From: Ruby Quiz <james@...>
Date: 2006-01-07T03:56:47+09:00
Subject: [QUIZ] Dice Roller (#61)

The three rules of Ruby Quiz:

1.  Please do not post any solutions or spoiler discussion for this quiz until
48 hours have passed from the time on this message.

2.  Support Ruby Quiz by submitting ideas as often as you can:

http://www.rubyquiz.com/

3.  Enjoy!

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

by Matthew D Moss

Time to release your inner nerd.

The task for this Ruby Quiz is to write a dice roller. You should write a
program that takes two arguments: a dice expression followed by the number of
times to roll it (being optional, with a default of 1). So to calculate those
stats for your AD&D character, you would do this:

	> roll.rb "3d6" 6
	72  64  113  33  78  82

Or, for something more complicated:

	> roll.rb "(5d5-4)d(16/d4)+3"
	31

[NOTE: You'll usually want quotes around the dice expression to hide parenthesis
from the shell, but the quotes are not part of the expression.]

The main code of roll.rb should look something like this:

	d = Dice.new(ARGV[0])
	(ARGV[1] || 1).to_i.times { print "#{d.roll}  " }

The meat of this quiz is going to be parsing the dice expression (i.e.,
implementing Dice.new). Let's first go over the grammar, which I present in a
simplified BNF notation with some notes:

	<expr> := <expr> + <expr>
	       |  <expr> - <expr>
	       |  <expr> * <expr>
	       |  <expr> / <expr>
	       |  ( <expr> )
	       |  [<expr>] d <expr>
	       |  integer
	
	* Integers are positive; never zero, never negative.
	* The "d" (dice) expression XdY rolls a Y-sided die (numbered
	  from 1 to Y) X times, accumulating the results.  X is optional
	  and defaults to 1.
	* All binary operators are left-associative.
	* Operator precedence:
	          ( )      highest
	          d
	          * /
	          + -      lowest

[NOTE: The BNF above is simplified here for clarity and space. If requested, I
will make available the full BNF description I've used in my own solution, which
incorporates the association and precedence rules.]

A few more things...  Feel free to either craft this by hand or an available
lexing/parsing library.  Handling whitespace between integers and operators is
nice.  Some game systems use d100 quite often, and may abbreviate it as "d%"
(but note that '%' is only allowed immediately after a 'd').