A ruby version of the Letters and Numbers "Numbers Round" solver. Check my previous gist for a Scala version with more info. REQUIRES RUBY 2.0! due to use of lazy enumerators.

lan.rb

class Equation

  attr_reader :ops

  def initialize(ops)
    @ops = ops
  end

  def calc
    calc_rec(@ops)
  end

  def calc_rec(equation)
    chunk = equation.take(3)
    if chunk.size == 1
      chunk.first
    else
      calc_rec(equation[3..-1].unshift(calc_chunk(*chunk)))
    end
  end

  def calc_chunk(a, op, b)
    a = a.to_i
    b = b.to_i
    case op
      when "+" then a + b
      when "-" then a - b
      when "*" then a * b
      when "/" then a / b
    end
  end

end

class Solver

  def initialize(cards, goal)
    # cards is an array of ints
    @cards = cards
    @goal = goal
  end

  def possibilities
    # should return a lazy enumerator of all possible equations
    # made from cards and ops
    hands = @cards.permutation.lazy
    opMix = %w{+ - * /}.repeated_combination(@cards.size - 1)
    hands.flat_map do |h|
      opMix.flat_map do |o|
        Equation.new(h.zip(o).flatten.compact)
      end
    end
  end

  def best_equation
    best = nil
    best_diff = Float::INFINITY
    possibilities.each do |p|
      a = p.calc
      if (a - @goal).abs < best_diff
        best = p
        best_diff = (a - @goal).abs
        break if best_diff == 0
      end
    end
    best
  end

end

s = Solver.new [25, 10, 50, 7, 2, 4], 900
puts s.best_equation.ops.join " "