evaluate expression from string (and follow order of operations)

math_eval_with_ooo.rb

require 'json'

class StringCalc
	def initialize(str)
		@tokens = str.split('')

		# puts @tokens.to_json
	end

	def run
		tokens = reduce(@tokens, %w(/ *))
		# puts tokens.to_json
		tokens = reduce(tokens, %w(+ -))
		# puts tokens.to_json
		tokens.first.to_i
	end

	def reduce(tokens, ops)
		new_tokens = []
		iter = tokens.each
		while true
			begin
				token1 = iter.next
				# puts "token1 #{token1}"
				if token1 =~ /\d/
					new_tokens << token1
				else
					token2 = iter.next
					# puts "token2 #{token2}"
					if ops.include?(token1)
						lhs = new_tokens.pop
						# puts "lhs #{lhs}"
						new_tokens << process(lhs, token1, token2)
					else
						new_tokens << token1 << token2
					end
				end
			rescue StopIteration
				return new_tokens
			end
		end
		new_tokens
	end

	def process(lhs, op, rhs)
		lhs.to_i.public_send(op, rhs.to_i).to_s
	end
end

puts StringCalc.new("5+4*3").run