#!/usr/bin/env ruby

# (c) 12/2006 Yoann Guillot
# this code is licenced under the terms of the WTFPL
# full text available at http://sam.zoy.org/wtfpl/

# a simple math expression parser & evaluator
# does not handle unary operators or not fully parenthesised expressions


require 'strscan'

class Node
	def initialize(l, op, r)
		@l, @op, @r = l, op, r
	end

	def eval(values)
		l = if @l.kind_of? Node
			@l.eval(values)
		else
			values[@l]
		end
		r = if @r.kind_of? Node
			@r.eval(values)
		else
			values[@r]
		end
		l.send(@op, r)
	end

	def self.gettok(ss)
		ss.scan(/\s*/)
		if ss.scan(/\(/)
			e = getexpr(ss)
			raise 'missing )' unless ss.scan(/\)/)
			e
		else
			ss.scan(/\w+/)
		end
	end

	def self.getexpr(ss)
		l = gettok(ss)
		ss.scan(/\s*/)
		# put all operators here
		return l if not op = ss.scan(/[-+&|]/)
		r = gettok(ss)
		new(l, op, r)
	end

	def self.calc(str, values)
		e = getexpr(StringScanner.new(str))
		if e.kind_of? self
			e.eval(values)
		else
			values[e]
		end
	end
end

values = { 'foo' => [1, 2, 3], 'bar' => [3, 4, 5], 'baz' => [4] }

p Node.calc('(foo | bar) - baz', values)