Dice Roller (#61) We don't need no steenking leexer/parsers


#1

Leexer/parsers? We ain’t got no Leexer/parsers. We don’t need no
Leexer/parsers. I don’t have to show you any steenking Leexer/parser.
Just call eval and use Ruby’s fine lexer/parser (apologies to Mel
Brooks, John Huston and Banditos Mexicanos everywhere).

This approach uses regex substitutions to first munge the input
expression to deal with the default cases (like d6 to 1d6 and 1% to
1d100), then it substitutes ** for d and hands it over to the
evaluator and prints the result.

Eval does what we want after an override of Fixnum.** to give us our
dice-rolling d-operator behavior. Conveniently, ** has the desired
precedence relative to the other operators, plus it is binary and
left-associative. This feels so evil. Seduced by the Dark Side I am.

Note that we get lot’s of additional behavior we don’t need, but the
original spec said the BNF was incomplete, so I was lazy and left the
undef[ining] of >>, << et al is ‘as an exercise…’ I don’t know
enough D&D to know if they might be useful.

BTW. In the spirit of ‘why can’t we all just get along?’: the dice
rolling algorithm is ported from this Python code at
http://www.onlamp.com/pub/a/python/2002/07/11/recipes.html?page=3

from random import randrange
def dice(num,sides):
return reduce(lambda x,y,s=sides:x + randrange(s), range(num+1))+num

here is the Ruby:

#!/usr/bin/env ruby

roll.rb

fix up Fixnum to override ** with our desired d behavior

class Fixnum
def ** (sides)
# validation
if sides<1
raise “Invalid sides value: ‘#{sides}’, must be a positive
Integer”
end
if self<1
raise “Invalid number of rolls: ‘#{self}’, must be a postitive
Integer”
end
# roll the dice
(0…self).to_a.inject{|x,y| x + rand(sides)}+self
end
end

dice_expression = ARGV[0]

default number of rolls is 1, substitute d6 => 1d6

dice_expression = dice_expression.gsub(/(^|[^0-9)\s])(\s*d)/, ‘\11d’)

d% => d100

dice_expression = dice_expression.gsub(/d%/,'d100 ')

this feels so dirty…substitute d => **

dice_expression = dice_expression.gsub(/d/, “**”)

(ARGV[1] || 1).to_i.times { print "#{eval(dice_expression)} " }


#2

On Mon, Jan 09, 2006 at 03:57:20AM +0900, Paul N. wrote:
[…]
} here is the Ruby:
}
} #!/usr/bin/env ruby
} #
} # roll.rb
} #
}
} # fix up Fixnum to override ** with our desired d behavior
} class Fixnum
} def ** (sides)
} # validation
} if sides<1
} raise “Invalid sides value: ‘#{sides}’, must be a positive
Integer”
} end
} if self<1
} raise “Invalid number of rolls: ‘#{self}’, must be a postitive
Integer”
} end
} # roll the dice
} (0…self).to_a.inject{|x,y| x + rand(sides)}+self

I think I may be misunderstanding something here. You use 0…self, when
I
would think it would have to be either 0…self or 1…self to get the
right
number of rolls. Am I off? In my solution I used 1…self in much the
same
way.

} end
} end
[…]

–Greg


#3

No, the mis-understanding is all mine. You are right, otherwise it
will include an extra roll. It should be:
(1…self).to_a.inject(0){|x,y| x + rand(sides)}+self

Further proof of the value of good unit tests. (Since (1…1).to_a
returns a single-member array, you need to provide inject with an
initial value to make it work.)

Regards,

Paul.


#4

This isn’t so much a solution to the quiz, but more of an addition to
the whole dice rolling thing. Assume you have an array of numbers
between 1-6… outputs the face of the dice in ascii art, one for each
element in the array:

class Array
def to_dice
logic = [
lambda{|n| '±----+ '},
lambda{|n| (n>3 ? '|O ’ : '| ')+(n>1 ? ’ O| ’ : ’ | ')},
lambda{|n| (n==6 ? '|O ’ : '| ')+
(n%2==1 ? ‘O’ : ’ ')+(n==6 ? ’ O| ’ : ’ | ')},
lambda{|n| (n>1 ? '|O ’ : '| ')+(n>3 ? ’ O| ’ : ’ | ')}
]

	str=''
	5.times {|row|
		self.each {|n| str += logic[row%4].call(n) }
		str+="\n"
	}
	str
end

end

#Example:
puts [1,2,3,4,5,6].to_dice

-Joby


#5

Here’s my own solution, which isn’t nearly as pretty or clever as some
of the stuff I’ve seen. I had started revising it, but it started
looking more and more like Dennis Ranke’s solution, which wasn’t good
since I was looking at his code to see how to do certain things in
Ruby. =)

