river.rb

#!/usr/bin/env ruby

# "1 rabbit saw 3 elephants while going to the river"

# - rabbit was going to the river
# - elephants were going to the river
# - rabbit and elephants were going to the river

[
  { rabbits: { count: 1, going_to_river: true }, elephants: { count: 3, going_to_river: false } },
  { rabbits: { count: 1, going_to_river: true }, elephants: { count: 3, going_to_river: true } },
  { rabbits: { count: 1, going_to_river: false }, elephants: { count: 3, going_to_river: true } },
].each do |state|
  # "every elephant saw 3 monkeys going to the river"

  # - monkeys were going to the river
  # - monkeys were not going to the river

  # and, anywhere from 3 to 9 monkeys

 [false, true].each do |monkeys_going_to_the_river|

   (3..9).each do |monkeys|

     state = state.merge(monkeys: { count: monkeys, going_to_river: monkeys_going_to_the_river })

     # "each monkey had 1 tortoise in each hand"

     # Let's assume that each monkey has at most 2 hands
     # (no extra limbs, and the lower limb extremeties are "feet")
     # - anywhere from 0 to 2 hands, for each monkey independently
     # therefore anywhere from 0 to 2n hands

     (0..2 * monkeys).each do |monkey_hands|

       # Anywhere from 1 (assuming at least 1 hand) tortoise, to N tortoises.
       # They could all be holding the same tortoise.

       tortoises_range = if monkey_hands > 0
                           (1..monkey_hands)
                         else
                           [0]
                         end

       tortoises_range.each do |tortoises|

         # - tortoises were going to the river [if the monkeys were going the river]
         # - tortoises were being taken to the river against their will, so were not "going" of their own accord

         [false, true].each do |tortoises_going_to_the_river|
           next if tortoises_going_to_the_river && !monkeys_going_to_the_river

           state = state.merge(tortoises: { count: tortoises, going_to_river: tortoises_going_to_the_river })

           # "How many animals were going to the river?"
           answer = state.values.select {|animal| animal[:going_to_river]}.map {|animal| animal[:count]}.sum

           require 'json'
           puts JSON.generate(state: state, answer: answer)

         end

       end

     end

   end

 end

end