Shunting Yard algorithm

shunting_yard.rb

class ShuntingYard

	@@op_precedence = {
		'+' => 2,
		'-' => 2,
		'*' => 3,
		'/' => 3,
		'^' => 4
	}

	def op_is_left?(operator)
		return (operator =~ /\^/) == nil
	end

	def initialize(expression)
		@output = []
		@stack = []
		@expression = expression.delete(" ")
		@pc = 0
	end

	def result
		while has_tokens
			parse_number if token_is_number?
			parse_operator if token_is_operator?
			parse_left_parenthesis if token_is_left_parenthesis?
			parse_right_parenthesis if token_is_right_parenthesis?
		end

		consume_full_stack

		@output.join(" ")
	end

	private

	def token
		@expression[@pc]
	end

	def has_tokens
		return @pc < @expression.length
	end

	def token_is_number?
		return (token =~ /[0-9]/) != nil
	end

	def token_is_operator?
		return @@op_precedence.has_key?(token)
	end

	def token_is_left_parenthesis?
		return token == "("
	end

	def token_is_right_parenthesis?
		return token == ")"
	end

	def parse_number
		number = ""
		while token_is_number?
			number << token
			@pc += 1
		end
		@output.push(number)
	end

	def parse_operator
		while @stack.length > 0 and @@op_precedence.has_key?(@stack.last)
			if @@op_precedence[token] < @@op_precedence[@stack.last] or (op_is_left?(token) and @@op_precedence[token] <= @@op_precedence[@stack.last]) 
				@output.push(@stack.pop)
			else
				break
			end
		end
		@stack.push(token)
		@pc += 1
	end

	def parse_left_parenthesis
		@stack.push(token)
		@pc += 1
	end

	def parse_right_parenthesis
		@pc += 1
		while @stack.length > 0
			t = @stack.pop
			break if t == "("
			@output.push(t)
		end
		#TODO validade if ( was found
	end

	def consume_full_stack
		while @stack.length > 0
			@output.push(@stack.pop)
		end
	end
end

shunting_yard_spec.rb

require_relative "shunting_yard"

describe ShuntingYard do
	it "Only a number" do
		ShuntingYard.new('1').result.should == '1'
		ShuntingYard.new('12').result.should == '12'
		ShuntingYard.new('34534').result.should == '34534'
	end

	it "Simple Operation" do
		ShuntingYard.new('1+2').result.should == '1 2 +'
		ShuntingYard.new('33*345').result.should == '33 345 *'
	end

	it "Multiple Operations" do
		ShuntingYard.new('1+2*3').result.should == '1 2 3 * +'
		ShuntingYard.new('23+6*12/2^2').result.should == '23 6 12 * 2 2 ^ / +'
	end

	it "Operations with parenthesis" do
		ShuntingYard.new('(1+2)*3').result.should == '1 2 + 3 *'
		ShuntingYard.new('3 + 4 * 2 / (1 - 5) ^ 2 ^ 3').result.should == '3 4 2 * 1 5 - 2 3 ^ ^ / +'
	end
end