solve.rb

require 'minitest/spec'
require 'minitest/autorun'

class Solve
  attr_reader :expected, :numbers

  def initialize expected, *numbers
    @expected = expected
    @numbers = numbers
  end

  def solution
    if operators
      format_answer operators
    else
      "No solution found for #{format_answer ['?'] * number_of_ops}"
    end
  end

  private

  def format_answer ops
    left_side = numbers.zip(ops).flatten.join ' '
    "#{left_side}= #{expected}"
  end

  def number_of_ops
    numbers.count - 1
  end

  def op_combinations
    [:+, :-, :/, :*].repeated_permutation number_of_ops
  end

  def operators
    @operators ||= op_combinations.find do |ops|
      begin
        iterator = 0
        expected == numbers.reduce do |result, number|
          new_result = result.send ops[iterator], number
          iterator +=1
          new_result
        end
      rescue ZeroDivisionError
        false
      end
    end
  end

end

describe Solve do
  it "returns a single operator with 2 numbers" do
    Solve.new(5, 1, 4).solution.must_equal "1 + 4 = 5"
  end

  it "returns 2 operators with 3 numbers" do
    Solve.new(10, 1, 8, 1).solution.must_equal "1 + 8 + 1 = 10"
  end

  it "works with different operators" do
    Solve.new(7, 1, 8, 1).solution.must_equal "1 * 8 - 1 = 7"
  end
  
  it "tells you when there is no solution" do
    Solve.new(100, 1, 1, 1).solution.must_equal "No solution found for 1 ? 1 ? 1 = 100"
  end
end