pitchers_problem.rb

# Or: require 'facets/enumerable/find_yield'
module Enumerable
  def map_detect
    self.each do |member|
      if (result = yield(member))
        return result
      end
    end
    nil
  end
end

class Array
  # [:a, :b, :c, :d].merge(1 => :bb, 3 => :dd) #=> [:a, :bb, :c, :dd]
  def merge_from_hash(hash)
    map.with_index { |x, idx| hash.fetch(idx, x) }
  end
end

class PitchersProblem
  def initialize(total)
    @total = total
  end
  
  def solve(current, final, solution = [])
    if current == final
      [solution + [current]]
    else
      (0...@total.size).to_a.permutation(2).map do |i1, i2|
        free2 = @total[i2] - current[i2]
        if current[i1] > 0 && free2 > 0
          moved = [current[i1], free2].min
          new_current = current.merge_from_hash({
            i1 => current[i1] - moved, 
            i2 => current[i2] + moved,
          })
          if !solution.include?(new_current)
            solve(new_current, final, solution + [current])
          end
        end
      end.compact.flatten(1)
    end
  end
end

require 'pp'
problem = PitchersProblem.new([10, 7, 3])
p problem.solve([10, 0, 0], [5, 5, 0]).min_by(&:length)