Well, there wasn't any discussion or submissions this week, save my own.  Guess
that means you'll have to suffer through my code this week.

There are really two aspects to a game of Yahtzee:  Dice rolling and score
keeping.  With dice rolling, you need to be able to handle a roll of multiple
dice and re-rolls of selected dice.  You'll need to be able to display and
examine this roll, of course.  Then, you'll need to be able to sum all the dice
or just certain dice.

Finally, when it's time to score those rolls, you need some way to match die
against patterns.  Three of a Kind, Four of a Kind, Full House and Yahtzee are
repetition patterns.  You're looking for any number to appear on the dice a set
number of times.  With Full House, you're actually looking for two different
dice to appear a different number of times.

Small Straight and Large Straight need another form of pattern matching: 
Sequence patterns.  Here you're searching for a run on the dice of a specified
length, but the actual numbers in the run don't matter.

Here's the class I coded up to cover those needs:

	# Namespace for all things Yahtzee.
	module Yahtzee
		# An object for managing the rolls of a Yahtzee game.
		class Roll
			#
			# Create an instance of Roll.  Methods can then be used the
			# examine the results of the roll and re-roll dice.
			#
			def initialize(  )
				@dice = Array.new(5) { rand(6) + 1 }
			end
			
			# Examine the individual dice of a Roll.
			def []( index )
				@dice[index]
			end
		
			# Count occurrences of a set of pips.
			def count( *pips )
				@dice.inject(0) do |total, die|
					if pips.include?(die) then total + 1 else total end
				end
			end
		
			# Add all occurrences of a set of pips, or all the dice.
			def sum( *pips )
				if pips.size == 0
					@dice.inject(0) { |total, die| total + die }
				else
					@dice.inject(0) do |total, die|
						if pips.include?(die) then total + die else total end
					end
				end
			end
		
			#
			# Examines Roll for a pattern of dice, returning true if found.  
			# Patterns can be of the form:
			#
			#     roll.matches?(1, 2, 3, 4)
			#
			# Which validates a sequence, regardless of the actual pips on
			# the dice.
			#
			# You can also use the form:
			# 
			#     roll.matches?(*%w{x x x y y})
			#
			# To validate repititions.
			#
			# The two forms can be mixed in any combination and when they
			# are, both must match completely.
			#
			def matches?( *pattern )
				digits, letters = pattern.partition { |e| e.is_a?(Integer) }
				matches_digits?(digits) and matches_letters?(letters)
			end
		
			# Re-roll selected _dice_.
			def reroll( *dice )
				if dice.size == 0
					@dice = Array.new(5) { rand(6) + 1 }
				else
					indices = [ ]
					pool    = @dice.dup
					dice.each do |d|
						i = pool.index(d) or
							raise ArgumentError, "Dice not found."
						indices << i
						pool[i] = -1
					end
				
					indices.each { |i| @dice[i] = rand(6) + 1 }
				end
			end
		
			# To make printing out rolls easier.
			def to_s(  )
				"#{@dice[0..-2].join(',')} and #{@dice[-1]}"
			end
		
			private
		
			# Verifies matching of sequence patterns.
			def matches_digits?( digits )
				return true if digits.size < 2
			
				digits.sort!
				test = @dice.uniq.sort
				loop do
					(0..(@dice.length - digits.length)).each do |index|
						return true if test[index, digits.length] == digits
					end
			
					digits.collect! { |d| d + 1 }
					break if digits.last > 6	
				end
			
				false
			end
		
			# Verifies matching of repetition patterns.
			def matches_letters?( letters )
				return true if letters.size < 2
			
				counts = Hash.new(0)
				letters.each { |l| counts[l] += 1 }
				counts = counts.values.sort.reverse
			
				pips = @dice.uniq
				counts.each do |c|
					unless match = pips.find { |p| count(p) >= c }
						return false
					end
					pips.delete(match)
				end
			
				true
			end
		end
	end

The descriptions and comments above should make that class pretty transparent, I
hope.

The method matches?() is my dice pattern matching system.  It understands arrays
of letters and/or numbers, feeding the correct sets to the private methods
matches_digits?() and matches_letters?().

Letters are used to check repetition.  For example, the pattern used to match a
Full House is %w{x x x y y}.  That requires three of any one number and two of a
different number.

