From: Ruby Quiz <james@...>
Date: 2005-11-25T00:24:39+09:00
Subject: [SUMMARY] Euchre Hands (#55)

My wife can't stand how Euchre reorders the suits.  "Everyone know Jacks come
after Tens," she would say.  Funny how most of us except the odd Ace concept
where the low card is frequently the high card, or even both.  Once you start
moving Jacks though, a lot of people have trouble following the game.

Of course, my wife would also say, "How can you call that Jack a Club (assuming
Clubs are Trump), when it has Spades printed on it?"  She's got me there.

My wife's analysis of Euchre's card order is really right on:  It's unusual. 
Because of that, the traditional sort() tool won't get you all the way to a
solution.  That makes us think about other options.

Don't get me wrong, many solutions bent sort() to their will.  However, as I
said in the quiz, I was looking as much at how to know the answer is correct as
I was at just having an answer.  To me, the weighted sorts are hard to prove and
no one did it to my satisfaction.  Given that, I want to look at the other major
approach people hit on.

The following is the beginning of Bob Showalter's solution:

	#!/usr/local/bin/ruby -w
	
	# Euchre hand sorter
	
	CARDS = %w(A K Q J T 9)
	suits = %w(Spades Hearts Clubs Diamonds)
	
	# read and check input data
	trump = gets.chomp
	raise "Invalid Trump Suit" unless suits.include? trump
	hand = []
	5.times do
	  card = gets.chomp
	  raise "Invalid card #{card}" unless
	    card.length == 2 &&
	    CARDS.include?(card[0,1]) &&
	    suits.find { |suit| suit[0,1].downcase == card[1,1] }
	  raise "Duplicate card #{card}" if hand.include? card
	  hand << card
	end
	
	# ...

Bob begins by defining the traditional card order, and the four suits.

The next chunk of code reads the trump suit and hand into variables.  This code
is careful to ensure proper input.  Suits must be one of the known four and
cards must be two characters, have a known face and suit, and not be duplicated.

	# ...
	
	# rotate trump suit to front
	suits.push(suits.shift) while suits.first != trump
	
	# if hand is void in second suit, swap second and fourth
	# (this keeps output in alternating colors)
	unless hand.find { |card| card[1,1] == suits[1][0,1].downcase }
	  suits[1], suits[3] = suits[3], suits[1]
	end
	
	# ...

This chunk of code does two things.  First, the suits are "rotated" until trump
is the first suit.  Note that this is why "CARDS" was declared as a constant but
"suits" is just a variable.  Good hints for the reader in this code.

The second thing the code does is the trick many hit on to defeat my extra
red/black requirement.  If we have four suits ordered red, black, red, black (or
black, red, black, red), the only edge case we can do anything about is when the
player doesn't have the second suit.  Here's the mapping of possible orderings
(assuming Spades is trump) to show what I mean:

	all four suits               (black, red, black, red ordering works fine)
	
	Spades, Hearts, and Clubs    (already correct)
	Hearts, Clubs, and Diamonds  (already correct)
	Spades, Hearts, and Diamonds (can't move trump to fix)
	Spades, Clubs, and Diamonds  (the fixable case)
	
	just two suits               (moves won't change anything)
	a single suit                (already correct)

In the fixable case, swapping the (missing) second and (present) fourth suits
resolves the issue.  That's what you're seeing in the code above.

	# ...
	
	# generate a sort order
	deck = []
	suits.each do |suit|
	  CARDS.each do |card|
	    deck << card + suit[0,1].downcase
	  end
	end
	
	# ...

Given that the correct suit order has been established, the entire Euchre deck
is built, in traditional card order.

	# ...
	
	# move bowers to front
	deck.insert(0, deck.delete_at(3))
	deck.insert(1, deck.delete_at(15))
	
	# ...

With the full deck in proper suit order, the only thing that remains is to shift
the bowers to the head of the pack.  Their location is assured, thus the use of
magic numbers above, though index() is an alternative.

	# ...
	
	# output sorted hand (n.b. Array#& (intersection) seemed to work, but
	# is the order guaranteed?)
	puts trump
	puts deck.select { |card| hand.include? card }

With the work out of the way, showing the results is trivial.  Print trump and
then just walk the deck printing any card in the hand.

I think that solution is very straight-forward and easy to accept as correct,
even without a large group of tests.  Let's look at one more variation of the
same theme, by Dominik Bathon:

	order = {
	    "Spades"   => "JsJcAsKsQsTs9sAdKdQdJdTd9dAcKcQcTc9cAhKhQhJhTh9h",
	    "Hearts"   => "JhJdAhKhQhTh9hAsKsQsJsTs9sAdKdQdTd9dAcKcQcJcTc9c",
	    "Clubs"    => "JcJsAcKcQcTc9cAhKhQhJhTh9hAsKsQsTs9sAdKdQdJdTd9d",
	    "Diamonds" => "JdJhAdKdQdTd9dAcKcQcJcTc9cAhKhQhTh9hAsKsQsJsTs9s"
	}
	
	trump = gets.strip
	cards = readlines.map { |l| l.strip }
	o = order[trump].dup
	# do we have a card of the 2nd suit
	unless cards.any? { |card| card[1] == o[15] }
	    # if not replace second suit by the last
	    o[14, 12] = o[36, 12]
	end
	puts trump, cards.sort_by { |card| o.index(card) }

This is basically the same thing, with prebuilt decks.  The code here just
selects which of the four possible decks to use base on the trump indication in
the input.

Again we see the second and fourth suits swapped, if needed.  This time it is
done with a simple String copy.

Finally, cards are sorted, by their position in the complete deck.

My thanks to all the card sharks who solved the quiz and even taught me
alternate hand ordering schemes.

Tomorrow we're off to the Pinewood Derby, so everyone pick up a car kit sometime
this evening...