In any case, I’ve decided to post my original, not-so-clever version.
But I’m glad to see all the entries; it’ll be interesting to write up
a summary. I’ve certainly learned a lot.


class Dice

TOKENS = {
:integer => /[1-9][0-9]*/,
:percent => /%/,
:lparen => /(/,
:rparen => /)/,
:plus => /+/,
:minus => /-/,
:times => /*/,
:divide => ///,
:dice => /d/
}

class Lexer
def initialize(str)
@str = str
end

 include Enumerable
 def each
    s = @str
    until s.empty?
       (tok, pat) = TOKENS.find { |tok, pat| s =~ pat && $`.empty? }
       raise "Bad input!" if tok.nil?
       yield(tok, $&)
       s = s[$&.length .. -1]
    end
 end

end

class Parser
def initialize(tok)
@tokens = tok.to_a
@index = 0
@marks = []
end

 def action
    @marks.push(@index)
 end

 def commit
    @marks.pop
 end

 def rollback
    @index = @marks.last
 end

 def next
    tok = @tokens[@index]
    raise "Out of tokens!" if tok.nil?
    @index += 1
    tok
 end

end

def initialize(str)
@parser = Parser.new(Lexer.new(str))
@dice = expr
end

def roll
@dice.call
end

def expr
# fact expr_
expr_(fact)
end

def expr_(lhs)
# ‘+’ fact expr_
# ‘-’ fact expr_
# nil

 @parser.action

 begin
    tok = @parser.next
 rescue
    res = lhs
 else
    case tok[0]
    when :plus
       rhs = fact
       res = expr_(proc { lhs.call + rhs.call })
    when :minus
       rhs = fact
       res = expr_(proc { lhs.call - rhs.call })
    else
       @parser.rollback
       res = lhs
    end
 end

 @parser.commit
 res

end

def fact
# term fact_
fact_(term)
end

def fact_(lhs)
# ‘*’ term fact_
# ‘/’ term fact_
# nil

 @parser.action

 begin
    tok = @parser.next
 rescue
    res = lhs
 else
    case tok[0]
    when :times
       rhs = term
       res = fact_(proc { lhs.call * rhs.call })
    when :divide
       rhs = term
       res = fact_(proc { lhs.call / rhs.call })
    else
       @parser.rollback
       res = lhs
    end
 end

 @parser.commit
 res

end

def term
# dice
# unit term_

 begin
    res = dice(proc { 1 })
 rescue
    res = term_(unit)
 end

 res

end

def term_(lhs)
# dice term_
# nil
begin
res = term_(dice(lhs))
rescue
res = lhs
end

 res

end

def dice(lhs)
# ‘d’ spec

 @parser.action

 tok = @parser.next
 case tok[0]
 when :dice
    rhs = spec
    res = proc { (1 .. lhs.call).inject(0) {|s,v| s += 

rand(rhs.call)+1 }}
else
@parser.rollback
raise “Expected dice, found #{tok[0]} ‘#{tok[1]}’\n”
end

 @parser.commit
 res

end

def spec
# ‘%’
# unit

 @parser.action

 tok = @parser.next
 case tok[0]
 when :percent
    res = proc { 100 }
 else
    @parser.rollback
    res = unit
 end

 @parser.commit
 res

end

def unit
# ‘(’ expr ‘)’
# INT (non-zero, literal zero not allowed)

 @parser.action

 tok = @parser.next
 case tok[0]
 when :integer
    res = proc { tok[1].to_i }
 when :lparen
    begin
       res = expr
       tok = @parser.next
       raise unless tok[0] == :rparen
    rescue
       @parser.rollback
       raise "Expected (expr), found #{tok[0]} '#{tok[1]}'\n"
    end
 else
    @parser.rollback
    raise "Expected integer, found #{tok[0]} '#{tok[1]}'\n"
 end

 @parser.commit
 res

end
end

main

d = Dice.new(ARGV[0] || “d6”)
(ARGV[1] || 1).to_i.times { print "#{d.roll} " }