Numbers are used to check sequence patterns.  As another example, the pattern to
match a Small Straight is [1, 2, 3, 4].  That requires that there be four
different numbers shown on the dice, each exactly one apart from one of the
other numbers.  Which numbers are shown doesn't matter.

As an interesting aside, the above class proved tricky to unit test.  Well, for
me anyway.  I didn't end up posting my tests because I was ashamed of the hack I
used.  Perhaps this should be a separate quiz...

Scoring is pretty simple.  We just need a Scorecard object that holds categories
we can add points to and totals based on those categories.  We need to be able
to print that, of course, and allow the user to identify categories using some
form of label.  Here's what I came up with for that:

	# Namespace for all things Yahtzee.
	module Yahtzee
		# A basic score tracking object.
		class Scorecard
			# Create an instance of Scorecard.  Add categories and totals,
			# track score and display results as needed.
			def initialize(  )
				@categories = [ ]
			end
		
			#
			# Add one or more categories to this Scorecard.  Order is 
			# maintained.
			#
			def add_categories( *categories )
				categories.each do |cat|
					@categories << [cat, 0]
				end
			end
		
			#
			# Add a total, with a block to calculate it from passed a
			# categories Hash.
			#
			def add_total( name, &calculator )
				@categories << [name, calculator]
			end
			
			#
			# The primary score action method.  Adds _count_ points to the
			# category at _index_.
			#
			def count( index, count )
				@categories.assoc(category(index))[1] += count
			end
		
			# Lookup the score of a given category.
			def []( name )
				@categories.assoc(name)[1]
			end
			
			# Lookup a category name, by _index.
			def category( index )
				id = 0
				@categories.each_with_index do |(name, count_or_calc), i|
					next unless count_or_calc.is_a?(Numeric)
					id += 1
					return @categories[i][0] if id == index
				end

				raise ArgumentError, "Invalid category."
			end
			
			# Support for easy printing.
			def to_s(  )
				id = 0
				@categories.inject("") do |disp, (name, count_or_calc)|
					if count_or_calc.is_a?(Numeric)
						id += 1
						disp + "%3d: %-20s %4d\n" % [id, name, count_or_calc]
					else
						disp + "     %-20s %4d\n" %
							[name, count_or_calc.call(to_hash)]
					end
				end
			end
		
			# Convert category listing to a Hash.
			def to_hash(  )
				@categories.inject(Hash.new) do |hash, (name, count_or_calc)|
					if count_or_calc.is_a?(Numeric)
						hash[name] = count_or_calc
					end
					hash
				end
			end
		end
	end

Using that isn't too tough.  Create a Scorecard and add categories and totals to
it.  Categories are really just a name that can be associated with a point
count.  Totals are passed in as a block of code that can calculate the total as
needed.  The block is passed a hash of category names and their current points,
when called.  Moving into the "main" section of my program, we can see how I use
this to build Yahtzee's Scorecard:

	# Console game interface.
	if __FILE__ == $0
		# Assemble Scorecard.
		score = Yahtzee::Scorecard.new()
		UPPER = %w{Ones Twos Threes Fours Fives Sixes}
		UPPER_TOTAL = lambda do |cats|
			cats.inject(0) do |total, (cat, count)|
				if UPPER.include?(cat) then total + count else total end
			end
		end
		score.add_categories(*UPPER)
		score.add_total("Bonus") do |cats|
			upper = UPPER_TOTAL.call(cats)
			if upper >= 63 then 35 else 0 end
		end
		score.add_total("Upper Total") do |cats|
			upper = UPPER_TOTAL.call(cats)
			if upper >= 63 then upper + 35 else upper end
		end
		LOWER = [ "Three of a Kind", "Four of a Kind", "Full House",
				  "Small Straight", "Large Straight", "Yahtzee", "Chance" ]
		bonus_yahtzees = 0
		LOWER_TOTAL = lambda do |cats|
			cats.inject(bonus_yahtzees) do |total, (cat, count)|
				if LOWER.include?(cat) then total + count else total end
			end
		end
		score.add_categories(*LOWER[0..-2])
		score.add_total("Bonus Yahtzees") { bonus_yahtzees }
		score.add_categories(LOWER[-1])
		score.add_total("Lower Total", &LOWER_TOTAL)
		score.add_total("Overall Total") do |cats|
			upper = UPPER_TOTAL.call(cats)
			if upper >= 63
				upper + 35 + LOWER_TOTAL.call(cats)
			else
				upper + LOWER_TOTAL.call(cats)
			end
		end
		
		# ...

I make a little use of the fact that Ruby's blocks are closures there,
especially with the Bonus Yahtzees total.  I simply have it refer to a
bonus_yahtzees variable, which the game engine can increase as needed.

