(11341178, 10)
  end
  
  def test_should_have_two_matches_in_different_positiion
    assert_equal 2, matches(12345621, 10)
  end
  
  def test_should_have_two_duplicate_matches_in_different_positiion
    assert_equal 2, matches(11345611, 10)
  end
  
  def test_should_have_three_matches_in_same_positiion
    assert_equal 3, matches(12341238, 10)
  end
  
  def test_should_have_three_duplicate_matches_in_same_positiion
    assert_equal 3, matches(11141118, 10)
  end
  
  def test_should_have_three_matches_in_different_positiion
    assert_equal 3, matches(12345321, 10)
  end
  
  def test_should_have_three_duplicate_matches_in_different_positiion
    assert_equal 3, matches(11145111, 10)
  end
  
  def test_should_have_four_matches_in_same_position
    assert_equal 4, matches(12341234, 10)
  end
  
  def test_should_have_four_duplicate_matches
    assert_equal 4, matches(11111111, 10)
  end
  
  def test_should_have_four_matches_in_different_position
    assert_equal 4, matches(12344321, 10)
  end
  
  def test_should_have_one_match_when_digit_appears_twice_on_lhs_but_once_on_rhs
    assert_equal 1, matches(11345178, 10)
  end
  
  def test_should_have_one_match_when_digit_appears_once_on_lhs_but_twice_on_rhs
    assert_equal 1, matches(12345118, 10)
  end
  
  def test_should_have_two_matches_when_digit_appears_three_times_on_lhs_but_twice_on_rhs
    assert_equal 2, matches(11145118, 10)
  end
  
  def test_should_have_two_matches_when_digit_appears_twice_on_lhs_but_three_times_on_rhs
    assert_equal 2, matches(12315111, 10)
  end
  
  def test_should_have_three_matches_when_digit_appears_four_times_on_lhs_but_three_times_on_rhs
    assert_equal 3, matches(11115111, 10)
  end
  
  def test_should_have_three_matches_when_digit_appears_three_times_on_lhs_but_four_times_on_rhs
    assert_equal 3, matches(11141111, 10)
  end
  
end

## James test output
Loaded suite /Users/jamesmead/Desktop/matches.rb
Started
....................
Finished in 0.002072 seconds.

20 tests, 20 assertions, 0 failures, 0 errors

## Nathan test output
Loaded suite /Users/jamesmead/Desktop/matches.rb
Started
F.....FFFF..FFFF..FF
Finished in 0.026939 seconds.

  1) Failure:
test_should_have_four_duplicate_matches:86
<4> expected but was
<16>.

  2) Failure:
test_should_have_one_match_when_digit_appears_once_on_lhs_but_twice_on_rhs:98
<1> expected but was
<2>.

  3) Failure:
test_should_have_one_match_when_digit_appears_twice_on_lhs_but_once_on_rhs:94
<1> expected but was
<2>.

  4) Failure:
test_should_have_three_duplicate_matches_in_different_positiion:78
<3> expected but was
<9>.

  5) Failure:
test_should_have_three_duplicate_matches_in_same_positiion:70
<3> expected but was
<9>.

  6) Failure:
test_should_have_three_matches_when_digit_appears_four_times_on_lhs_but_three_times_on_rhs:110
<3> expected but was
<12>.

  7) Failure:
test_should_have_three_matches_when_digit_appears_three_times_on_lhs_but_four_times_on_rhs:114
<3> expected but was
<12>.

  8) Failure:
test_should_have_two_duplicate_matches_in_different_positiion:62
<2> expected but was
<4>.

  9) Failure:
test_should_have_two_duplicate_matches_in_same_positiion:54
<2> expected but was
<4>.

 10) Failure:
test_should_have_two_matches_when_digit_appears_three_times_on_lhs_but_twice_on_rhs:102
<2> expected but was
<6>.

 11) Failure:
test_should_have_two_matches_when_digit_appears_twice_on_lhs_but_three_times_on_rhs:106
<2> expected but was
<6>.

20 tests, 20 assertions, 11 failures, 0 errors