/r/dailyprogrammer #73 intermediate

rpn.rb

#!/usr/bin/env ruby

class RPNError < ArgumentError
   def initialize sym, stack
      args = stack.reverse.map.with_index {|el, i|
         "#{el}.#{sym} (#{i} for #{el.method(sym).arity})"
      }.join(', ')
      super "wrong number of arguments: #{args}"
   end
end

stack = []

ARGV[0].scan(/(-?[0-9]+)|(self|true|false)|(\S+)/).each do |num, keyw, sym|
   if num
      stack.push num.to_i
   elsif sym
      args_n = nil
      obj, i = stack.each.with_index.find {|el, i|
         arity = el.method(sym).arity
         args_n = stack.size - i - 1
         arity < 0 ? (arity.abs-1 <= args_n) : (arity == args_n)
      }
      raise RPNError.new(sym, stack) unless obj
      stack.push obj.send(sym, *stack.pop(args_n+1).drop(1))
   else
      stack.push({:self => stack, :true => true, :false => false}[keyw.to_sym])
   end
end

puts "=> #{stack.first.inspect}"