alien-flowers.rb

#!/bin/ruby
# https://www.hackerrank.com/contests/worldcodesprint/challenges/colorful-ornaments

require 'set'

A,B,C,D = gets.strip.split(' ').map(&:to_i)

RR = %w(R R)
RB = %w(R B)
BB = %w(B B)
BR = %w(B R)

PAIRS = [RR, RB, BB, BR]

class Problem
    attr_reader :valid_chains
    NEXT_PAIRS = {
        'B' => [BB, BR],
        'R' => [RR, RB]
    }
    TARGET_PAIRS = {
        RR => A,
        RB => B,
        BB => C,
        BR => D
    }
    def run
        @valid_chains = 0
        generate_next_char('R', { RR => 0, RB => 0, BB => 0, BR => 0 })
        generate_next_char('B', { RR => 0, RB => 0, BB => 0, BR => 0 })
    end

    def generate_next_char(last, counts)
        chains = [
            [last, counts]
        ]
        while chains.any?
            last, counts = chains.pop
            
            if valid?(counts)
                @valid_chains += 1
                next
            end
            
            NEXT_PAIRS[last].each do |possible_next_pair|
                next if counts[possible_next_pair] >= TARGET_PAIRS[possible_next_pair]
                chains.push [
                    possible_next_pair[1],
                    counts.merge({
                        possible_next_pair => counts[possible_next_pair] + 1
                    })
                ]
            end
        end
    end
    
    def valid?(counts)
        counts == TARGET_PAIRS
    end
end

problem = Problem.new
problem.run
puts problem.valid_chains % (10**9+7)