Dice Roller (#61)

Ruby
1

Hi! This is my first entry to a RUBY-QUIZ. Sure I could have opted to
use
‘eval’, but I figured I might as well learn a little about expression
parsing. Fun stuff!

My entry is focused around The Shunting Yard Algorithm, allowing me to
parse, transform and evaluate in the same step. I do not keep the
‘postfix’
transform, so it is slightly inefficient to perform multiple ‘rolls’.
That
is an optimization for another day.

I also got to try out some new regexp stuff. Of particular note is the
use
of (?=d) in the expression used to search for d’s that need an implicit
1
lvalue. I was having problems with 5dddd7 type commands until I
discovered
this allowed the target d NOT to be consumed by the regexp.

As a final note I am a Ruby N., and a Java developer by day, so any
tips
or comments on my coding would be appreciated. Thanks!

  • John

$DEBUG = false

Dice Roller entry point

def roll_dice( dice_command, roll_count )
begin
puts “Executing #{roll_count} roll(s) of #{dice_command}”
results, total = Dice.new( dice_command ).roll( roll_count )
puts “Result: [#{results.join(’, ')}] => #{total}”
rescue Exception => e
puts “Roll error: #{e}”
end
end

class Dice

operator => [precendence, associativity]

@@operators = { “d” => [3, :right],
“*” => [2, :left] , “/” => [2, :left],
“+” => [1, :left] , “-” => [1, :left]
}

Initialize the Stacks and load the dice instructions

def initialize( dice_command )
if $DEBUG
alias :d :dnd_roll_loaded
else
alias :d :dnd_roll_random
end

@operator_stack, @value_stack = [], []
prepare_instructions( dice_command )

end

def roll( roll_count )
results = (1…roll_count).collect { execute }
[results, results.inject {|sum, item| sum + item } || 0]
end

private

The infix command is parsed into tokens and then executed using

The Shunting Yard Algorithm. Evaluation is done “on-the-fly” as

items are placed on the value stack (acting as the post-fix

“output”).
def execute
@operator_stack.clear
@value_stack.clear

# Process the tokens in L -> R order
# Look for non-digit characters and numbers
@instructions.scan(/\D|\d+/) do | token |
  case token
    when "("
      @operator_stack.push token

    when /\d+/ # any number
      @value_stack.push token.to_i
     
    when /[-\+*\/d]/ # the operators
      finished = false
      until finished or @operator_stack.empty?
        if higher_operator(token)
          finished = true
        else
          resolve_expression
        end
      end
      @operator_stack.push token

    when ")"
      resolve_expression while @operator_stack.last != "("
      @operator_stack.pop
     
    else
      raise "Invalid token found: #{token}"
  end
end

resolve_expression while !@operator_stack.empty?

raise "Unexpected problem. #{@value_stack.size} values remain after

execution."
unless @value_stack.size == 1
@value_stack.pop
end

def resolve_expression
opr, rhv, lhv = @operator_stack.pop, @value_stack.pop,
@value_stack.pop
raise “No more values left for #{opr} to consume!” unless rhv && lhv

value = (opr == "d") ? value = d( lhv, rhv ) : lhv.send( opr, rhv )
@value_stack.push value.to_i

end

def dnd_roll_random( roll_count, die_value )
(1…roll_count).inject(0) { |value, item| value + ( rand(die_value)

  • 1
    ) }
    end

    def dnd_roll_loaded( roll_count, die_value )
    roll_count * die_value
    end

    def higher_operator(opr)
    if associativity(opr) == :left
    precedence(opr) > precedence(@operator_stack.last)
    else
    precedence(opr) >= precedence(@operator_stack.last)
    end
    end

    def precedence(opr)
    @@operators[opr] ? @@operators[opr][0] : 0
    end

    def associativity(opr)
    @@operators[opr] ? @@operators[opr][1] : :left
    end

    def prepare_instructions( dice_command )

    1) Eliminate all whitespace.

    2) Substitute d100 for d%

    3) Insert the implied 1 if a d is the first character

    or is preceded by an operator other than ‘)’

    @instructions = dice_command.gsub(/\s+/, ‘’)
    @instructions.gsub!(/d%/, ‘d100’)
    @instructions.gsub!(/([-+*/(d]|\A)(?=d)/, ‘\11’)
    puts “Normalized instructions: #@instructions” if $DEBUG

    raise “Unmatched left / right parenthesis” unless
    @instructions.scan(/(/).size == @instructions.scan(/)/).size
    end
    end

Argument parsing

if $0 == FILE
raise “DiceRoller dice_command [roll_count=1]” unless (1…2).include?(
ARGV.length )
roll_dice(ARGV[0], ARGV[1] ? ARGV[1].to_i : 1)
end