about_scoring_project.rb

require File.expand_path(File.dirname(__FILE__) + '/edgecase')

# Greed is a dice game where you roll up to five dice to accumulate
# points.  The following "score" function will be used calculate the
# score of a single roll of the dice.
#
# A greed roll is scored as follows:
#
# * A set of three ones is 1000 points
#   
# * A set of three numbers (other than ones) is worth 100 times the
#   number. (e.g. three fives is 500 points).
#
# * A one (that is not part of a set of three) is worth 100 points.
#
# * A five (that is not part of a set of three) is worth 50 points.
#
# * Everything else is worth 0 points.
#
#
# Examples:
#
# score([1,1,1,5,1]) => 1150 points
# score([2,3,4,6,2]) => 0 points
# score([3,4,5,3,3]) => 350 points
# score([1,5,1,2,4]) => 250 points
#
# More scoring examples are given in the tests below:
#
# Your goal is to write the score method.

def grouping(xs, &block)
  unless xs.empty?    
    ys = xs.take_while {|x| x == xs.first}
    rest = xs.drop(ys.count)

    ys = yield ys if block_given?
    block = lambda {|x| x} unless block_given?

    [ys] + grouping(rest, &block)
  else
    xs
  end
end

def score(dice)
  scores = grouping(dice.sort) do |rolled|
    score_roll rolled.first, rolled.count
  end

  scores.reduce 0, :+
end

def score_roll(die, count)
  if count >= 3
    points = die == 1 ? 1000 : (die * 100)
    points + score_roll(die, count - 3)
  else
    points = {1 => 100, 5 => 50}.fetch(die, 0)
    points * count
  end
end

class AboutScoringAssignment < EdgeCase::Koan
  def test_score_of_an_empty_list_is_zero
    assert_equal 0, score([])
  end

  def test_score_of_a_single_roll_of_5_is_50
    assert_equal 50, score([5])
  end

  def test_score_of_a_single_roll_of_1_is_100
    assert_equal 100, score([1])
  end

  def test_score_of_multiple_1s_and_5s_is_the_sum_of_individual_scores
    assert_equal 300, score([1,5,5,1])
  end

  def test_score_of_single_2s_3s_4s_and_6s_are_zero
    assert_equal 0, score([2,3,4,6])
  end

  def test_grouping_empty_array_returns_empty_array
    assert_equal [], grouping([])
  end

  def test_grouping_single_element_array_returns_nested_array
    assert_equal [[5]], grouping([5])
  end

  def test_grouping_of_array_returns_subarrays
    assert_equal [[1,1], [5,5]], grouping([1,1,5,5])
  end

  def test_score_of_a_triple_1_is_1000
    assert_equal 1000, score([1,1,1])
  end

  def test_score_of_other_triples_is_100x
    assert_equal 200, score([2,2,2])
    assert_equal 300, score([3,3,3])
    assert_equal 400, score([4,4,4])
    assert_equal 500, score([5,5,5])
    assert_equal 600, score([6,6,6])
  end

  def test_score_of_mixed_is_sum
    assert_equal 250, score([2,5,2,2,3])
    assert_equal 550, score([5,5,5,5])
  end

end

Ah, I'm on 1.8.7 and spent a lot of time looking for something that already did what I used grouping for. It's called partition-by in Clojure. I'm not sure I would have seized upon slice_before, that's a very clever trick with your returning clause. And you're right about our solutions being similar, I believe that reduce was abandoned for inject in 1.9.
The last thing I would direct your attention to would be to my scoring function's {1 => 100, 5 => 50}.fetch(die, 0), which is a one line if/then/else statement. I was pretty proud of that. My next refactoring would be to put it in the Integer class, exactly as you did.

Every language needs a Test-Driven Learning Experience like rubykoans.com. Heck, every developer should have to keep their own diary of distilled problems they've solved and test-cases around them. Kind of the pilot's flight-log of software development.