From: Christian Neukirchen <chneukirchen@...>
Date: 2006-01-13T00:22:30+09:00
Subject: Re: [QUIZ.SUMMARY] Dice Roller (#61)

Matthew Moss <matthew.moss.coder@gmail.com> writes:

> First off we have the random number generation; most solutions had this or a
> very similar variant. So the call 3.d(6) will roll and accumulate three
> six-sided dice, and the expression 3.d(6) is almost a valid dice expression.
> It is in Christian's initialization method that dice expressions are turned
> into Ruby expressions:
>
>     def initialize(dice)
>        @src = dice.gsub(/d(%|00)(\D|$)/, 'd100\2').
>                    gsub(/d(\d+)/, 'd(\1)').
>                    gsub(/(\d+|\))d/, '\1.d').
>                    gsub(/\d+/) { $&.gsub(/^0+/, '') }
>
>        @dice = eval "lambda { #@src }"
>     end
>
> (I've left out a bit of error checking; see the full solution to see
> everything.)  The first substitution fixes up percentage and double-zero
> values as 100. The second turns 'd' as a binary operator into 'd' as a
> method call. The third substitution provides the default dice count of 1
> if it wasn't specified. And the last substitution removes leading zeroes of
> integers; this last substitution prevents the Ruby from interpreting values
> as octal.

The third substitution doesn't provide a default count (Dice#d does that),
instead, it converts 2d(6) to 2.d(6).

> The morphed express is saved away in a lambda, which allows Christian to
> reevaluate the expression repeatedly without performing those substitutions
> every time roll is called.

I could have evaluated @src each time without substituting anew, but a
lambda only makes it need to *parse* once.

Thanks for the nice writeup,
-- 
Christian Neukirchen  <chneukirchen@gmail.com>  http://chneukirchen.org