Let's step into that engine now.  Here's the section that handles dice rolling:

		# ...
		
		# Game.
		puts "\nWelcome to Yahtzee!"
		scratches = (1..13).to_a
		13.times do
			# Rolling...
			roll = Yahtzee::Roll.new
			rolls = 2
			while rolls > 0
				puts "\nYou rolled #{roll}."
				print "Action:  " +
				      "(c)heck score, (s)core, (q)uit or #s to reroll?  "
				choice = STDIN.gets.chomp
				case choice
				when /^c/i
					puts "\nScore:\n#{score}"
				when /^s/i
					break
				when /^q/i
					exit
				else
					begin
						pips = choice.gsub(/\s+/, "").split(//).map do |n|
							Integer(n)
						end
						roll.reroll(*pips)
						rolls -= 1
					rescue
						puts "Error:  That not a valid reroll."
					end
				end
			end
			
			# ...

Most of that code is for processing user interface commands.  The actual dice
roll handling is just calls to the correct methods of Roll at the correct times.

Finally, here's the scoring portion of the game:

			# ...
	
			# Scoring...
			loop do
				if roll.matches?(*%w{x x x x x}) and score["Yahtzee"] == 50
					bonus_yahtzees += 100
					
					if scratches.include?(roll[0])
						score.count(roll[0], roll.sum(roll[0]))
						scratches.delete(choice)
						puts "Bonus Yahtzee scored in " +
						     "#{score.category(roll[0])}."
						break
					end
					
					puts "Bonus Yahtzee!  100 points added.  " +
					     "Score in lower section as a wild-card."
					bonus_yahtzee = true
				else
					bonus_yahtzee = false
				end
				
				print "\nScore:\n#{score}\n" +
				      "Where would you like to count your #{roll} " +
				      "(# of category)?  "
				begin
					choice = Integer(STDIN.gets.chomp)
					raise "Already scored." unless scratches.include?(choice)
					case choice
					when 1..6
						score.count(choice, roll.sum(choice))
					when 7
						if roll.matches?(*%w{x x x}) or bonus_yahtzee
							score.count(choice, roll.sum())
						end
					when 8
						if roll.matches?(*%w{x x x x}) or bonus_yahtzee
							score.count(choice, roll.sum())
						end
					when 9
						if roll.matches?(*%w{x x x y y}) or bonus_yahtzee
							score.count(choice, 25)
						end
					when 10
						if roll.matches?(1, 2, 3, 4) or bonus_yahtzee
							score.count(choice, 30)
						end
					when 11
						if roll.matches?(1, 2, 3, 4, 5) or bonus_yahtzee
							score.count(choice, 40)
						end
					when 12
						if roll.matches?(*%w{x x x x x})
							score.count(choice, 50)
						end
					when 13
						score.count(choice, roll.sum)
					end
					scratches.delete(choice)
					break
				rescue
					puts "Error:  Invalid category choice."
				end
			end
		end
		
		print "\nFinal Score:\n#{score}\nThanks for playing.\n\n"
	end

The first if block in there is watching for Bonus Yahtzees, which are the
hardest thing to track in a Yahtzee game.  If a second Yahtzee is thrown, it
increments the bonus_yahtzee variable (so the Scorecard total will change), then
it tries to score the Yahtzee in the correct slot of the Upper section.  If that
slot is already full, it warns the code below to allow wild-card placement by
setting the boolean variable bonus_yahtzee.

The rest of the scoring code is a case statement that validates dice patterns
and scores them correctly.  It looks like a lot of code, but it's very basic in
function.  I'm really just stitching Roll and Scorecard together here.

That's all there is to my version of Yahtzee.  I didn't do the extra challenges,
obviously.  It's pretty easy to add Triple Yahtzee to this version.  The AI is a
bigger challenge, if you want it to play well.  Those I'll leave as a challenge
for the reader.

You already know what tomorrow's quiz is.  You asked for it.