largest_multiple.rb

#!/usr/bin/env ruby
# Copyright (c) 2016 Pete Hanson
# frozen_string_literal: true
# :reek:UtilityFunction: { exclude: %w(largest_multiple) }

NUMBER = %(
  73167176531330624919225119674426574742355349194934
  96983520312774506326239578318016984801869478851843
  85861560789112949495459501737958331952853208805511
  12540698747158523863050715693290963295227443043557
  66896648950445244523161731856403098711121722383113
  62229893423380308135336276614282806444486645238749
  30358907296290491560440772390713810515859307960866
  70172427121883998797908792274921901699720888093776
  65727333001053367881220235421809751254540594752243
  52584907711670556013604839586446706324415722155397
  53697817977846174064955149290862569321978468622482
  83972241375657056057490261407972968652414535100474
  82166370484403199890008895243450658541227588666881
  16427171479924442928230863465674813919123162824586
  17866458359124566529476545682848912883142607690042
  24219022671055626321111109370544217506941658960408
  07198403850962455444362981230987879927244284909188
  84580156166097919133875499200524063689912560717606
  05886116467109405077541002256983155200055935729725
  71636269561882670428252483600823257530420752963450
)

# Sequences and their products
class SequenceProducts
  def initialize number, ndigits
    @ndigits = ndigits
    @sequences = sequences number
  end

  def largest_product
    @sequences.max_by { |sequence| self.class.product sequence }
  end

  def self.product sequence
    sequence.inject :*
  end

  private

  def sequences number
    digits = number.scan(/\d/).map(&:to_i)
    digits.each_cons @ndigits
  end
end

if __FILE__ == $PROGRAM_NAME
  (1..13).each do |ndigits|
    sequence = SequenceProducts.new(NUMBER, ndigits).largest_product
    product = SequenceProducts.product sequence
    puts "#{product} = #{sequence.join ' x '}"
  end
end

# Answer: 23,514,624,000 = 5 x 5 x 7 x 6 x 6 x 8 x 9 x 6 x 6 x 4 x 8 x 9 x 5